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Seeking help with Maple Tree SS+ Clone build

Hi all,

I'm finishing up a build of a pair of Maple Tree SS+ clones with an extra set of output tubes and a few tweaks. The basic design is unchanged as I'm not an expert, just a semi-talented tech with enough knowledge to be dangerous. Unfortunately the damn thing doesn't work, and I'm stumped so I'm hoping the experts here can critique the design and give me some ideas how to proceed.

The problem (I have built 2, and both behave identically): oscillation starting at the voltage amp stage 6SJ7 (you can see it on the plate and it propagates from there). It's a saw-tooth wave that looks like a capacitor charging and discharging at rythmic (but not constant) intervals. Like this: ......|\|\|\......|\|\|\..... etc
The oscillation will increase in frequency when you power down until the caps discharge but the waveform shape remains constant.

If you remove the 6SJ7 the rest of the circuit is stable and quiet.

I've quadruple checked my wiring and layout, ground scheme, ABC+ supply, etc. It matches the schematic. Power supply is clean and smooth.

Here's the base design, taken from their freely available owners manual (NOTE the 221K resistor in the power supply is actually 2.21K)
Maple Tree SS+.jpg


Now here's my schematic:
SS2 Amp Schematic.jpg


My suspicion: NFB loop may be off because I'm using a 3.3K OPT vs 6.6K to suit the extra tubes?
All testing has been done in UL but switching to triode doesn't change the fault.
I've tried using only 2 output tubes (per original schematic), same problem.

Any and all suggestions for improvements are welcome!
 
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I highly doubt that simply rewiring the amp will do anything. +1 for positive feedback.

My second thought was motor-boating. I see no decoupling caps. But I don't think motor-boating would go away when the global NFB loop is opened, so you're dealing with instability rather than anything supply related.

Tom
 
Jaytor is correct. The signal is out of phase but no one has told you what to do. Make this quick test. Unground the speaker common and ground the 8 ohm connection. Connect the feedback from the now floating common. If that fixes it, do this. Reconnect the secondary as it was and do only one of the following as a permanent fix.

1) Swap the plate and screen connections to the primary upper and lower, or.
2) Swap the grid connections to the output tubes, or.
3) Swap the plate connections of the 6SL7 driver tube. (probably the easiest)
4) As a last resort, reduce the feedback by raising the feedback resistor. (Also disconnect those smaller cathode capacitors.)
Let us know how it went.
 
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UPDATE 2: No matter what connecting NFB would set it off. I tried reversing the phase but that just introduced high frequency squeal so can confirm the phasing of the OPT is correct.
Removing the 0.047 cap (local feedback on V1?) resolves the fault and NFB then behaves as it should.
Reconnecting that cap, with NFB connected, mutes the input and introduces the sawtooth oscillation.
 
OK I figured it out! The phasing WAS incorrect, but I was thrown off because when I flipped it the NFB was disconnected and it immediately started oscillating... but flip the phase AND connect the NFB and all is good.

Still ironing out a few bugs but first proper load test gave me 60W just below clipping (no distortion analyzer so don't ask the THD numbers...)
 
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As is it sounds superb, but I'm open to any suggested tweaks. Like I said, I'm not a designer so I'm just muddling through and welcome suggestions.

Are you referring to the 510K ahead of V2 or the 475Ks in the output section?

If I halve the value how much more current would I need?

I flip flopped the output of the phase splitter, switching the OPT wiring would be a chore.

Final power run 54W into 8 in UL and 27W triode. I expected 50-60 so bang on.
 
I think you have done a very good job making a pair of beautiful amps. Excellent fit and finish on all three units. Looking at the PS it's hard to tell if the 375 & 145 volts are divided down in the PS or amp chassis where they preferably should be. I only mention this because the amps should operate stable (no oscillation) without feedback connected. (Assuming I read post 7 correctly) Maybe sometime later find a NOS pair of red RCA 5693 (6SJ7) pentodes, much lower microphonics. And maybe even some 5691 (6SL7) red base dual triodes.
 
Don't look too closely, machining with hand tools means nothing is quite lined up! Next build I'm taking the machining to a shop.

The split is in the PS, the ABC voltages are fed through the umbilical (DC for heaters are local in the amps) - I will note that I adjusted the voltages after speaking with Maple Tree to confirm the proper ratio so that was likely contributing to the instability I had before. Amazingly they helped me out even though I'm just copying their work and not a real customer, kudos to them! You'll notice I changed the resistor values between the stages to get there with the huge caps I used.

Should I add some local capacitance inside the amp? Easy for me to do if it will improve anything.

I'm really happy with the performance of the Amperex and Tung Sol combo, I might try the RCA reds but honestly it is great as is.
 
"Should I add some local capacitance inside the amp? Easy for me to do if it will improve anything."

Normally I would say yes. However if it's working well and to your satisfaction, then don't fix it. But putting the down stream filter capacitors in the amplifier chassis when using a remote PS is accepted as proper practice. Sometimes having a distant audio ground return can be problematic.
 
Ah makes sense from the ground perspective. I thought about adding some film caps inside the amp for a bit of extra filtering closer to the tubes, I might do so in the future. All of my builds are constantly evolving as I test and tweak, or want to try something new. That's half the reason I used a 100W OPT on a 50W amp, in case I want to rebuild it into something more powerful in the future. The other reason is more iron should have better bass response, maybe.