Stereo Amp build based on Hammond organ circuit

Latest developments....
I copied same values to another channel and channel #2 is now better then #1 that I did all experiments on.
Channel #1 has ringing pattern that I can't control by manipulating feedback / step network values. Is there a way to improve it or it is a signature of output transformer?
All values are identical between the channels. Both channels are stable and show very similar frequency response. A tad shy up-to 32Hz or so.

Any suggestions?


Channel #1
1KHz / 8KHz

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Channel #2
1KHz / 8KHz

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try swapping the tubes just in case its something there.

I have also run into quirks with layout that makes the channels perform very slightly differently. Usually its some stray capacitance from lead routing or something of that nature. Frankly I wouldn't worry about that little bit of ringing though.
 
I did try swapping drivers with no change.
I have tried swapping output tubes before but ringing was always there on channel #1, no matter what. Even before I had modified R/C stages.
Wouldn't you agree channel #2 square waves look better? I'm a bit of perfectionist and would like to find solution. Any ideas to try? What area of the circuit is in question here?
One thing I might try - moving wiring with chopsticks while watching the scope. Also, I'll hook-up scope to output grids and see if that ringing is coming from driver / inverter.
Suggestions welcomed!
 
Ringing is present right out of the voltage amp on pin #6.
Removing feedback removes ringing from VA output and from output tube grids. Output from transformer seems to have few bumps though. Looks like this ringing generates somewhere within output transformer and gets re-introduced into VA via feedback. Possible? Anyway to block / tame it?

Strange thing to me is that this channel is quieter without signal since it is physically further away from leading choke in power supply. Trailing choke is right next to ringing transformer.
One thought... Could choke polarity play into it? I mean starting and ending end of choke windings connection in relation to leading choke connection.
 
Keep your perfectionism in check. Tuning a tube amp is about compromise. Those small bumps are due to winding resonances. I've rarely seen left and right channel output transformers with identical HF characteristics.

The most important thing is for both channels have the same or very close frequency response characteristics and that both channels are stable into whatever load you throw at them
 
Quick update.
Just finished soldering all flying leads in place and ran final frequency response test. Both channels equally show very slight, not even worth noting, decrease below 32Hz and above 15KHz. No stability issues. Squares look good too. I have to get me better signal generator that is good for over 2v of output. My Jackson puts out distortion on the negative swing and I can't even set AC balance with it. I don't feel like going over recapping old bulky box. Modern transistor types should be better option, right? I just set my trimmers to match original 47k resistance for now. I will do more tests when I get better sig generator. Maybe even distortion analyzer, who knows.

Ultimate test... Cornwalls! No hum. So far, I like what I hear.
Kevin, Supertramp's "Just another day" sounds phenomenal! I read they used Wurlitzer electric piano. That legendary brand's birthplace is just 80 miles away from me! If caps will improve after they break-in, even better. 7195s glow happy purple in the dark.
Chassis stays cool after addition of 5v fan in the bottom plate.

Thanks everyone for your help and support!
This project took me long time but things I've learned from you in the process are invaluable. Now I have much better idea how to avoid major mistakes that kept me from finishing this amp sooner.
 
Slight loss at the very bottom is not unusual, and not a big deal. Also not uncommon to lose a bit up top. If its not more than 1db down at either end you've got very good response.

Newer should give enough output, but you can whack together a simple op-amp buffer to give some more output if needed.
 
Slight loss at the very bottom is not unusual, and not a big deal. Also not uncommon to lose a bit up top. If its not more than 1db down at either end you've got very good response.

Newer should give enough output, but you can whack together a simple op-amp buffer to give some more output if needed.

I could amplify my handheld generator output but it doesn’t have very clean signal either. I’d rather look for better overall performance alternative.
Any suggestions on what models to look for?
 
I think you've got a fine performing amp. Excellent work!

I use a BK precision 4017A signal generator for everything except distortion measurements. It was a little expensive (bought it new) but I use that thing ALL the time so it was well worth it. Its response is flat from 8 Hz to over 300 KHZ. Outputs 20V p-p open circuit, and 10V p-p into a 50Ω load. I like this unit because it has a manual variable sweep frequency control, not just stepped at certain predefined points.

In all but one case it had enough drive for everything I needed. In that one case, I used my tube preamp to amplify the output of the signal generator to get the extra gain I needed.

Wish I could afford a nice HP or Agilent programmable sweep generator, but the BK works quite well for the weekend tinkering stuff I do.
 
Any of the people known for making test gear should be fine, provided they aren't known for making awful test gear. I like HP stuff but its not cheap or especially new at this point.
 
I think you've got a fine performing amp. Excellent work!

I use a BK precision 4017A signal generator for everything except distortion measurements. It was a little expensive (bought it new) but I use that thing ALL the time so it was well worth it. Its response is flat from 8 Hz to over 300 KHZ. Outputs 20V p-p open circuit, and 10V p-p into a 50Ω load. I like this unit because it has a manual variable sweep frequency control, not just stepped at certain predefined points.

In all but one case it had enough drive for everything I needed. In that one case, I used my tube preamp to amplify the output of the signal generator to get the extra gain I needed.

Wish I could afford a nice HP or Agilent programmable sweep generator, but the BK works quite well for the weekend tinkering stuff I do.

I've heard that BK brand is recognized. Are you familiar with other BK models?
4017A seems as an overkill for what I will ever be using it for. What lower model would still have flat response but not go as high up in frequency?
For example, 3011A shows similar specs up to 100KHz. Not sure if those are good enough. Seems like it should be.
 
I'm not familiar with the other models enough to comment, but judging from my unit, I wouldn't hesitate to recommend any BK Precision signal generator. I've had mine for close to 10 years and it hasn't given me any trouble at all.

Just a thought--you might want to look at a used Heathkit. I can't remember the model of mine off hand, but I know it goes to 100 KHz, but only in stepped increments. It does sine and square. I can't remember how much I paid for it; it was not expensive though.
 
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I use an HP 339A. This is an expensive unit even used. But it is very nice--all analog.
 
I'm running a Sound Tech ST1700a. Wonderful piece, but fairly senior at this point. I think the 1700b or the 1710 also has intermodulation distortion measurement. Forget which, but it looks like the 1700a with an extra gauge on the front. I did have some trouble with a funky optocoupler and a fussy adjustment to get it to lock into low distortion mode. Its still slightly finnicky and wants to be warmed up a bit to lock in fully. I don't use that mode, nothing i measure is sufficiently clean to matter.
 
BK precision 4017A is on the way to me. I got a new one for just over $200. I see them sell for close to $400 on average, so I think I got a good deal.
That seller has one more if anyone is interested.
 
Can also go soundcard based audio analyser. The right cheap USB device and free software will typically beat any vintage analyser.
 
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