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Old 11th February 2011, 05:18 PM   #21
kevinkr is offline kevinkr  United States
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Ok, I am getting 50-70mV of 60Hz hum on the output of the input stage, none of the other stages seem to add any hum.


Hehe yeah the isolation table is great .... but the two pumps for it are super noisy.
Look at the filament supply wiring and have you applied the dc bias I suggested to the filament supply?
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Old 11th February 2011, 05:22 PM   #22
HFGuy is offline HFGuy  Canada
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I have the heater circuit referenced to the cathode of KT88. Remember there is no 60Hz being created on the output of the 2nd 6SN7, just the first.


BTW. I am pretty convinced now that after I get this all working I will convert it back to the first schematic.
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Old 11th February 2011, 05:30 PM   #23
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I have the heater circuit referenced to the cathode of KT88. Remember there is no 60Hz being created on the output of the 2nd 6SN7, just the first.


BTW. I am pretty convinced now that after I get this all working I will convert it back to the first schematic.
I've done the same trick and as long as there is no significant audio present it works just fine.

Now you need to go back and do some additional trouble shooting. The tube is sufficiently far from the power transformers that magnetic pick up is unlikely to be the culprit. My first suspicion would be a ground loop, and my next induction from the filament wiring. (Or - horrors, a terrible wiring error.. I've done it with such consequences..) You haven't got more than one point grounded to the chassis do you? RCA jacks are either the only point connected to chassis or they are on isolation washers returned to the single common ground point - can't have both.

Yes I would go for fixed bias! And all component values as specified in the original design. Once it is working if you feel so inclined you can look for "improvements" or not.

Keep plugging away you are making real progress. Victory is at hand..
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Old 11th February 2011, 05:38 PM   #24
HFGuy is offline HFGuy  Canada
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Yeah I have made progress for today. I think I will call it a day. I think I am going to put the hum thing down for now and figure out the NFB thing. I think I am applying PFB atm.... but that will wait for tomorrow. I am just happy you guys helped me solve the parasitic oscillation problem. Now I just have to solve new oscillation

Yes, I am pretty set on going back to the original schematic. I just want to solve the NFB/PFB thing and maybe the 60Hz hum before I take it apart.

This project had been a major undertaking for me, it was my first power amp and first tube project and I think i went alittle overboard. After doing this I have learned a lot and the 2nd monoblock will be MUCH improved.
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Old 11th February 2011, 05:53 PM   #25
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This project had been a major undertaking for me, it was my first power amp and first tube project and I think i went alittle overboard. After doing this I have learned a lot and the 2nd monoblock will be MUCH improved.
... that's a major drawback of stereo: you have to do it again.

I may need to build an 8 channel buffer if I go to a digital x-over for my active speakers. Not looking forward to it.
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Old 11th February 2011, 06:00 PM   #26
HFGuy is offline HFGuy  Canada
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Too true. My last project was a voltage reference (noise floor is something aorund -150dB, the gear we have here cannot actually measure any lower). Measured the first prototype channel and felt I was done ..... until i remembered i had to build 3 more channels.

Click the image to open in full size.
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Old 11th February 2011, 06:04 PM   #27
kevinkr is offline kevinkr  United States
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Too true. My last project was a voltage reference (noise floor is something aorund -150dB, the gear we have here cannot actually measure any lower). Measured the first prototype channel and felt I was done ..... until i remembered i had to build 3 more channels.

Click the image to open in full size.

Very cool. I have designed references and VI (voltage/current source) for ATE applications with similar performance. Very difficult to measure, but as part of design validation you have to.. Another life away from audio..
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Old 11th February 2011, 06:45 PM   #28
HFGuy is offline HFGuy  Canada
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Ok I just checked and I am about 99% sure I am applying PFB. I have checked the wiring about 100x and am pretty sure it matches the schematic. So can someone verify this for me.

Input stage = first inversion
Phase splitter = 2nd inversion
3rd gain stage = 3rd inversion
Output stage = 4th inversion


So the input signal and the feedback signal should be in-phase. Since the feedback signal is applied to the cathode will result in NFB. But i just measured the input signal and the feedback signal, and they are inverted ! Can someone confirm if the output stage is actually inverting (I am not 100% on OPTs) ?

Here is the OPT (showing dot convention)

Click the image to open in full size.
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Old 11th February 2011, 06:58 PM   #29
kevinkr is offline kevinkr  United States
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See post # 16, you have confirmed that the input and feedback are out of phase - all you need to do is swap either the connections to the grids of the output tubes or if it is tidier swap 1 & 5 and 2 & 4 on the primary. Since three of the stages are differential your convention is not necessarily correct and any one of the differential stages can be flipped in phase by swapping the input or output connections, but NOT both. Simplest might be to swap the transformer primary. Your call, but do it only in one (any) of those three stages. (2nd - 4th)
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Old 11th February 2011, 08:05 PM   #30
HFGuy is offline HFGuy  Canada
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Ok i just swapped 2 of the phases and now it works. There are still problems but I am making serious progress. The problems I can see now

- Hum ? Not so sure the problem is still there
- Very limited power, the waveform gets all screwy after I get to about 6W (Ill take some photos later tonight of whats happening.
- When i connect the ground lead of one of my scope probes in an attempt to measure other nodes in the circuit the output goes all screwy again.


On small signal it looks like the cutoff frequency is about 150KHz.
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