Bottlehead Kits > Crack-a-two-a
Tube Rolling
TurbOSquiD77:
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!
TurbOSquiD77:
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
Paul Birkeland:
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
TurbOSquiD77:
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
--- Quote ---"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! "
--- End quote ---
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.]
--- Quote ---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?
--- End quote ---
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)
Chris65:
--- Quote from: TurbOSquiD77 on February 14, 2017, 01:56:37 PM ---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.
--- End quote ---
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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