Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Crack-a-two-a => Topic started by: TurbOSquiD77 on November 24, 2016, 09:04:26 AM
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Hello everyone and happy holiday!
Starting my Crackatwoa build today. Can I roll these tubes? I did in the Crack but just wanted to double check for use with it's big brother.
Svetlana 6AS7G/6H13C "Winged C"
Philips Miniwatt ECC82 made in Heerlen , Holland, 1962.
https://i.imgur.com/2RWRxxZ.jpg
https://i.imgur.com/SjBE6OD.jpg
Any suggestions you think would be fun to try with the shunt voltage reg. tubes?
Cheers! ;D
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The allowable tubes are identical between the Crack and Crack-a-two-a.
For the shunt reg tubes, I tend to use 6005's.
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The allowable tubes are identical between the Crack and Crack-a-two-a.
For the shunt reg tubes, I tend to use 6005's.
For rolling the Shunt reg tube, would the effect of the overall sound be as pronounce as rolling the input and power tube? Thank you
Krit.
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In general rolling the shunt reg tube is audible but much more subtle than rolling the signal tubes. Probably something best tried for yourself rather than relying upon opinions.
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In general rolling the shunt reg tube is audible but much more subtle than rolling the signal tubes. Probably something best tried for yourself rather than relying upon opinions.
Thanks Doc. I don' have crackatwoa yet, just gathering some info. Since its on my next to purchase list, after I experiment on mod on my Crack to see how each mod effects the sound. I am lerning so much and its so much fun! I am proud to say that I am bottlehead addict now.
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Is it okay to run a GE 12BH7A in the Crackatwoa? I'm also using the 6AS7G from above, and just recieved GE 6005/6AQ5W's :)
How about for the E80CC?
I'm unsure how to decipher the modifications needed to run various tubes, as most of the posts about tube rolling are for the Crack. I shall continue my studies. lol. If someone could point me in the right direction for popular tubes, input and power, it surely would help the initial struggle.
Thank you - all help is appreciated!
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Actually, I might have an explanation for why the 12BH7A wouldn't be a direct replacement for the 12AU7. If you find anything incorrect, or something that could be explained more effectively please advise...etc. Pulled this from datasheets, but I noticed some numbers were not shown for certain tubes.
12AU7 - Vf 6.3 Volts / If 0.3 Ampere
CV4003 - Vf 6.3 Volts / If 0.3 Ampere
ECC82 - Vf 12.6 Volts / If 0.15 Ampere - Tapped heater also permits operation at 6.3V 0.3A. (I do not understand why, but I currently run this tube with the 6AS7G both pictured above)
12BH7A - Vf 12.6 Volts / If 0.3 Ampere - Durch Heizfaden-Mittenanschluss auch Betrieb mit 6.3 V und 0.6 A möglich. Google translate: By filament center connection also operation with 6.3 V and 0.6 A possible. (I do not understand why)
Other specs:
Plate Voltages, Current, and Resistance
Grid Voltage
Amplification Factor
Transconductance
Links for more info on the following questions are much appreciated :) Forum threads and other stuff. It's tough to find a needle in a haystack, especially when it's a specific needle like the C2A. Wait, the Crackatwoa is an amplifier - not a needle. Anyways.....
What would be the correct way to go about calculating if a tube type will work with the stock Crack/Crackatwoa? How about modded (output caps, TwoQuiet, and in use with other input/power tubes)?
What justifies if a tube is within an allowed range of value in order to be used without further modifications?
What tube specifications are most important to pay attention to? Although, I'm sure all of them influence something....
I could insert the tubes I'd like to use, do a full voltage/resistance check from the manual, and compare that to values of the stock tubes.
Seems logical, but after the tests I would not know where to make adjustments. I can spend some time on it and post all values if need be!
Is there a standard way to implement changes needed to run tubes of certain specs? (the resistor at location X needs to be changed if the voltage at location Y is +/- outside optimal range...etc.)
I will spend time learning basic circuitry, but I was wondering the order in which power flows through the amplifier.....
Example: Power comes in, goes to [object/location/terminal], then goes to [object/location/terminal]....etc.
Would this be an effective way to troubleshoot/adjust component values?
Example: Location X voltage too high, so replace the resistor just before it to decrease voltage at location X.
I'd like to implement switches, like some Crack builders have, to allow use of different tubes.
I'd also like to implement a switch allowing me to change between 2 separate sets of output capacitors. Or just make connectors and need to power off, disconnect/connect them, power on.
Definitely liking this project - just a bit 8)
Pics soon!
Thank you
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The 12BH7 and E80CC will work in the C2A, but the two R1 resistors on the small PC board will need to be swapped out with 470 Ohm parts.
Filament voltage isn't a constraint on the driver tube, there's a bit of headroom on that winding.
As far as what will and won't work, the first consideration is whether the tube in question is pin compatible. For instance, the 6CG7 is electrically a drop-in substitute for the 12AU7 in the C2A, but its pinout is different, so you would have to rewire the socket to get it to work.
The next consideration is the overall operating point. If you look at the plate curves and see where the 1.5V grid bias line hits 3.5mA, how close are you to 75V? If you are off one way or the other, then current has to be adjusted. In the C2A, since it is shunt regulated, there is a bit of a limit to how far you can adjust current one way or the other, though this shouldn't be much of an impediment ultimately due to the range of plate current generally needed for these small dual triodes.
The last consideration is gain. Just because a tube is pin compatible with a 12AU7 and can be made to produce the appropriate plate voltage doesn't mean it will be a good fit in the circuit. For example, a 12AX7 with 0.5V of bias an 0.75mA of current will make the appropriate plate voltage to work in the Crack, but the extra 16dB of gain will be a nuisance.
-PB
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Thank you Paul.
These cool?
http://www.mouser.com/ProductDetail/Vishay-Dale/RN55E4700FB14/?qs=sGAEpiMZZMtlubZbdhIBIEq9S5QvvTQUOPhdG1TlS6g%3d
Description seems incorrect - they are indeed 125 mW (1/8 W) by part number.
I've found this:
http://db.audioasylum.com/cgi/m.mpl?forum=bottlehead&n=119635
"This comes up periodically. We use specifric resistors in specific places for specific reasons.
Carbon composition is the best for low inductance, which is important in grid stoppers which are effective at hundreds of MHz. They are noisy, but only if they have DC current through them, and the noise is proportional to the resistance. So as grid stoppers (no current) and as plate stoppers (small resistance, and located after the first signal amplification stage so that noise is 30dB less important) they are OK. In both applications, drift of the resistance is not important.
Plate resistors are usually metal film, which is generally very quiet even with DC current, and they are also readily available in 1% or better precision. They do have a bit of inductance, but it's not significant at audio frequencies. We also use them in the RIAA equalization network, where precision and low noise are important. We avoid them when small amounts of inductance are important.
Wirewound is the quietest, but usually has the most inductance. We us them in power supplies where the small inductance helps isolate diode switching transients and power line RFI noise.
Metal oxide is noisy and usually imprecise, but they are very sturdy (withstand voltage transients and momentary overloads without damage) so they make good power supply bleeders where the large parallel capacitance can shunt the noise to ground.
Hope that helps! "
Could you briefly elaborate on the physical locations of where resistor types function best?
[Resistor Type] = [Circuit Section] = [Physical Part - C4S Boards, sube sockets, RIAA network...etc.]
If you look at the plate curves and see where the 1.5V grid bias line hits 3.5mA, how close are you to 75V?
Could you elaborate on this? Basically follow the 1.5V grid bias line until 3.5mA and see what voltage you're at?
Does this apply to input tubes only? Or power as well? Is the driver tube considered the input or power tube? I'm guessing power, but the word "input" throws my understanding off.
This info, in addition to studying circuitry, greatly aids in the initial learning process.
-Regards 8)
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These cool?
http://www.mouser.com/ProductDetail/Vishay-Dale/RN55E4700FB14/?qs=sGAEpiMZZMtlubZbdhIBIEq9S5QvvTQUOPhdG1TlS6g%3d
Description seems incorrect - they are indeed 125 mW (1/8 W) by part number.
Vishay Dale RN55 are in practice rated up to 0.5W as they derated for military usage. The commercial equivalent is the CMF55.
Yes, they are nice, I use them extensively.
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Could you briefly elaborate on the physical locations of where resistor types function best?
[Resistor Type] = [Circuit Section] = [Physical Part - C4S Boards, sube sockets, RIAA network...etc.]
Grid stoppers are connected to the tube grids. You can look at the datasheet for each tube to see which pins are grids. If you stick with metal film you're pretty much safe for just about anything you may run into (other than high power, where there tend not to be metal film resistors big enough)
Could you elaborate on this? Basically follow the 1.5V grid bias line until 3.5mA and see what voltage you're at?
Yes, the C2A driver circuit has a fixed bias voltage of 1.5V (the LED) and a constant current source set at about 3.5mA. With these two numbers, you can look at a tube's plate curves to see what plate voltage you'd get with 1.5V of bias an 3.5mA of current.
Does this apply to input tubes only? Or power as well? Is the driver tube considered the input or power tube? I'm guessing power, but the word "input" throws my understanding off.
In the C2A design, this applies to the 12AU7 (you could call it a preamp or driver tube, it is not the power tube, the 6080 is the power tube).
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Vishay Dale RN55 are in practice rated up to 0.5W as they derated for military usage. The commercial equivalent is the CMF55.
Yes, they are nice, I use them extensively.
Nice! Thank you. So that's a yes on the RN, CN, or either? I'm guessing either, but I want to be sure that if I'm ordering a 125 mW (1/8 W) resistor, that is what I'm getting.
Closest CM I can find is CMF55470R00FKR6 (1/2 W 470 ohms). Though, I read somewhere that wattage isn't quite that important. True, or only in some cases?
Yes, the C2A driver circuit has a fixed bias voltage of 1.5V (the LED) and a constant current source set at about 3.5mA. With these two numbers, you can look at a tube's plate curves to see what plate voltage you'd get with 1.5V of bias an 3.5mA of current.
Helps a ton! How might I apply this to the power circuit?
Where should I take measurements from if (after replacing the R1 resistors on the low current C4S board with 470ohm 1/8W) I'd like to insert a 12BH7 or e80cc and test to ensure all is well?
Thank you :o
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Closest CM I can find is CMF55470R00FKR6 (1/2 W 470 ohms). Though, I read somewhere that wattage isn't quite that important. True, or only in some cases?
You can use Ohm's Law to determine this. 3.5mA passes through 470 Ohms, P=(I^2)*R, so P=(0.0035^2)*470, so the resistor will dissipate 0.005 Watts of power, and for long life, we would want a resistor that's rated for at least 0.015 Watts of power. So a resistor that's rated for at least 1/60th of a Watt will work (for this position at least).
Helps a ton! How might I apply this to the power circuit?
That's quite a can of worms. You really should leave this alone. Trying to alter the operating current of the output stage can have adverse effects on the operation of the amplifier, and the presence of the shunt regulator means that the whole amp would need to be redesigned if this is messed with. Additionally, there are very few dual triodes with enough transconductance to offer the low output impedance available with the 6080.
Where should I take measurements from if (after replacing the R1 resistors on the low current C4S board with 470ohm 1/8W) I'd like to insert a 12BH7 or e80cc and test to ensure all is well?
OA and OB on the small center C4S board.
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You can use Ohm's Law to determine this.
That's quite a can of worms.
OA and OB on the small center C4S board.
Sounds good to me, and thanks for explaining. :D
Good to know about the power circuit - I shall not touch! *Imagines Gandalf
Just threw a Tung Sol 5998 in the C2A. Loving it 8) sounds more airy and detailed.
New sound due to the higher transconductance? The amp seems to have higher gain - louder all around (running Beyer T1.2's and HD6XX/50's). Also have DT-990 600 Ohm's but don't use as often.
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OA and OB on the small center C4S board.
I've replaced the R1 resistors on the low current board with 470 Ohm Dale's.
Here are my voltages (the numbers on the right are mine):
Tubes:
Tung-Sol 5998 (also grabbed voltage of OA and OB on Low board, and OB on High board with the 6080 in place for comparison.)
GE 12BH7A
GE 6005's
OA and OB on Low Current C4S are half of what they should be.
OB on the High Current C4S is low as well.
Terminal Voltage (DC)
Low Current C4S
IA 150V 150v
OA 60-90V 35.1v Damn near the same with 6080
KregA 3-6V 5.42v
bRegA 150V 150v
IB 150V 148.9v
OB 60-90V 36.3v Damn near the same with 6080
KregB 3-6V 5.45v
bRegB 150V 148.9v
High Current C4S
IA 190V 193.2v
OA 150V 148.9v
bA 0V .1mV
IB 0V 0-3mV
OB 90-110V 51.4v 68.5v with 6080
bB 150V 148.9v
Thank you!
Travis
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Is it possible that I could run with the 237 Ohm resistors at R1 on the low current board? Trying voltage test....
6080, 12BH7A, 6005's, 237 Ohm resistor at R1 on Low Current board:
Terminal Voltage (DC)
Low Current C4S
IA 150V 150V
:o OA 60-90V 47.6V
KregA 3-6V 5.73V
bRegA 150V 150V
IB 150V 148.9V
:o OB 60-90V 48.8V
KregB 3-6V 5.86V
bRegB 150V 148.9V
High Current C4S
IA 190V 192V
OA 150V 150V
bA 0V .3mV
IB 0V .6mV
:o OB 90-110V 77.1V
bB 150V 150V
Tung-Sol 5998, GE 12BH7A, GE 6005's, 470 Ohm resistor at R1 on Low Current board:
Terminal Voltage (DC)
Low Current C4S
IA 150V 150V
:o OA 60-90V 35.1V Damn near the same with 6080
KregA 3-6V 5.42V
bRegA 150V 150V
IB 150V 148.9V
:o OB 60-90V 36.3V Damn near the same with 6080
KregB 3-6V 5.45V
bRegB 150V 148.9V
High Current C4S
IA 190V 193.2V
OA 150V 148.9V
bA 0V .1mV
IB 0V 0-3mV
:o OB 90-110V 51.4V (5998) 68.5V (6080)
bB 150V 148.9V
I'll give her a listen with the 237 Ohm resistors at R1 Low Current board - 12BH7/5998. :)
Happy weekend!
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.... and while doing further research I've found that:
"Yeah, ~105V is on the high side, 55V is very low, 88V is in the safe zone.
Those 470 Ohm resistors you linked to will work well with the E80CC. For the 12BH7, try a 200 Ohm resistor to bring the plate voltage up.
-PB"
I was thinking that running the 237 Ohm resistor with the 12BH7 and 5998 was okay, and the voltage drop was normal - something to do with the 5998 maybe. I was wrong and should not be running the amplifier out of the allowed range of measurements.
I listened with 237 Ohm resistors at R1 - 12BH7(A)/5998 and I didn't like it as much as the Philips ECC82. I'm going to throw the ECC82 back in until I grab some 200 Ohm resistors - then I will try the 12BH7 again. I'd rather not run low voltage for an extended period of time. From what PB was explaining, it's not good especially with the 5998's higher transconductance and lower plate voltage.
Definitely trying to find a good single E80CC! Sounds awesome from what I read. 8)
-T
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For the books: 5998/ECC82/6005's - using 237 Ohm resistors at R1 on the Low Current C4S board.
Terminal Voltage (DC)
Low Current C4S
IA 150V 150V
:) OA 60-90V 76.2V
KregA 3-6V 5.70V
bRegA 150V 150V
IB 150V 148.9V
:) OB 60-90V 76.5V
KregB 3-6V 5.92V
bRegB 150V 148.9V
High Current C4S ( / indicates left and right High Current board measurements)
IA 190V 192.6/192.3V
OA 150V 148.9V/150V
bA 0V .3mV/.5mV
IB 0V .2mV/.5mV
:o ::) OB 90-110V 84.9V/84.3V
bB 150V 148.9V/150V
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I'm sorry, I'm mixing up the resistor mod for the E80CC with what you need to do for the 12BH7.
Try an R1 of 124 Ohms on the low current C4S R1. Post your measurements for Kreg on each side after you've done this. R1A on the high current C4S boards may also need an adjustment in value as well.
-PB
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I'm sorry, I'm mixing up the resistor mod for the E80CC with what you need to do for the 12BH7.
Try an R1 of 124 Ohms on the low current C4S R1. Post your measurements for Kreg on each side after you've done this. R1A on the high current C4S boards may also need an adjustment in value as well.
-PB
Will do! Would you happen to know what value resistor I'd be looking at to adjust R1A on the high current boards? Ballpark range if exact value is unknown - I'm looking to order some resistors of different values to have on hand. :)
6SN7/(GT) looks interesting as well. I read the 12BH7 is most of the 6SN7 minus a bit of tubey sound.
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If you want to buy some resistors, values between 11 and 12 ohms are likely what that R1 should change to. Also changing the 270 Ohm 5W resistors out to 300R 5W resistors will alleviate some C4S thermal load.
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Also changing the 270 Ohm 5W resistors out to 300R 5W resistors will alleviate some C4S thermal load.
Awesome. Looking at Mouser for 300R 5W:
Vishay/Dale RE60G3000C02 chassis mount wirewound 1%
Ohmite 805F300E chassis mount wirewound 1%
Vishay/Draloric AC05000003000JAC00 Cement Filled Ceramic 5%
Ohmite TUM5J300E Cement Filled Ceramic 5%
Xicon 280-CR5-300-RC Cement Filled Ceramic 5%
I read inductive types are good, and chassis mount types need proper chassis surface area for the heat sink to function at optimal temps. Unsure if 1% tolerance is favored.
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Interesting....
The amp has been on for 5 hours. No headphones, and no RCA's connected (listening to other amp).
I go upstairs for an hour and come down to the power transformer dissipating heat into the air more than usual.
The chassis is hotter than usual. Slight smell (if any).
Rapidly fluctuating pissed off cyborg noise from the power transformer. It's done this before, and during my voltage tests, but not as audible.
Took quite a while for the PT-8 to cool down.
Fuse looks good.
Sleep.
;D
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An angry power transformer will come with voltages that aren't what they are supposed to be.
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Agreed, haha.
Forgot to solder an LED and resistor after cleaning up some solder joints. Fixed, voltage test, all is good. E80CC showed up today so I thought I'd give her a shot, and then the 12BH7A.
5998/E80CC/6005's:
470 ohm at r1 on low current board - with e80cc
Terminal Voltage (DC)
Low Current C4S
IA 150V 150.1V
OA 60-90V 67.3V
KregA 3-6V 5.45V
bRegA 150V 149V
IB 150V 149.1V
OB 60-90V 64.1V
KregB 3-6V 5.37V
bRegB 150V 150.1V
High Current C4S ( / indicates left and right High Current board measurements)
IA 190V 193/192.9V
OA 150V 149/150V
bA 0V .3/.5mV
IB 0V .3/.5mV
:o OB 90-110V 74.4/76.8V
bB 150V 149/150V
5998/12BH7A/6005's:
124 ohm at r1 on low current board
Terminal Voltage (DC)
Low Current C4S
IA 150V 150V
OA 60-90V 63.3V
:o KregA 3-6V 6.40V
bRegA 150V 148.9V
IB 150V 148.9V
OB 60-90V 65V
:o KregB 3-6V 6.59V
bRegB 150V 150V
High Current C4S ( / indicates left and right High Current board measurements)
IA 190V 195/194.7V
OA 150V 148.9/150V
bA 0V .3/.5mV
IB 0V .3/.5mV
:o OB 90-110V 75.3/73.2V
bB 150V 148.9/150V
I have a few 1/4 W resistors between 11 and 12.1 Ohms. Also picked up a few different brands of 5W 300R wirewound ceramic/cement resistors.
Thank you Paul I appreciate you giving your knowledge. Hit me with explanations if you could. I'll make sense of them, and I like figuring things out.
-T
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Yeah, you could go down to 12.1 for R1A on the high current C4S for each side. This will give the 6AQ5's more current, as we want the cathode follower and shunt regulator to run roughly the same current.
When you pull more current through the circuit, you pull more current through the power supply resistors. That is the reason for my recommendation to go to a 10W part. When you set the C4S to deliver more current, the MJE5731A will have to dissipate more heat. By going to a 300 Ohm resistor in each leg of the power supply, that effect is mitigated by lowering the voltage available at the input of the C4S.
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Awesome, I will try and see how she performs!
Just to clarify.... should those be 5 or 10 watt 300R resistors in the power supply? I have a few types of 5W ceramic/cement.
Is Kreg okay, or will the other changes adjust the voltage here?
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5W parts will be OK for the 300 Ohm power supply resistors.
The Kreg voltage will change when you put the 12.1 Ohm resistors in. This change will not need to be undone if you go back to the 12AU7 or E80CC.
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Nice, good to know! Thank you
-T
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5W parts will be OK for the 300 Ohm power supply resistors.
The Kreg voltage will change when you put the 12.1 Ohm resistors in. This change will not need to be undone if you go back to the 12AU7 or E80CC.
Hey, hope the show was awesome!
I finally got to switch out the power supply resistors to 300R 5W, 124 Ohms at R1 low current, and 12.1 Ohms at R1A on the high current boards. Those wirewounds were quite the task!
Here are my numbers:
5998/12bh7a - 124 ohm at r1 on low current board - 12.1 ohm at R1 on high current - 300R psu resistors
Terminal Voltage (DC)
Low Current C4S
IA 150V 149.2V
OA 60-90V 63V
KregA 3-6V 5.25V
bRegA 150V 149.2V
IB 150V 148.7V
OB 60-90V 64.8V
KregB 3-6V 5.58V
bRegB 150V 148.7V
High Current C4S ( / indicates left and right High Current board measurements)
IA 190V 189.9/190V
OA 150V 148.7/149.2V
bA 0V .4/.5mV
IB 0V .3/.5mV
:o OB 90-110V 75.2/72.8V
bB 150V 148.7/149.2V
What causes OB to read low?
-T
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Got around to measuring with the 6080. All are exactly or close to the same readings except OB on the high current boards. Is this due to the 5998 having higher plate resistance, and more than twice the moo's? I read that this won't affect heat, and that what does affect heat is the current the tube is pulling from the transformer. Apparently the 6080/6AS7G and 5998 are close according to a source found on head fi.
5998:
Class A Amplifier
Plate Voltage ................................. 110 V
Grid No. 1 Voltage Derived from
Cathode Bias Resistor ....................... 105 Ω
Amplification Factor .......................... 5.4
Plate Resistance (approx) ..................... 350 Ω
Transconductance .............................. 15.5K μ
Plate Current ................................. 100 mA
6AS7G:
Class A Amplifier
Plate Voltage ................................. 135 V
Grid No. 1 Voltage Derived from
Cathode Bias Resistor ....................... 250 Ω
Amplification Factor .......................... 2
Plate Resistance (approx) ..................... 280 Ω
Transconductance .............................. 7000 μ
Plate Current ................................. 125 mA
Let's measure with the 6080 vs the 5998 measurements in my post just before this one:
6080/12bh7a - 124 ohm at r1 on low current board - 12.1 ohm at R1 on high current - 300R psu resistors
Terminal Voltage (DC)
Low Current C4S
IA 150V 149.2V
OA 60-90V 62.9V
KregA 3-6V 5.26V
bRegA 150V 149.2V
IB 150V 148.7V
OB 60-90V 64.8V
KregB 3-6V 5.58V
bRegB 150V 148.7V
High Current C4S ( / indicates left and right High Current board measurements)
IA 190V 189.1/189V
OA 150V 148.7/149.2V
bA 0V .3/.5mV
IB 0V .3/.5mV
8) OB 90-110V 87.3/87V
bB 150V 148.7/149.2V
Is this okay or should I look into bringing high current OB's voltage back up when running the 5998? However, OB on the low current board is fine. Interesting! I see pcb traces going through R2B, R1B, Y/Y+, and in the opposite direction being output caps/headphone jack/OB low current board, as well as pin 6 and 3 on the 5998 socket.
Possibilities:
R2B (22KΩ 5W)
R1B (31.6Ω ⅛W)
Y/Y+ (33KΩ ⅛W)
Nice post btw - just for everyones reference:
http://bottlehead.com/smf/index.php?topic=4965.0
Quote from: TurbOSquiD77 on February 14, 2017, 03:42:42 pm
Helps a ton! How might I apply this to the power circuit?
That's quite a can of worms. You really should leave this alone. Trying to alter the operating current of the output stage can have adverse effects on the operation of the amplifier, and the presence of the shunt regulator means that the whole amp would need to be redesigned if this is messed with. Additionally, there are very few dual triodes with enough transconductance to offer the low output impedance available with the 6080.
Not about to go that route if it involves much more than component value changes. That is, if changing R2B..etc is where this is heading. Just hoping to work towards keeping her healthy :)
Huzzuh!!!!
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When OA and OB on the small C4S board are a little low in their range (like yours) that will bring the OA and OB voltages of the high current C4S boards that feed the 6080 down a little bit as well.
To nudge them back up, the R1 on the center small C4S board will have to be lowered a bit. If you're at 124R now, try a 113R and see if that perks up your plate voltages (and consequently 6080 cathode voltages) a little bit.
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Cool, I will try and dial her in. Thanks Paul
Will having the cathode voltage low at OB on the high current boards be a problem if I continue to use the 5998?
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Nah, better a little low than really high, though there's some thermal headroom on those heatsinks.
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Numbers are looking good after the adjustments - will share soon!
Quick question before hitting the hay....
I've replaced the stock 270R 5W power resistors with 300R 5W. Easy, kind of!
I am now (after some patience and time and in the end rebuilding the entire power supply with new parts anyways [UF4007-GP-TP Ultra Fast rectifiers - found em! ;)]) running 300R 10W.
Holy hell that's a tight fit - they are pretty long.
All voltages checked out great - same or better numbers than those previously posted.
I think I'm hearing some clipping. Very light, seems intermittent, and at odd times on the same track. But sometimes at the exact same time, on the same track. It's just a little snap/pop right? But, the C2A should be more than enough for Beyers and Senns. hmmm
Happy listening, pics to come.
-T
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Seems as if my soldering iron (not me of course) caused some damage to the Nichicons - metal is showing a bit. I have new ones on the way. Power transformer is making more noise than usual. However, all voltages check out great.
Can I redo the incoming voltage test with everything connected? Since I've modified the power resistors with 300R 10W's. I also had to curve the rectifiers in the power supply quite a bit to get them around the resistors. This might be contributing to something but not sure.
I've also been battling dirty power coming from my PC. What a nightmare. Many steps have been taken, and next is a power conditioner.
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Yeah, this is part of the process of doing strange modifications. There may be problems that come up that will take some time and effort to resolve on your own (I haven't tried the mods, so I'm not sure what might be happening).
Your incoming line voltage is very, very unlikely to change appreciably. If you live in an area where this happens, you will already be very aware!
What happened to the UF4007's that came with the kit? Was there an issue that required their removal? From where did you source the new diodes?
The power transformer shouldn't be making any additional noise unless you've put a considerable additional load on it. This would generally be reflected by low voltage measurements and hot parts. Just as a test, you can pull all the tubes out and turn the kit on, then listen for the noise again.
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Yeah, this is part of the process of doing strange modifications. There may be problems that come up that will take some time and effort to resolve on your own (I haven't tried the mods, so I'm not sure what might be happening).
Your incoming line voltage is very, very unlikely to change appreciably. If you live in an area where this happens, you will already be very aware!
Totally understand! I think I've created my own confusion....
Long story short I believe my new dac is the source of a very small pop/clipping sound which is randomly present in my music. New dac in conjunction with installing 10W power resistors (and second guessing my fitment/soldering, damn things are long!) made me think that the C2A power supply was the source of the issue.
What happened to the UF4007's that came with the kit? Was there an issue that required their removal? From where did you source the new diodes?
I rebuilt the entire power supply by properly desoldering, removing, cleaning with Caig Deoxit D5/D100, re positioning brand new parts along with the 10W 300R, soldering with Cardas silver solder (with lead), and cleaning with Deoxit D5/D100 and a pick to scrape the flux, topping it off with Deoxit Shield. Cleaned the transformer leads as well in the same manner. Everything looks super good! :)
Sourced all parts from Mouser, the exact values of the stock kit.
294-270K-RC
http://www.mouser.com/Search/ProductDetail.aspx?R=294-270K-RCvirtualkey21980000virtualkey294-270K-RC
UF4007GP-TP
http://www.mouser.com/Search/ProductDetail.aspx?R=UF4007GP-TPvirtualkey54720000virtualkey833-UF4007GP-TP
The power transformer shouldn't be making any additional noise unless you've put a considerable additional load on it. This would generally be reflected by low voltage measurements and hot parts. Just as a test, you can pull all the tubes out and turn the kit on, then listen for the noise again.
It seems as if the power transformer and I have a love/hate relationship. Every time I change something, rebuild, or upgrade I find myself with the notion that the transformer is louder and/or hotter than before. Maybe it's because I'm worried. Maybe it's because I goofed (yep once lol). But I remember that when I did goof, the transformer was much more crunchy pissed cyborg sounding and HOT than it is now. Transformer hum is always audible, but only when I put my ear 6-12" away. It's a little bit higher pitched of a hum, not low and smooth. It does get hot... typically I can grab the transformer for 5 seconds or longer before needing to let go. That is, when she's been on for a couple hours pumpin' jams. I never see smoke, but do see heat dissipating which can be mistaken for smoke depending on lighting.
Those 10W resistors are huge! Can I connect the diodes to the same terminals, but in a different fashion? Such as, upper vs. lower terminal. This would help positioning them immensely, and also allow me to straighten the diodes. As of now, a couple of the diodes are curved to fit around the resistors, as the resistor leads won't allow me to move any further away from the diodes. Or, I can just use 5W resistors if 10W isn't absolutely needed, and without consequence.
Thanks Paul,
I'll try to keep the questions to whether or not I can do something, or what parts to use, rather than spewing my issue before taking all the troubleshooting steps. I have come to the conclusion my issue is not the C2A, it is my Amanero USB in the dac that is causing a random pop noise. C2A sounds good right now :)
-T
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The 10W power supply resistors aren't necessary, just use the 5W ones.
Deoxit is a cleaner for connectors. I would recommend against using it while soldering. The solder Flux exists to clean and prep the joint for you, and if Deoxit leaves behind any corrosion inhibitors, it may actually make soldering more difficult.
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Mental note made. May only use Deoxit for post solder joint cleaning - it does work very well to remove flux residue. I did not realize the solder flux cleaned the joint so well before hardening. Thought it was just a little thing*
Popping issue fixed by using ASIO Bridge software. Sounding good now :)
Will the diodes be okay being curved a bit? From a physics perspective I'm not quite sure.
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Mental note made. May only use Deoxit for post solder joint cleaning - it does work very well to remove flux residue.
AFAIK, those active contact cleaners should also be washed away with a normal solvent, because the the residues can cause corrosion themselves!
I don't know deoxit specifically, but I know that from the "Kontakt" brand that is popular here, that there are 3 types of spray that are best used together:
- "Kontakt 60" for deoxidation
- "Kontakt WL" for general cleaning
- "Kontakt 61" for protection against oxidation
From what you read online it's a common mistake that deoxidizer is used without washing it away.
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Thanks troplin! I picked up some CRC contact cleaner (quick dissolve kind) and have been using it with a foam swab so I don't over spray.
Update on resistor value changes for 12BH7A and E80CC:
5998/12bh7a
113 ohm at r1 on low current board - 12.1 ohm at R1 on high current board
300R 10W psu resistors
Jupiter Copper Foil .10uF 600V on low current board
Rebuilt power supply with standoffs to accomodate 10W power resistors
Terminal Voltage (DC)
Low Current C4S
IA 150V 149.3V
OA 60-90V 70.5V +7 from previous 124 ohm R1
KregA 3-6V 5.56V
bRegA 150V 149.2V
IB 150V 148.7V
OB 60-90V 72V +7 from previous 124 ohm R1
KregB 3-6V 5.51V
bRegB 150V 148.7V
High Current C4S ( / indicates left and right High Current board measurements)
IA 190V 190.5/190.6V
OA 150V 148.7/149.2V
bA 0V .3/.5mV
IB 0V .3/.5mV
OB 90-110V 81/79.2V +6 from previous 124 ohm R1
bB 150V 148.7/149.2V
5998/ e80cc
453 ohm at r1 on low current board - 12.1 ohm at R1 on high current
300R 10W psu resistors
Jupiter .10uF 600V on low current board
Rebuild power supply with standoffs
Terminal Voltage (DC)
Low Current C4S
IA 150V 149.3V
OA 60-90V 68.4V
KregA 3-6V 4.24V
bRegA 150V 149.3V
IB 150V 148.7V
OB 60-90V 64.9V
KregB 3-6V 4.58V
bRegB 150V 148.7V
High Current C4S ( / indicates left and right High Current board measurements)
IA 190V 189.7/189.6V
OA 150V 148.7/149.3V
bA 0V .3/.6mV
IB 0V .3/.5mV
OB 90-110V 74.8/76.8V
bB 150V 148.7/149.3V
Moving from 470 ohms to 453 ohms at R1 yielded a value of 1V greater at OA, OB on low current board, and .5 ohms (left) 0 ohms (right) at OB on the high current boards. So, 17 ohms less yielded 1V greater or less. I have 412, 422, and 432 ohm values available to try. 412 ohms maybe?
Are there any reasons why I should not go to a lower value with R1 on the low current board? I want to make sure this position isn't relied upon other than voltage levels, or if voltage is the only thing I'm concerned with here. I could possibly go lower for the 12BH7A as well - less than 100 ohms?
Regards,
-T
Ps. She's sounding really, really good ;D and the power transformer makes next to no noise for the first couple hours, but starts to make a bit of noise after that - it does get quite hot to the touch but not enough to burn me.
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None of those voltages look particularly concerning. What you have to look out for is having something like 50V or 100V coming out of the small center PC board, or having unusually low Kreg voltages in the regulator.
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Anyone rolled the 6AQ5's to any effect? I picked up a 5-tube sleeve of Sylvania Gold Brand 6005's to try when my build is complete.
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The 6AQ5s don't have a ton of influence over the sound, but let us know what you find.
-PB
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I certainly will.
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So, almost four months later, I finally found some time to sit down and do a comparison of the 6AQ5/6005 tubes I have on hand.
My Audio chain:
Lossless FLAC via Foobar2000 > Topping D50 DAC > customized Crackatwoa (Sylvania 6SN7W short bottle driver, Tung Sol 421A power tube) > ZMF Auteur > my head. Everything is plugged into a Furman PST-8D power conditioner and my DAC is linear power supplied, because, unfortunately, I am insane.
Here are the shunt regulator tube players:
Sylvania 6AQ5 (year unknown, stock tubes)
Sylvania Gold Brand 6005 (no date codes on boxes or tubes, I believe they are mid 60’s)
GE Five-Star 6005 ‘57 (white print)
GE Five-Star 6005 ‘62 (red print)
So I should start out by saying that when I first received the tubes, I did a quick AB comparison between the Sylvania Gold Brand and the ’57 GE Five-Star, nothing comprehensive at all, but at the time, I kind of thought the GE’s sounded better, so that is what I have been running since finishing my Crackatwoa. I did not realize how right I was at the time.
I used the same song for all of my listening tests, I find that acoustic singer/songwriter music works well for me when critical listening. Chose a song I am very familiar with, “Between the Bars†by Elliot Smith. God only know what kind of psychological trauma I have inflicted on myself listening to this song one-hundred times.
My comparison started with what I thought would be the two highest performers, the Sylvania Gold Brand and the ’57 GE Five-Star. After switching back and forth five or six times, listening to the first verse of the song, what I thought were subtle differences became more glaringly obvious. The Sylvania Gold Brand sounded very much more claustrophobic, dull, and lacked texture relative to the GE’s (all relative, of course). By comparison, the GE’s really increased the width of the track, adding far more texture to Smith’s double-tracked vocals, and more body to the music. I continued going back and forth between the tubes to try and confirm what I was hearing, doing my best to stave off confirmation bias. No doubt in my mind what I heard was legit, the GE’s were much better.
From there, I decided to compare the ’62 GE Five-Star and ’57 GE Five-Star, figuring they would most likely sound identical. Well, that wasn’t true either. While the differences were not as glaring as they were between the Sylvania Gold Brand and the ’57 GE’s, the ’57 GE’s still bested the ’62 GE’s. The width and texture on the ’62 GE’s was better than on the Gold Brands, but the ’57 GE’s still managed the largest sound stage and detail retrieval out of the three sets of tubes.
Next I compared the Sylvania 6AQ5 stock tubes to the Sylvania Gold Brand. To my surprise, the stock tubes sounded better than the Gold Brand, to me, despite them not being the more premium “ruggedized†6005 model. Differences between these two tubes were, again, smaller than between the ’57 GE’s and the Gold Brand, but appreciable. My hierarchy was starting to form.
Lastly, I compared the ’62 GE’s to the stock tubes. The GE’s did better on width and texture than the stock tubes, but not an enormous difference there. I did a few more back-and-forth listens to the ’57 GE’s to reconfirm my initial thoughts, and they did that.
So, the ranking goes as follows:
GE Five-Star 6005 ‘57 > GE Five-Star 6005 ‘62 > Sylvania 6AQ5 (stock) > Sylvania Gold Brand 6005
It is somewhat fortunate I compared the best and worst tubes right off the bat. Had I not heard the stark differences, maybe I would not have continued. Now, these differences are not night-and-day (not like switching between stock Crack and Crack+SB, or 6080 to 5998 power tube, for example), but they are definitely appreciable and significant when you are listening for them. This comparison got me REALLY excited, as I had not considered rolling the shunt regulator tubes a viable option to increase the sound quality of the Crackatwoa, and I’ve just about maxed out its performance in every other area. Of course, this was a sighted test, nothing scientific here, but I feel comfortable enough with my observations that I am going to roll a number of other shunt tubes and report back. Here is what I have coming so far:
-Siemens EL90 ‘65
-RCA EL90 ‘70 (commissioned Telefunken to manufacture these tube in Ulm, Germany, has Telefunken manufacture information on one side, RCA branding on the other)
-Tung Sol 6AQ5A smoked glass
-Tesla 6L31
-Tungsram 6AQ5
-Philips EL90
Some of these tubes are stateside, some are coming internationally. Unlikely anyone cares that much about this, but I’ll update here with impressions anyway :)
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So, almost four months later, I finally found some time to sit down and do a comparison of the 6AQ5/6005 tubes I have on hand.
My Audio chain:
Lossless FLAC via Foobar2000 > Topping D50 DAC > customized Crackatwoa (Sylvania 6SN7W short bottle driver, Tung Sol 421A power tube) > ZMF Auteur > my head. Everything is plugged into a Furman PST-8D power conditioner and my DAC is linear power supplied, because, unfortunately, I am insane.
Here are the shunt regulator tube players:
Sylvania 6AQ5 (year unknown, stock tubes)
Sylvania Gold Brand 6005 (no date codes on boxes or tubes, I believe they are mid 60’s)
GE Five-Star 6005 ‘57 (white print)
GE Five-Star 6005 ‘62 (red print)
So I should start out by saying that when I first received the tubes, I did a quick AB comparison between the Sylvania Gold Brand and the ’57 GE Five-Star, nothing comprehensive at all, but at the time, I kind of thought the GE’s sounded better, so that is what I have been running since finishing my Crackatwoa. I did not realize how right I was at the time.
I used the same song for all of my listening tests, I find that acoustic singer/songwriter music works well for me when critical listening. Chose a song I am very familiar with, “Between the Bars†by Elliot Smith. God only know what kind of psychological trauma I have inflicted on myself listening to this song one-hundred times.
My comparison started with what I thought would be the two highest performers, the Sylvania Gold Brand and the ’57 GE Five-Star. After switching back and forth five or six times, listening to the first verse of the song, what I thought were subtle differences became more glaringly obvious. The Sylvania Gold Brand sounded very much more claustrophobic, dull, and lacked texture relative to the GE’s (all relative, of course). By comparison, the GE’s really increased the width of the track, adding far more texture to Smith’s double-tracked vocals, and more body to the music. I continued going back and forth between the tubes to try and confirm what I was hearing, doing my best to stave off confirmation bias. No doubt in my mind what I heard was legit, the GE’s were much better.
From there, I decided to compare the ’62 GE Five-Star and ’57 GE Five-Star, figuring they would most likely sound identical. Well, that wasn’t true either. While the differences were not as glaring as they were between the Sylvania Gold Brand and the ’57 GE’s, the ’57 GE’s still bested the ’62 GE’s. The width and texture on the ’62 GE’s was better than on the Gold Brands, but the ’57 GE’s still managed the largest sound stage and detail retrieval out of the three sets of tubes.
Next I compared the Sylvania 6AQ5 stock tubes to the Sylvania Gold Brand. To my surprise, the stock tubes sounded better than the Gold Brand, to me, despite them not being the more premium “ruggedized†6005 model. Differences between these two tubes were, again, smaller than between the ’57 GE’s and the Gold Brand, but appreciable. My hierarchy was starting to form.
Lastly, I compared the ’62 GE’s to the stock tubes. The GE’s did better on width and texture than the stock tubes, but not an enormous difference there. I did a few more back-and-forth listens to the ’57 GE’s to reconfirm my initial thoughts, and they did that.
So, the ranking goes as follows:
GE Five-Star 6005 ‘57 > GE Five-Star 6005 ‘62 > Sylvania 6AQ5 (stock) > Sylvania Gold Brand 6005
It is somewhat fortunate I compared the best and worst tubes right off the bat. Had I not heard the stark differences, maybe I would not have continued. Now, these differences are not night-and-day (not like switching between stock Crack and Crack+SB, or 6080 to 5998 power tube, for example), but they are definitely appreciable and significant when you are listening for them. This comparison got me REALLY excited, as I had not considered rolling the shunt regulator tubes a viable option to increase the sound quality of the Crackatwoa, and I’ve just about maxed out its performance in every other area. Of course, this was a sighted test, nothing scientific here, but I feel comfortable enough with my observations that I am going to roll a number of other shunt tubes and report back. Here is what I have coming so far:
-Siemens EL90 ‘65
-RCA EL90 ‘70 (commissioned Telefunken to manufacture these tube in Ulm, Germany, has Telefunken manufacture information on one side, RCA branding on the other)
-Tung Sol 6AQ5A smoked glass
-Tesla 6L31
-Tungsram 6AQ5
-Philips EL90
Some of these tubes are stateside, some are coming internationally. Unlikely anyone cares that much about this, but I’ll update here with impressions anyway :)
Well, it's been nine months since I originally posted on the shunt regulator tube performance. While I did briefly compare the other tubes I had on the way months ago, since no one ever showed any interest, I never finished this post.
Today, I decided to do a re-hash of those comparisons and report on them (albeit, in a much more abbreviated format) in case some Bottlehead forum lurker is waiting with bated breath for my final verdict.
I compared the best set from the first round, the GE five-star 6005, with the remaining tubes.
Right to the point, here are my final thoughts.
GE five-star 6005 > RCA/Telefunken EL90 = Tungsram 6AQ5 = Philips EL90 > Tung Sol 6AQ5A >>> Siemens EL90
I know "better" is subjective, so I should clarify, what I am looking for in the comparison is detail retrieval, soundstage, clarity.
The GE's outclass the rest in these departments, although I would say they make the sound slightly more dry.
The big shocker for me was the Siemens. Holy distortion! Definitely make the sound more "tubey" but at the cost of detail and soundstage. I may give them another shot, since that could be appealing in some cases, but for pure technicality, not so much.
The Telefunken's, Tungsrams, and Philips tubes were too close to call, I could not tell a difference after a few listens. Maybe nuances could be gleaned with continued listening, but then I think that is trying too hard.
Tung Sol's were pretty disappointing, lacking technical prowess, but not so tubey (like the Siemens) that they are memorable.
So, the GE five-star 6005 are THE BEST performing shunt regulator tubes in the Crackatwoa. I have no idea why from an engineering perspective why that would be, but it is easily picked out. These are dirt cheap on Ebay, looks like GE made a lot of them, so I would suggest anyone with a Crackatwoa give them a shot.
Once again, this is not a Speedball-level upgrade in sound, but it is an appreciable difference.
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What Kreg voltages do you get with the Siemens tubes?
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Good work - thanks for posting!
I will however make a couple points - not to disagree, just to say that these explorations can be complicated. But first let me reiterate - I do not for a minute question that you heard what you heard. I am always grateful when listening tests are reported. It's one of the great things about the internet, we all get to benefit from vastly greater varieties of experience.
We all know that tubes lose performance as they age. My main point is that tubes also change as they break in. It usually takes 50-200 hours of conducting significant current to fully "form" the cathode. Until that happens, the emission capability is limited, and there is often some noise as well - sometimes just a subtle hiss, occasionally quite dramatic pops and crackles. The cathode is formed at the factory, but it's expensive and time consuming, so only the minimum is done. And it appears that the cathode loses its capability when unused for a long time. From your description, I speculate that the '57 GE was burned in before the serious comparisons started, and none of the others have been.
Second, a subsidiary point is that higher quality cathodes take longer to break in. This at least was understood in the "golden age" literature. Small amounts of silicon in the cathode base metal (nickel) speed the formation of the cathode, but also make it age and lose emission faster. So only the premium tubes have the high-purity cathode base material (expensive), and they take much longer to form before leaving the factory (also expensive).
I have no idea whether these effects influenced your comparisons, of course. But it's possible. Always keep an open mind!
On the subject of the regulator tube affecting the sound, this is much more likely when a cathode follower is involved. That's because the signal current loop includes the power supply (regulator in the C2A case). Back when we were developing the hybrid shunt regulator, I build a cathode follower with several candidate regulator topologies which could be switched for comparisons. We definitely heard differences! For comparison, the Mainline uses a current-source plate load which provides something like 50dB isolation from its (regulated) power supply. So does the BeePre.
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What Kreg voltages do you get with the Siemens tubes?
I'll check some point today or tomorrow, PB!
Good work - thanks for posting!
I will however make a couple points - not to disagree, just to say that these explorations can be complicated. But first let me reiterate - I do not for a minute question that you heard what you heard. I am always grateful when listening tests are reported. It's one of the great things about the internet, we all get to benefit from vastly greater varieties of experience.
We all know that tubes lose performance as they age. My main point is that tubes also change as they break in. It usually takes 50-200 hours of conducting significant current to fully "form" the cathode. Until that happens, the emission capability is limited, and there is often some noise as well - sometimes just a subtle hiss, occasionally quite dramatic pops and crackles. The cathode is formed at the factory, but it's expensive and time consuming, so only the minimum is done. And it appears that the cathode loses its capability when unused for a long time. From your description, I speculate that the '57 GE was burned in before the serious comparisons started, and none of the others have been.
Second, a subsidiary point is that higher quality cathodes take longer to break in. This at least was understood in the "golden age" literature. Small amounts of silicon in the cathode base metal (nickel) speed the formation of the cathode, but also make it age and lose emission faster. So only the premium tubes have the high-purity cathode base material (expensive), and they take much longer to form before leaving the factory (also expensive).
I have no idea whether these effects influenced your comparisons, of course. But it's possible. Always keep an open mind!
On the subject of the regulator tube affecting the sound, this is much more likely when a cathode follower is involved. That's because the signal current loop includes the power supply (regulator in the C2A case). Back when we were developing the hybrid shunt regulator, I build a cathode follower with several candidate regulator topologies which could be switched for comparisons. We definitely heard differences! For comparison, the Mainline uses a current-source plate load which provides something like 50dB isolation from its (regulated) power supply. So does the BeePre.
Thanks for your input Paul, a completely valid point, it's something I had briefly considered myself. Unfortunately, burning in each set for 100 or so hours would be a process, but you've piqued my interest. You are right, the GE's have definitely spent the most time in my amplifier, especially compared to this second round of tubes. If our esteemed tube manufacturer's reputations are to be believed, I would expect high quality from the likes of Telefunken, Siemens, and Philips (Holland), which didn't necessarily show up in my listening.
I'll give it a shot and report back, 100 or so hours for each set and see if my impressions change. Heck, I'll be listening anyway, might as well pretend I am being productive!
Really interesting regarding the cathode follower topology, I guess my ears can be trusted after all (for now). We'll see what some burn time yields.
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I think I have completed my shunt regulator tube collection for the Crackatwoa, or at least, I don't want for any others.
Introducing the Telefunken 6005, manufactured in Ulm, Deutschland, based on the General Electric 6005. Internal construction is very similar, although the getter is raised and parallel to the top mica, unlike the GE 6005. Also, the plate is sectioned only in one place, rolled like a hard-candy wrapper, whereas the GE 6005 plate is sectioned in two places, like two half-circles meeting.
These tubes offer the greatest clarity, space, and bass definition of any shunt regulator tubes I have used in the Crackatwoa. Listening right now with a Tung Sol 6SN7GT round plate input, MOV A1834 output. Incredible.
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Has anyone ever tried a 6v6 tube with an adapter for the 6005?
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I have been rolling tubes in my C2A and find that I love the GEC A1834/6AS7G. I came across an adapter to run two GEC A2293/CV4079 tubes and got it as a cheap experiment to see how similar it might sound to my all-star favorite.
I measured the voltages before running these tubes and everything is happy except the “OB†measurements on the high current boards are 79.4V and 82.2V; below the recommended range of 90-110V. Is it safe to run these out of range?
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You would need to post the OA and OB on the center C4S board and the OB voltages on the outer high current C4S boards.
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Center OA 67.6V
Center OB 71.2V
OB HC 147.0V & 147.9V
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OB HC 147.0V & 147.9V
Those voltages are what I would expect for OA on those boards.
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Sorry, I got confused there. Yes, those are the OA measurements. Here are the measurements:
Center OA 67.6V
Center OB 71.2V
OB HC 82.2V & 79.4V
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Yeah so you have about 10V of bias there which isn't so bothersome. If you had something like 67V coming out of one of the small board outputs and 70V on one of the high current OB terminals, that could have some wonky effects on the operation of the amp.