jokerman777

my bad could've just read to pages ago more carefully where the discussion on the matter already happened... I did a quick sim on Joamat's "new bias servo", 5Vpp on one side into 32ohm load, ofc not the same amp circuit, not opa445, and not assuming sim == real life behavior, but probably enough to convince me it's not gonna be free lunch. Below is before and after (also tried 300ohm load and difference is much smaller though). Now there is also KG's optocoupler version but between adding another 8 x opa445 and more and just doing thermal coupling right in the first place, well... 😃 And many thanks for the advice and detailed explanation! Yeah I'm aware of the generous amount of feedback used here, much like the original CFA2 right? where CFA3 is changed to open loop design with optional feedback config. I didn't "choose" to go with this way on purpose and haven't built/listened to enough stuff to form a particular preference on the camps between no feedback/some feedback/a lot of feedback (which kinda has a bad reputation). I'm pretty much just randomly exploring options now, for possibly a smaller scale project than CFA3 for my need. Might still jump back on doing the original if I got enough messing with spice

Thanks @JoaMat, this data is really helpful. From your measurement it does look like I really needed either bias servo or do it like the original for hopefully less drift. Nice soft start behavior too

So I have spent the thanksgiving break playing around with ltspice, mainly wanted to do a 3-5W class A into 32 ohm balanced amp and started off with Kevin's CFA2/CFA3 design, and comes down to something like this, basically a slightly reduced CFA2: Just thought I want to put here for some input before trying to actually make it, which I'd much appreciate if any 🙂 I mainly have questions around: "floating tail" input (just like uberamp/preamp2025 input) instead of separate CCS (like dynalo/CFA3 input): Works in sim but in reality I don't know if I can just send feedback this way and make it fully differential with CF. When input is single ended where one side is ground, due to the feedback it seems to be amplifying the voltage disproportionally on two sides while overall differential gain is maintained. A problem or just an expected behavior that I can live with? (most of time I won't use single ended input) thermal: Is it essential to mount all the transistors in darlington triples to heatsink? Dissipation wise the inner ones don't really run all that hot. Im tempted to just mount the output pairs, planning to run them at around 200mA each phase margin: unsure if I'm measuring open loop gain correctly, am just doing vout / (vin - vfb) on one leg, where vfb is the feedback entry point. other suggestions?

Some if not all 6dj8/ecc88 datasheet has spec'ed max plate voltage at 130V, which the top tubes operate right around that range, I'd think e88cc/6922 is actually safer bet.

Anyone has some experience or tips simulating CFA3 SS mode in Ltspice? The transient analysis is barely moving for me, with or without servo, single ended or balanced input, not with arbitrarily large time step size neither, only way I tried to make run through is making open loop gain too low such that the close loop gain is no longer maintained by the ratio of the feedback resistors. I wonder if it's the cross feedback thing making it so slow. For reference CFA2 with current feedback runs fine; CFA3 open loop mode also runs fine, I'm getting gain of about 3.3 with gain resistors set to 10k || 1.9k, is this expected? (I use Jfet on the input) Thanks! Edit: Also attaching my ltspice sim file CFA3_ltspice.zip

I ended up using STP8NK80Z(FP) in place of those FQP ones and they work all good if that's what you asking? Should be available from major vendors still.

Very cool! How do you couple output stage to headphones, doesn't look like output transformers are used?

😃Was trying not to dump more flat-earth theories of T2 to the thread as I already did a bunch, but here are what (I think) my problems were: I didn't find the cause of the oscillations appeared at the output, but they are killed by upping the feedback caps from 5pf to 10pf. Some "AC components" I measured around output plate CCS might actually be oscillations excited by me sticking my handheld meter probes across floating nodes - I recall when I measure referencing ground it was different but better, but in the end when I probe with oscilloscope I see nothing there. When I trim both channels in place, turns out the 10M90S in -500V rail reached the boundary of current limiting so it's trying to sag and gives a sawtooth wave on that output rail voltage, hence appeared at the amp output. Lowering the current limiting resistors fixed it. When it comes to getting preamp tubes for T2, probably the most important and minimum requirement for the circuit to work is to have well-balanced triode sections for each individual tubes. Some permutations of the quad JJ tubes I got will give too much imbalance that is outside the correction range of the servo. Also I find my left channel having more tolerance on input tube mismatch for the servo to correct than the right channel - suppose the better matched the rest of the circuit is, the more tolerance on input tube mismatch you will get.

Fixed this amp a while back and been running it day to day ever since. So far everything works and seems to be stable, I finally get to enjoy the music coming out of it 🙂 It's definitely quite a thing building and troubleshooting it in the past 8 months or so, major lesson learned is that I could've gotten a scope on day 1 if I could turn back time. Many thanks to Kevin for providing such a design - the major heavy lifting of bringing back a great amp. Thanks to GeorgeP for supplying the hardware with all the building advices, or I would never have considered building one. And a special thank to Simmconn, who spent a lot of extra time not only helped me with diagnosing the issues, but also taught me a lot of the nuts and bolts about proper troubleshooting practices in general as well as demystifying the T2 circuit to me through patiently explaining. Of course there are a few more to mention, thanks to the folks out here who kindly helped me. It takes 10+ headcase veterans to help a newbie like me building a working T2 😄

@G600 I got my boards from GeorgeP and it should be whatever is the latest version of Kevin's design with original parts (as opposed to "modern version") Haven't got back to this thread for a while but I made some progress over the holiday time identifying the problem with a lot of help from simmconn as well as finally using a scope. -Balance: turns out by changing the permutation of my preamp tube locations in certain case I can get both channels balanced, likely some of my 6dj8 has better matched sections than others which were kicking the servo out of working range. -Offset: Replacing R73 with 10k pot I was able to trim out the remaining offset to near zero no problem. -Oscillation: quite possibly my last bit of problem, that I was observing low frequency (60-80Hz) sawtooth waves at all outputs. left-channel has seemingly matched waveforms on O+ and O- with hundreds of millivolts peak-to peak and right-channel has unmatched waveforms with one measures 1.42V pk-pk and the other much less and also reversed pattern along time axis. Slightly trimming the battery voltages doesn't seem to affect this behavior. When I got time again I will inspect things around the balance servo as well as breaking the loop to see if I can find out where it comes from. 🙂

Might have gotten a tiny bit more clue, been trying random things while waiting on a batch of K216/J79 replacement parts to arrive ($$$😢). I first tried replacing Q6/Q7, Q11/Q12 with KSA1156s, nothing changed, then I tried putting both C12/C13 in there instead of either, nothing changed (thus removed one again). Then I tried removing 5pf feedback caps across R78, R79: -Q33 still measures weird, plate CCS still violates Kirchhoff's law and are imbalanced, one J79 still has positive Vgs -Output offset went straight to -80V -Balance servo seems to be working in this case, with O+/O- within 1V difference and C/D being 1/10 of that. -Setting meter to AC measure mode and probe between O+ and ground, 18V @ 400kHz... O- doesn't give any significant reading, near 0V at near 0 frequency. So first of all it's possible that by removing these caps I just introduced new/worse problems. But it also makes some sense if oscillation is a part (if not all) of my original problem? if O+/O- were swinging some big time AC that would kinda explain why DC measure around CCS voltage doesn't check out, and why Q33 is having invalid Vbe from DC measure to be turned on yet Q32 still seems to be conducting fine? I certainly have absolute 0 knowledge on how to track down and troubleshoot stability issues, probably next step for me is just mess around with anything that have a time constant in it - capacitors, right after my previous battles against all diodes and resistors. I'm planning on trying: 1. Replace C1 with mica, I don't know what this RC network does but I seen other people built working T2 having mica cap there (could be wrong, only eyeballed from shape), and I used a film. 2. put 5pf feedback cap back and then try slightly higher values for comparison 3. maybe recap balance servo just in case I messed up some .47u/.1u values, hard to tell what I had there from looking Not too hopeful if this will fix anything but at least cure some boredom while waiting on those rare MOSFETs..

@simmconn Thanks for your explanation, once again 🙂 Sorry for the confusion and yes I meant D6/D7 (or D10/D11). Specifically, taking one side of CCS as an example: 1. V(D4) + V(D5) = 3.6V, Vbe(Q6) = 0.45V, V(R17) = 3.2V, this is all good. 2. Probe across R18-R20, all gives around 2.4V 3. Probe across B-E of Q8-Q10, all gives 0.57V 4. Probe across D6: 1.8V 5. Probe across D7: 0.7V (and probe D6, D7 together gives about 2.6V) D6, D7 look as bright, first round of trouble shooting I already replaced all LEDs and elevated them from contacting the board. Amp powered off and I diode test them they all fine with around 1.83V of Vf. So 2-5 makes no sense only thing looks ok is Vbe across C3675s. I will go ahead and check board connectivity tho. (Everything looks wrong when one probe is stick to O+?)

nvm, staring at the schematic a little bit more Q34-Q36 probably do this, referencing -560V supply negative battery voltage would like to be about 3x of V(R74) makes sense that it likes to stay somewhere between -540V and -550V

I been focusing on the "working" channel in the past two week, with +/- offset of -5V/-10V (they will slowly drift up to around -9V/-15V or so over time) - I removed R65, R66, R69, R70, they measured fine, rest of the resistors in the string also measures fine. - I removed R88, R90, they measured fine, then R89 and R91 in circuit also measured fine. - Probing around offset servo referencing -560V supply, Q33 still in very weird state, I then removed Q33, it measured fine on transistor tester. All these parts I put new ones in anyway. I then tried running the amp with R88, R90 disconnected, this should take the balance servo out of the loop, well output measures around +30V/-40V.. So this is not really a working channel, the seemingly low offset is just because the original offsets are quite high but center around 0, and the balance servo is already working pretty hard to bring them together. And without balance servo the situation of Q33 is the same. C/D might be ok - they likes to stay at same potential and is usually around the middle point between 1/10 of +/- offset when servo is connected back. Moving on trying to find out the cause of the imbalance - voltage at the gate of Q24 and Q25 are about 5-6V apart, thus probably so are the voltages at cathode of the output tubes. Then here is another weird thing: one of the J79 has point something volts of positive Vgs. Then going to the plate load CCS of EL34, since this should have major impact on cathode voltage of the output tubes. R17/R27 measures fine, with around 3.2V drop across them, this checks out with 3.6V drop of D4, D5/D8, D9 and 0.45V Vbe of Q6/Q11. R18-20/R28-R30 however on one side all measured around 2.37V-2.4V, on the other side all measure around 2.2V. So there are 2 problems - first being they are not balanced, second being the voltage drop doesn't quite seem right - when I directly measure Vbe of Q8-Q10/Q13-Q15, they are all around 0.57V, which is fine. D6,D7/D8,D9 should have about 3.6V across them, so voltage across all these resistors should be around 3V instead? I then try measure voltage across D8 and D7, D8 measures fine with around 1.8V drop, D7 however measures only 0.7V... I don't think this is right as D7 is visually as bright as D8, also this should instead gives around 2V drop across all the 820ohm resistors, one more mystery.. For the imbalance of current I thought of thermal again, as one set of the CCS is closer to EL34 than the other. Had a friend came over help me elevating almost all resistors, then I re-soldered every joint on the component side of the board due to fear of cold joint after pulling - Power on again and nothing changes. So now it sums up to: 1. Balance servo might be ok or at least I will leave it alone for now 2. Weirdness around Q33 and I don't know what it's doing. 3. Positive Vgs on one J79 4. Things in plate load CCS measures weird and off. I don't know if I have a single problem or multiple, and not quite sure among them which is the cause and which is the effect. To my understanding, the majority of the current carried by plate CCS - thru R17-R20, this should be independent of any feedback stuff in rest of the amp? If that's the case I should try to fix this first, but I don't know what to do about it, all the resistor values measures fine when amp is off. And also an unrelated question - what is the factor in the circuit that prevents negative battery voltage from changing? When trying to adjust the trimmer closer to 740V, up to some point apparently negative voltage likes to stay where they are around -540V while positive voltage can still be trimmed down. (I also tried trimming battery voltages in opposite direction, up to 10V difference, this doesn't seem to affect output imbalance and they still likes to stay around 5V apart.)

Thank you! I will take them out and check! This reminded me that, depending on how I set the trimmers, when turning the amp on with a bunch of things not conducting, O+ and O- can be as high as the positive 500V rail voltage, that certainly exceeds the 350V rating of R88, R90, R65, R66, R69, R70 I used, also voltage across C12/C13 can be a little above their 1kv rating. So I shall probably just replace them all anyway.