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Bottlehead Kits => Legacy Kit Products => Topic started by: ssssly on May 13, 2023, 07:16:38 AM

Title: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 13, 2023, 07:16:38 AM
Roughly 20 years ago I was introduced to the Bottlehead community via the DIY speaker building community. And after building my first Bottlehead kit, a pair of Paramours, I was hooked.

With considerable assistance from PJ, PB, CB, Voltsecond, Wardsweb and others, I built Foreplays (I, II and III), S.E.X I and II, Paramounts, an Eros, two BeePres, a Crack, some Quickies and repaired and/or customized piles of classic Dynaco, HH Scott, Altec and Macintosh gear.

Then, I was injured in the line of duty. Loosing much of the motor control in my right hand and finding myself unable to hold a soldering iron still enough build anything. So in true do it yourself fashion, I went to school to learn mechatronics engineering. Figured if I couldn't hold an iron, at least I could build the robots to hold one for me. And while successful in building said robots. It still wasn't as satisfying.

Then it was suggested to me to adopt smithing as a form of physical therapy. So I learned metalworking. Welding, then blacksmithing, then copper and silver smithing. Working from larger to smaller until now. Where I have reached the point that I can once again build by hand (Not to mention the robots).

Prior to my injury I had plans to build a SR-45.

Now, at long last I give you "The Snow Creek Shuffle-Episode I: The Phantom SR45" build thread.

PS..my apologies to PB and PJ if this spurs requests for the long oop SR45 board.





 
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 13, 2023, 07:22:23 AM
I like big caps and I cannot lie.

To provide them some extra breathing room I decided to go with some spare Paramount top plates. Sanded to a low polish, 2000 grit, then primed and painted Hammer tone copper.

Machined Teflon tube sockets.

Custom resin printed hole shrinker.

All brass hardware.

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 13, 2023, 07:28:19 AM
PT-7 power transformer.

Prototype OT-2 output transformer and PC-3 plate choke I traded PJ some OG PT-2s for.

NOS paper wound C-7X in Rust-Oleum white.

Underside of the resin hole shrinker.

Random unbranded RCA and binding posts I picked up at an electronics market in Seoul, Korea.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 13, 2023, 07:31:32 AM
Black resin printed hole drilling guides/standoffs to accommodate the OT-2 and PC-3 on the Paramount Chassis
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 13, 2023, 07:33:43 AM
Safety ground wiring.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Thermioniclife on May 13, 2023, 08:34:40 AM
Quite an inspirational tale. Well done, looking good and best wishes.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on May 13, 2023, 11:13:33 AM
I remember selling an extra set or two of those boards a long time ago, but I couldn't remember who ended up with them.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 13, 2023, 01:19:15 PM
 
I remember selling an extra set or two of those boards a long time ago, but I couldn't remember who ended up with them.

Also when I convinced QueenBee to do one last run of PT-2s for the project. That included the PT-2s I traded with PJ for the OT-2s and PC-3s in this amp and now power his 6J5 Paramours.

The remaining 2 NOS PT-2s I have will be going into my Paramour rebuild in a later episode of "the Shuffle".

I'm planning on doing them with fully 3D printed and electroformed in place wiring. As an interim proof of concept for a method of 3D printing transformers that I've been developing.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: 2wo on May 13, 2023, 05:33:22 PM
If I remember correctly, the original SR45 kit used the then current C3S boards. Took a lot of jumpers but doable...John
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Doc B. on May 14, 2023, 05:21:50 AM
3D printed transformers gets my attention. Will be interested to hear how this develops.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 14, 2023, 06:26:23 AM
3D printed transformers gets my attention. Will be interested to hear how this develops.

Doc,

Would be more than happy to include anyone from Bottlehead in the project in any way they would like. My EE work is focused on digital motion controllers and I/O for them. I'm designing a manufacturing method based on known designs. Y'all know way more about tx design and implementation than I.

So if there is a transformer design you would like to test via this method, I'd be happy to.

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 14, 2023, 06:28:28 AM
If I remember correctly, the original SR45 kit used the then current C3S boards. Took a lot of jumpers but doable...John

Sorry, what is doable?
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 14, 2023, 07:35:36 AM
Speaking of TX design and implementation.

PT-7 has marked 150v, 6.3v and 3.15v out with corresponding 0v for the 150v and 6.3v.

Need 300v and 2.5v.

First blush, just instal a voltage doubler for 300v. Or is there a better way here?

Drop the 3.15v to 2.5v with a voltage divider, no problem.

Where should I pull the 0V leg for that though? 





Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Joppa on May 14, 2023, 08:24:42 AM
Yes, the PT-3/7/10 uses doublers for high voltages. For lower voltages (such as Crack), the winding is used with a fullwave bridge.

In the MonAmour, the 6.3v center-tapped winding was rectified with the usual FWCT arrangement; it's OK to use half the winding to power a 2A3 or 45 with AC. Be aware that either arrangement biases the 6.3v terminals to the cathode voltage of the 2A3 or 45 - around 50-60 volts depending on the specific design. It's important that the winding be AC grounded because it also serves as an electrostatic shield for the high-voltage winding. Usually the cathode bypass cap provides the grounding function
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 14, 2023, 10:15:30 AM
To make sure I understand you correctly, pin 4 is the center tap for the secondary and therefore common for both the 6.3v and 3.15v.

Normally those legs would be phase corrected with full wave bridge rectifiers but since both legs are being used as AC heaters, it's not necessary.

And the cathode bypass cap is sufficient for the ground path for AC shielding in that configuration for the otherwise standard SR45 circuit.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Joppa on May 14, 2023, 11:13:58 AM
The center tap is terminal 10 on the PT-7. The labeling is somewhat ambiguous.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 16, 2023, 01:11:11 PM
Mains and heaters wired. Only 1k resisters I had on hand were 5w. Overkill but it's putting out 2.6v.

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on May 16, 2023, 02:43:53 PM
What's the 1K resistor doing there?
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 16, 2023, 02:57:15 PM
First leg of a divider to drop the filament voltage to 2.5v.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Joppa on May 16, 2023, 05:28:31 PM
You won't get 1.5 amps through a 1000-ohm resistor ...  :^D
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on May 16, 2023, 06:24:17 PM
You can use two 1N5820 diodes to make a full wave rectified supply to feed the #45, then a small value resistor, then a 10,000uF cap.  The value of the resistor and the actual 10,000uF cap you use will interact together to result in an optimal value, but I think you could start with a 0.22 ohm/3+W resistor to see what happens. 
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 16, 2023, 08:07:30 PM
You can use two 1N5820 diodes to make a full wave rectified supply to feed the #45, then a small value resistor, then a 10,000uF cap.  The value of the resistor and the actual 10,000uF cap you use will interact together to result in an optimal value, but I think you could start with a 0.22 ohm/3+W resistor to see what happens.

Sounds like a wonderful idea. And I have everything but the resistors.

But i needed to order parts this week anyway.

What would you suggest as a reasonable range of resistors to buy to dial it in? Given the 10,000uf caps I have are the Xicons from a BeePre.

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on May 17, 2023, 05:15:24 AM
I'd buy a 0.15, 0.2, and 0.3 or so ohms.  If you're ordering resistors, maybe look at getting a pair of 10,000uF 25V caps.  Those caps will live a pretty hard life in that position, so a physically larger cap with higher ripple current capabilities will live a lot longer.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 17, 2023, 08:18:25 AM
Dang it. I'm going to find something to do with those BeePre parts.

Though I suppose I should just knock it up and see what the real world voltage drop and ripple is with those diodes first anyway. Spec sheet says .475v peak drop which would only leave me with ~2.2v. So I'm guessing not much voltage left to play around with under load.  And fingers crossed that real world drop is under .6v combined.

And I should probably get the scope and boards out anyway. Need to design and test the voltage doubler for the HV side.

And now I'm sitting here thinking: I need to order stuff anyway, why not try to throw pi filter in there. Or model it up and see if I can do a constant current heater to prolong the life of the 45s. In other words why I always have 10 projects going at once.

Right. Order some good 10000uf filter caps and resistors. Build it and see what it sounds like.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on May 17, 2023, 02:28:37 PM
Which diodes did you select for the filament supply? 

I'm not entirely certain that constant current regulation for a filament actually prolongs the life of a tube, but YMMV.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 18, 2023, 05:50:29 AM
Which diodes did you select for the filament supply? 

I'm not entirely certain that constant current regulation for a filament actually prolongs the life of a tube, but YMMV.

1N5820s.

I'm not convinced either way either. And upon a minute of further thought, the circuit I was thinking of would require a second heater in series or a dummy load. So if I decide to try the current route, it would be in a different form.

I'm just going to build the full wave rectifier with over specd caps, see what I get for ripple and turn on voltage and go from there. I want to get them built and see what they sound like more than I want to fiddle with theoretical benefits at this point.

Oh, there is a connection labeled ground on the board you made me that is connected to the center tap in the hookup instructions. But that was for a PT-2. Would you still suggest connecting it to the center tap at terminal 10 on the PT-7?

From my incomplete understanding of the differences between the two transformers I was thinking ground.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on May 18, 2023, 09:22:18 AM
The PT-7 has only one secondary.  If you ground it, that will zero bias the #45 tube and things will go awry in a huge hurry.  That ground connection should connect to your audio ground and the heater winding floats (it will be biased up to the bias voltage of the #45 tube since it's serving as the filament winding).
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 19, 2023, 06:58:49 AM
Sweet. Thanks. That's what I thought.

I'm just happy I finally guessed correctly.

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 25, 2023, 07:31:25 AM
When life gives you silly big capacitors, 3d print them a cap strap while you are waiting for your Mouser order.

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: 2wo on May 25, 2023, 05:31:53 PM
Well, that's pretty cool...John
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on May 26, 2023, 06:14:55 AM
Why...thank you. I tried to make it aesthetically pleasing as well as functional. Unfortunately the Parafeed cap didn't quite clear. It's stuffed tight into the corner between the plate choke and the SR PCB.

Moved the bracket over a couple mm and printed V5 last night. In the ultrasonic now.

And if you think that's cool wait until I get to the Paramour rebuild portion of the shuffle. I'm going to do fully 3D printed and electroformed in place wiring.

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on June 03, 2023, 10:20:47 AM
I mocked up the full wave rectifier for the 45 filament and...I don't really like it. Even with the low Vf of the 1N5820s it just seemed to struggle to provide a stable 2.5v. Set to 2.5v it always wanted to sag and hover around 2.1-2.2. Set to 2.7v around 2.1-2.3v and set to 2.8v it would swing from 2.2-2.8v.

After trying to troubleshoot it for a day I was back to thinking just drop a simple LM317 based constant current heater circuit in there.

Then I remembered seeing some ADP linear voltage regulators that operate around the operating points of the 45 filament. And I came across this little bugger.

https://www.digikey.com/en/products/detail/analog-devices-inc/ADP3339AKC-2-5-RL7/11534905

up to 6v in, 2.5v @ 1.5a out, and only needs 1uf esr agnostic stabilizing caps.

Been staring at the specs for some time now wondering why this won't work a treat and not finding any.

So give it to me...why won't this regulator work?

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on June 03, 2023, 11:35:08 AM
Heat through a regulator can be problematic.  Remember that you need to cover the dropout voltage plus some safety margin.

I don't know what "setting for 2.7" means.  You setup the full wave rectifier with a resistor between the rectifiers and the first cap, then adjust the resistor value to get yourself about 2.5V.  I would aim for 2.4V-2.6V.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on June 05, 2023, 04:57:30 AM
I calculated the values for the resistors using my pet AI. Since the resistors were the only variable I referred to them as "set" resistors.i bought a range of resistors from 0.2-0.5ohm. According to the calculations greater than 0-0.2 ohm should result in ~2.7v. I'm now getting ~1.5v loaded with a tube instead of a dummy load.

It says the dropout is only 0.23v at 1.5A for that regulator. Actually why it caught my attention for more than a second. That and the 85/105c ambient/junction temp. Which leads me to believe it could be fine temp wise.

But since I have never seen them used in a heater, I assumed there was something I was missing.

But they are kind of an obscure oop regulator. And the only ones that operate at or near those points. So figured it could just be one of those things where nobody has used one because nobody was looking for one. Since most people would probably just use a transformer designed for what they need.

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Joppa on June 05, 2023, 05:39:26 AM
Regulated filaments can be tricky to design. It's the one area where PSUD is not as accurate as  you might wish, due largely to the inaccurate diode models. You may need to use a Schottky diode rated 15 amps or more. Then you need to subtract the peak ripple voltage from the average DC voltage to keep the regulator from dropping out. I subtract another 10% to allow for power line fluctuations. And the DCR of the transformer (including the reflected primary DCR) adds to the series resistance. And the high voltage winding rectifier/filter adds to the primary voltage losses.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on June 05, 2023, 11:38:25 AM
Regulated filaments can be tricky to design. It's the one area where PSUD is not as accurate as  you might wish, due largely to the inaccurate diode models. You may need to use a Schottky diode rated 15 amps or more. Then you need to subtract the peak ripple voltage from the average DC voltage to keep the regulator from dropping out. I subtract another 10% to allow for power line fluctuations. And the DCR of the transformer (including the reflected primary DCR) adds to the series resistance. And the high voltage winding rectifier/filter adds to the primary voltage losses.

Certainly seems that way. I have by far the least experience designing anything power supply related. Just not something you come across in mechatronics much unless you are designing large scale industrial, which I don't. And even then it's pretty much just IDing COTS stuff that fits the bill.

I may just slap the PT-2s I have left in these SR-45s and experiment with the PT-7s more when I do my Paramour rebuilds.

I currently have some 6BX7 amps powering the horns in my upstairs system and it's making it near impossible to find a good crossover point. They just sound completely different than the Paramounts powering the woofers. And seem to exaggerated the beamy-ness of the 500b horns.

So I kind of just want to get these built to get the 6Bs out of my upstairs system.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on July 02, 2023, 07:28:49 PM
And there we have it. Amp 1 wired up. On to checks tomorrow.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on July 03, 2023, 06:09:58 AM
Well these certainly weren't the numbers I was expecting.

DRV P (225)
KREG (18)
REG P (355)
KDR (7.8 )
B+ (355) that's actually what I was expecting
CK (397)

A1,2 (67)
A3 (318)
A4 (26.2)

B4,5 (3.2)

What's really boggling my brain at the moment is the heater voltages.

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on July 03, 2023, 06:12:58 AM
Which voltage is wrong?
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on July 03, 2023, 06:55:44 AM
The 45 socket voltages were not what I was expecting.

I was expecting A1,2 sub 2vac
A3 ~ 300vac


And the 45 got so hot so quick it kind of alarmed me, given 60x the voltage I was expecting on A1,2.

But if those numbers are good and my expectations were wrong, happy to chalk it up in the win column and see what it sounds like.



Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Doc B. on July 03, 2023, 07:55:29 AM
I'm not following this. I am not looking at the schematic, but on a 45 (or 2A3 or 300B) the filament is A1 and A4. A2 is the grid. To measure AC fil voltage on these tubes you measure AC across from A1 to A4. Note that the entire filament can be operating on AC voltage and also be DC biased with respect to ground. So you could measure 2.5 VAC across the filament pins and also some higher DC voltage with respect to ground on either of those pins.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on July 03, 2023, 08:55:38 AM
I used your funky numbering convention from an old paper P'mour schematic.  :P

1-2 filament, 3 plate, 4 grid.

Data sheet for the 45, P'mour & mount manuals are 1-4 filament, 2 plate, 3 grid. BeePree is 2 grid, 3 plate.

Bonehead problem solved. I was measuring to ground. Filament is 2.7vac.

However, the data sheet lists max 275v to the Plate and min 31.5 for the grid and I have 318v and 26.2v respectively.

I


Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Doc B. on July 03, 2023, 11:57:14 AM
I'm not helping much, huh. Pin 2 is plate, pin 3 is grid.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on July 03, 2023, 01:39:58 PM
I'm not helping much, huh. Pin 2 is plate, pin 3 is grid.

Always nice to see to chime in though.

This is my first 45 Amp. Do 45 tubes normally run considerably hotter to the touch than 2A3s or 300bs? Like...lick your fingers and remind yourself never to touch that again hot?
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on July 03, 2023, 02:31:18 PM
Well, it makes music. Much better sounding music than the 6b amp it replaced. Also surprisingly louder than that amp, which in theory should be in the 10watt range.

And far less hum. 4x less to be precise. And quieter than my Paramounts by a milivolt.

Is that 318v at the plate in the ballpark of what it should be or should I reduce it?

Sounds great. Just don't want it to be a tube eating machine if that's not the intended operating point.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on July 03, 2023, 05:14:11 PM
What value did you use for your cathode resistor? 

PJ originally drew up the SR-45 for use with the available 3K iron from Magnequest, but with what you have I would be aiming for a little less quiescent current.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on July 03, 2023, 06:55:09 PM
1.6k
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on July 04, 2023, 04:41:38 AM
And if you wouldn't mind, what is the math for calculating the value?

While I have largely considered my audio hobby as electric Legos, I would like to begin actually wrapping my head around the EE side of these circuit topologies.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on July 04, 2023, 04:57:45 AM
That seems pretty reasonable to me.  To figure out what cathode resistor value you want, you would start by marking the desired operating point on the plate curves, then you could have the bias voltage (67V) you desire and the operating current (about 40mA).  V=I*R, so you have 67=0.040*R, so you end up with 1675 ohms as the ideal calculated resistor.  The 1.6K and the hum pot arrangement gets pretty close.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Joppa on July 04, 2023, 05:48:41 AM
I'm away from my notes at present, so I haven't commented yet. Specifically, I don't remember the operating point I chose. But the voltages seem a bit too high. You could be over-heating the 45.

It remains unclear what the voltages are - that "26v at the grid" might be 26mV(?) - or else the grid resistor is not connected(?).
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on July 04, 2023, 06:29:39 AM
He's running about 250V plate-filament voltage and 40mA of current, so more or less at the limit for #45 operation.  I would presume since the apparent bias voltage is correct that the "26" is indeed mV.  With the PT3/7/10 doubler winding, regulating the B+ a bit higher keeps the top C4S a bit cooler and plays better with 4-5K output iron.

A little extra cathode resistance might not be the worst idea to bump the plate dissipation down a little bit.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on July 04, 2023, 07:15:38 AM
It is indeed 26mV. Fat fingered it. But that is also a dead cold measurement. The tube was getting so hot I wasn't leaving it running long between measurements.

I ended up swapping the PT-2s into it.

The output iron is 4.9k.

The resistors in the PCB were changed out to get the B+ to ~360v per PBs previous recommendation. Dual 1.8ks if I remember correctly.

Also changed the Ufs for Cree diodes. So .7v vs .2v drop per.

The 45 tube gets really hot, really quick. Way hotter than any 2A3 or 300B I've ever touched. Have to wait ~5 min before I can even think about touching it. And I'll yank a 2A3 or 300B immediately after turning off an amp. I have a welder's left hand and it burned me the first time.

Happy to take any additional measurements. I built it with plenty of room to poke around.

Which really makes the question, what was the intended operating point PJ chose?

Looking at tube charts, for anything but an EML 45, I'd guess 270v plate to ground. But I amiditly have little idea what I'm talking about here.

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on July 04, 2023, 07:29:06 AM
The modifications I specified for raising the B+ will not work with the PT-2.  This isn't the kind of circuit where you can change one think without a cascade of other changes being required, so I would put the other power transformers back if at all possible, and this is especially true if you left the voltage doubler in place and used that with the PT-2.


Yes, tubes get too hot to touch, that's normal. 
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on July 04, 2023, 10:06:49 AM
Nope. No voltage doubler.

Back home looking at the schematic and the original operating points are 320v plate, 13.5 v bias for the first half of the 6CM7, 200v plate, 8v bias for the second. And 300v for the 45 plate.

Looking at the tube curves im guessing my issue with the PT-2 running the 6cm7 at those voltages is the risk of running out of current with the 45 running beyond max.

And I'm guessing the reason for running the 45 at 300+v is to extend the bass and high treble response at a slight hit to linearity and tube life while maxing out amplification.

I however don't need bass or watts out of these amps. They will live as dedicated horn compression drivers amps. They'll never see below 500hz. And the compression drivers they will live hooked up to break up in the 12-15k region.

If my previous guesses are correct, would it then make more sense to decrease the plate voltage of the 45s to the 260v range, where the curves seem more linear, to keep or increase the voltage on the 6cm7 which seems more linear approaching 450v?

On these amps in particular id much rather rob Peter of bass and watts to pay Paul in linearity and midrange. And id greatly prefer to stick with the PT-2 at this point. Swapping out that transformer will be a PITA I'd rather avoid.



Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on July 04, 2023, 12:21:48 PM
The plate voltage is low with the PT-2 to provide compliance for the shunt regulator and the extra voltage isn't really helpful when running a 3K output transformer.  If you're using a 4-5K transformer, then you could take advantage of a little more B+ and run the #45 a lot closer to the typical book operating point into the typical load impedance.

The plate voltage of your amps is the voltage appearing at the plate minus the average of the voltage appearing at pins 1 and 4.  You are currently running well below 300V.

I would highly recommend putting the PT-7 transformers and the doublers back in place and leaving things as they are, with the caveat that you could probably go to 1.8K as a cathode bias resistor on the output stage if you want to.

If you want to go back to the PT-2, then the lower regulated B+ and 3K iron makes a lot more sense.

Again there is nothing alarming about how hot the #45s are getting.  They are rated for 10W of plate dissipation and you were right at 10W of plate dissipation under the conditions you initially posted.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Joppa on July 04, 2023, 01:34:29 PM
From the data sheet, 250v plate to cathode at 40mA would be -48v bias and thus a 1200 ohm cathode resistor 298v plate to ground.

320v plate to cathode at 40mA would be -67v bias and thus a 1675 ohm cathode resistor; 387v plate to ground. And 12.8 watts plate dissipation, which is too much.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on July 04, 2023, 02:24:15 PM
From the data sheet, 250v plate to cathode at 40mA would be -48v bias and thus a 1200 ohm cathode resistor 298v plate to ground.

320v plate to cathode at 40mA would be -67v bias and thus a 1675 ohm cathode resistor; 387v plate to ground. And 12.8 watts plate dissipation, which is too much.
A1/4 (67)
A2 (318)
This is 251V P-K and about 41mA of plate current, so 10.3W. 

With the PT3/7/10, nudging B+ up to more like 340V allows one to move a bit closer to the next operating point in the datasheet of 275V/36mA with 56V of bias, which calls for a bit less than 1.6K of cathode bias resistor.  To be honest I just borrowed the operating point from the #45 instructions in the Stereomour I manual ;)  Perhaps there actually is a bit of positive grid voltage at the #45 causing it to draw a bit more current than it should be?
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on July 05, 2023, 02:02:37 AM
Well, let's see.

I've got some 1.2k 12w resistors laying around. They are mills non inductive wirewound, not sand. But bypassed by a 100uf cap I wouldn't think the value should change much until the vhf range.

Gonna throw one in and get some better measurements with things warmed up more than when taking the first couple sets.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Joppa on July 05, 2023, 05:32:24 AM
Don't reduce the cathode resistance below its current 1600 ohm value until we understand the voltages right.

251v plate to cathode a 40mA is indeed a good operating point for a 3000 ohm load, but it is NOT consistent with the plate curves of a 45 at -67v bias. Not even close.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: 2wo on July 05, 2023, 06:30:11 PM
If you have some 100-200 ohm resistors, 1/2w or preferably higher you can add one in series to the cathode resistor and recheck your operating point till you get what you want then replace with a single HP resistor.

The added resistor is just for short term testing...John
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on July 05, 2023, 06:52:46 PM
I would also suggest using a clip lead to attach the grid of the #45 to the power supply ground to see if the voltage appearing at pins 1/4 changes at all.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on August 03, 2023, 11:03:12 AM
It always amazes me how busy retired life can be.

Quick summary since my last post.

Relabeled the 4 pin according to standard convention. A1,4 Filament, 2 Plate, 3 Grid. Hopefully this reduces vice increases confusion.

Swapped a 1.2k cathode resistor in and promptly let the magic smoke out of something, immediately turned off. Created a few new bad words. Went over everything 100 times trying to figure it out. Thought maybe there was a pulled ground or ground loop. Rewired star chassis ground and changed the signal ground to chassis ground point. Retested, no smoke but crazy voltages. A(1,4) 321v, (2)358v, 3(293) vDC measured to RCA ground lug. Swapped the 1.6k back in. Still same crazy voltages. Then when poking around to try to get a voltage reading I shocked the crap out of my pinky, which was nowhere near a bare wire or terminal. The lead to the B- side of the coupling cap was touching the metal casing of the cap.

Assumed that that was what the magic smoke was let out of and sent a couple hundred volts into my pinky. Replaced it. Made sure nothing was touching the case. Fired it up up again today and...

New measurements to RCA ground
(1.6k cathode resistors, 180k PC board resistors, 4.9k Output transformer, 40H plate choke):

DRV P     (228vDC)
KRG        (14.95vDC)
REG P     (358vDC)
KDR        (7.9vDC)
B+          (358vDC)
CK          (405vDC)

A1,4        (52.3vDC)
A2           (330vDC)
A3           (1.84vDC)

B1           (358vDC)
B2           (0.03vDC)
B3           (7.9vDC)
B4           (0.003vDC)
B5           (0.004vDC)
B6           (224.8vDC)
B7           (0.004vDC)
B8           -
B9           (15.7vDC)

A1-A4      (0.003vDC, 2.7vAC)
B4-B5      (0.001vDC, 6.5vAC)

With clip lead from A3 to PC Board G: A1,4 (50.9vDC w/ dead cold tubes immediately post leds lighting)
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on August 03, 2023, 01:35:28 PM
1.8k PC board resistors
I don't recall there being a 1.8K resistor on the PC boards, which part is this?

Your voltages look a lot better IMO, you have pretty close to 275V/34mA.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on August 04, 2023, 06:53:04 AM
That
I don't recall there being a 1.8K resistor on the PC boards, which part is this?

That's because I put the accent on the wrong sy llable.

180k

Looked better to me as well. Seems much happier now that the 45 and 6CM7 aren't shorted through a capacitor case.

That and I think the signal ground from the 45 was iffy before. When I rerouted it, the wire was tarnished. Used solid silver wire for the signal path and it got pushed further into the joint while soldering than I had cleaned back to solder it.  Was nice and shiney on the outside but when I desoldered it, the inside was full of nasty.

But the numbers looked close enough to me to build the second one and tweak the cathode resistor along the way.

Anything else I should be looking at that I missed?
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on October 22, 2023, 07:14:32 AM
And they live.

Was able to get the 45 measurements right with the 180k resistors on the board and a 900ohm cathode resistor. But it was driving the 6cm7 into oscillation. Dropped in a pair of 160k resistors on the PC board and a 1.6k cathode resistor and everything is happy now. 248v 1.5A at the 45 plate, no oscillation in the 6cm7, making beautiful music. They do have a touch of audible hum but I think it is either a ground location issue or the long wire from the inputs picking it up. So still a bit of tinkering to do but they sound great (just have to turn the volume up a bit).

Thanks again for all the help.

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on October 22, 2023, 09:02:34 AM
Do you have a hum pot on the 45 filaments?
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on October 23, 2023, 05:39:16 PM
Do you have a hum pot on the 45 filaments?

I do. It is completely independent of the of the hum pot setting. I does get louder with volume but not linearly. The hum lags behind total volume by a considerable amount.

I built the amps with different input wire routing to see if they would be audibly different. They are. But they are also both too loud. So I'm going to move the signal ground so its direct to chassis at the 9 pin socket terminal strip and see what that gets me. I have fairly high hopes.

Could also just need better rf shielding than kapton that close to the output transformer. Good dielectric, but not much for shielding. And I'm fairly certain I have mylar tape someplace.


Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on November 12, 2023, 06:29:25 AM
Hum solved. But raised more questions.

The culprit for the loud, asymmetrical hum was the ground wire from the PCB to the IEC ground. The noisier amp had that ground wire between the PT and C-7X. I cut it a touch too short when building that amp so routed it a bit different. Rerouting it to the outside of the chassis eliminated the loud hum.

But while poking around changing and testing stuff I noticed something else. My Paramount's were picking up hum from the SR-45s. With just the Paramounts on, to get hum out of the speakers I have to intentionally set the humpot to generate hum. But with the hummy SR45s on, the Paramounts also hummed. Loud.

So it seems that the SR45s were feeding hum into the the the wall socket, that was then being picked up by the Paramounts.

Installing a pair of diodes between the IEC and chassis ground on the SR45seliminated this. But I can't help thinking there is something else going on that needs to be addressed.

Particularly that now without the hum there is a faint hiss.

They are certainly listenable now. They sound good, very good. Damn good. In fact as good or better than I've heard anything powering an Altec horn driver. They seem to roll off the highs that can make Altec horns overly bright and shrill sometimes. All previous amps I've had biamping that horn had issues with metal percussion instruments like bells, xylophones or even pianos sometimes. Where the initial attack on the instrument could be so shrill at energetic volumes it physically hurt my ears. The SR45s are revealing enough to hear both the mallet strike and full ring out of the xylophones and piano in Chick Cores's Crystal Silence without any shrillness or fatiguing qualities. The only other amp Ive had in that system that got close had glowing blue voltmeters on the front and cost more than my car. And why I rarely listened to live jazz on that system.

But something must still be amis for them to want to feed that much hum into the wall socket without blocking diodes. So still some tinkering left on the ground paths. Perhaps having the shielding ground from the AC in and heater wires tied into the signal ground too close to the tx center tap ground??? Or the safety ground bundle where it passes by the C-7X needs additional shielding???

Then there's the hiss. The character of which makes me think it's unstable capacitance somewhere. That kind of low crackly hiss you get from the blank track of a dirty record without the pops. Or a leaky cap minus the whistles. They are totally listenable with the hiss as is, it's low and obscured by the constant white noise from the stream next to my house. But I can also tell that there is better to be had by blackening the background with it's removal.

So perhaps trying bypass caps caps to see if I can track it down to an individual cap??? Or flipping the leads on non polar capacitors to see if it's related to capacitance in the internal shielding??? Or I used some Dale non inductive wirewounds for the C4S resistors instead of metal or carbon film because that's what I had at 160K---would that cause low level hiss?
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Paul Birkeland on November 12, 2023, 07:53:34 AM
The problems you're experiencing likely have a solution that's more about how the amp is put together rather than a part that isn't working properly.  It's tough when an amp like this gets full of big caps, as we can't really see a lot of what's going on, and ultimately this project is custom enough that debugging procedures will end up being specific to just this amp. 

Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on November 12, 2023, 10:30:38 AM
Yeah. Totally.

Looking more for suggestions on methodology to track it down or start tracking it down based on experience. Not move wire from T1 to T2 to solve x problem.

If reversing or bypassing non polar caps and/or throwing shielding at ground wires/transformers doesn't work my next step, based on how I normally deal with unwanted noise in logic circuits, would be to mic the speakers to try to find a noise profile and then differentially probe each trace and component, working backwards from the output, to see where the noise is first introduced.

Or is there a better method here?
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: Doc B. on November 12, 2023, 11:48:34 AM
Hiss is usually noise from a tube in a low signal/high gain part of the circuit. Horns will exacerbate the issue. I would start there. Then move on to checking caps if you can't resolve it by rolling tubes.
Title: Re: The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga
Post by: ssssly on November 13, 2023, 05:23:19 AM
Rolling tubes doesn't affect it.

Thank you much. I shall start there.