Parasitic oscillation problem untraceable

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TriodeLuvr
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

JMPGuitars wrote:
Wed 01/06/21 7:14 am
TriodeLuvr wrote:
Wed 01/06/21 2:56 am
It's Class A
It's not class A. Cathode bias doesn't make an amp Class A.

https://www.aikenamps.com/index.php/the ... on-class-a
The phase inverter is Class A. The output stage is AB1. I modeled this in SPICE to produce an output waveform that makes the effect obvious. Below is what happens at the output when the phase inverter is unbalanced.

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Re: Parasitic oscillation problem untraceable

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Disconnected reverb at point X.
Voltage divider at point X
C6 from .047 to .022uf
PI Tail resistor 30k

Fizz fizz fizz.
On OD channel across the entire sweep of the voltage divider
On clean channel on overdriven volume

This can't be the PI anymore!!! This has to be on the 1st preamp stage??
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Wed 01/06/21 3:49 pm
Disconnected reverb at point X.
Voltage divider at point X
C6 from .047 to .022uf
PI Tail resistor 30k

Fizz fizz fizz.
On OD channel across the entire sweep of the voltage divider
On clean channel on overdriven volume

This can't be the PI anymore!!! This has to be on the 1st preamp stage??
Question: clean channel, is it better than it was or the same or worse? Same question for the OD channel?

Post all your voltages, especially your PI. If the voltage dividers there didn't improve anything, then make the preamp section going to X completely stock per schematic (before taking new voltages).
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

JMPGuitars wrote:
Wed 01/06/21 7:22 pm
Bieworm wrote:
Wed 01/06/21 3:49 pm
Disconnected reverb at point X.
Voltage divider at point X
C6 from .047 to .022uf
PI Tail resistor 30k

Fizz fizz fizz.
On OD channel across the entire sweep of the voltage divider
On clean channel on overdriven volume

This can't be the PI anymore!!! This has to be on the 1st preamp stage??
Question: clean channel, is it better than it was or the same or worse? Same question for the OD channel?

Post all your voltages, especially your PI. If the voltage dividers there didn't improve anything, then make the preamp section going to X completely stock per schematic (before taking new voltages).
Clean ch I think is the same
OD ch is worse

It's a sad thing that I don't have reference voltages. It would simplify troubleshooting...

I'm wondering about possible causes:
1. Tubes: preamp are ok because I tried a lot of different ones. 6v6? I don't have another set to check
2. Layout errors. I had to design the board myself because there are no examples that can be copied.. Google for gut pics of the imperial.
3. The relays system is wired correctly because it works fine, but as a shared part of the circuit it could be a possible cause. I could disconnect the turret nodes where the wires to the relays depart. I could disconnect the relays and jumper to test each channel separately.
4. Preamp or power amp problem? I could disconnect the preamp wires at point X and run the preamp of my modern classic into point X and vice versa? Should I additionally interconnect both chassis to minimalize ground potential issues?

Is there any good thinking here?
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Re: Parasitic oscillation problem untraceable

Post by crgfrench »

TriodeLuvr wrote:
Wed 01/06/21 11:13 am
I modeled this in SPICE to produce an output waveform that makes the effect obvious. Below is what happens at the output when the phase inverter is unbalanced.
That's very interesting, thanks! How did you get the 2:1 amplitude ratio to generate (what value load resistors would do this)?
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

crgfrench wrote:
Thu 01/07/21 12:38 am
TriodeLuvr wrote:
Wed 01/06/21 11:13 am
I modeled this in SPICE to produce an output waveform that makes the effect obvious. Below is what happens at the output when the phase inverter is unbalanced.
That's very interesting, thanks! How did you get the 2:1 amplitude ratio to generate (what value load resistors would do this)?
Dear friends 🙃...
I kindly ask to take this discussion on a dedicated thread. I'm trying to solve some issues with my amp in this thread. When there is another topic fluctuating through this thread it might be possible that the 'helping hands' miss some of the relevant information here. Don't get me wrong... I'm following your discussion eagerly, but I would like to follow it in a dedicated thread.
Thanx guys!!!👍👍👍
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

Bieworm wrote:
Thu 01/07/21 1:37 am

Dear friends 🙃...
I kindly ask to take this discussion on a dedicated thread.
My fault, sorry. Won't happen again.
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Thu 01/07/21 12:17 am
Clean ch I think is the same
OD ch is worse

It's a sad thing that I don't have reference voltages. It would simplify troubleshooting...

I'm wondering about possible causes:
1. Tubes: preamp are ok because I tried a lot of different ones. 6v6? I don't have another set to check
2. Layout errors. I had to design the board myself because there are no examples that can be copied.. Google for gut pics of the imperial.
3. The relays system is wired correctly because it works fine, but as a shared part of the circuit it could be a possible cause. I could disconnect the turret nodes where the wires to the relays depart. I could disconnect the relays and jumper to test each channel separately.
4. Preamp or power amp problem? I could disconnect the preamp wires at point X and run the preamp of my modern classic into point X and vice versa? Should I additionally interconnect both chassis to minimalize ground potential issues?

Is there any good thinking here?
There's a lot of guessing there. You can try swapping the position of the 6v6 tubes and see if that changes anything, otherwise you would obviously need to get a second set to compare.

https://www.aikenamps.com/index.php/wha ... distortion

Return everything to stock value at X. Then try changing C6 to .022µF, followed by a 470K resistor going to P5 instead of a direct connection. You can add a 100 to 220pF cap across the new 470K if it affects the tone negatively.

Thanks,
Josh
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

JMPGuitars wrote:
Thu 01/07/21 7:16 am
Bieworm wrote:
Thu 01/07/21 12:17 am
Clean ch I think is the same
OD ch is worse

It's a sad thing that I don't have reference voltages. It would simplify troubleshooting...

I'm wondering about possible causes:
1. Tubes: preamp are ok because I tried a lot of different ones. 6v6? I don't have another set to check
2. Layout errors. I had to design the board myself because there are no examples that can be copied.. Google for gut pics of the imperial.
3. The relays system is wired correctly because it works fine, but as a shared part of the circuit it could be a possible cause. I could disconnect the turret nodes where the wires to the relays depart. I could disconnect the relays and jumper to test each channel separately.
4. Preamp or power amp problem? I could disconnect the preamp wires at point X and run the preamp of my modern classic into point X and vice versa? Should I additionally interconnect both chassis to minimalize ground potential issues?

Is there any good thinking here?
There's a lot of guessing there. You can try swapping the position of the 6v6 tubes and see if that changes anything, otherwise you would obviously need to get a second set to compare.

https://www.aikenamps.com/index.php/wha ... distortion

Return everything to stock value at X. Then try changing C6 to .022µF, followed by a 470K resistor going to P5 instead of a direct connection. You can add a 100 to 220pF cap across the new 470K if it affects the tone negatively.

Thanks,
Josh
Thx
Does crossover only apply to output tubes or is this possible on a preamp stage?
Apart from all that... is a bad connection/solder or wrong proximity configuration of components on the board likely a cause of what we're hearing here? As I said before, the turrets on this board are the problem-taking-solder-turrets. So a connection might look good, but isn't actually. That would imply it could be anywhere.. or worse.. everywhere. In that case I'm looking at dismantling the board. Remove all turrets. Put on the goldplated ones. And reattach everything...😬 in any case, chopsticking and prodding gave me nada.
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Thu 01/07/21 8:05 am
Does crossover only apply to output tubes or is this possible on a preamp stage?
Apart from all that... is a bad connection/solder or wrong proximity configuration of components on the board likely a cause of what we're hearing here? As I said before, the turrets on this board are the problem-taking-solder-turrets. So a connection might look good, but isn't actually. That would imply it could be anywhere.. or worse.. everywhere. In that case I'm looking at dismantling the board. Remove all turrets. Put on the goldplated ones. And reattach everything...😬 in any case, chopsticking and prodding gave me nada.
If chopsticking gave you nothing, and tapping on components and filter caps doesn't tell you anything, then you haven't been told anything except that your guesses are just guesses. Thinking productively requires studying. Study the schematic. Consider what we know about voltage levels, and how amp circuits work. Then consider that if a solder connection was bad, chances are that you would likely get some kind of response while chopsticking. But even if there is a solder connect that's bad, how would it affect the circuit in a way that would cause this issue? If a stage is biased too hot, and it's the only one, then you would probably want to check the cathode connections and values. One way to check that of course is to look at your voltages. If all the stages are biased too hot, then you would want to look at your B+. There's a lot of logic you can apply to troubleshooting in general, but guessing won't get you there without a ton of luck.

I would like to see voltage levels once you do the stuff I said previously, and then we can go from there.
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

If you could supply a schematic version including all the changes you've done, that would help too.
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

This looks like it might contain strong higher-order harmonics. Is this clipping occurring at the output tubes, or is it in one of the preamp stages? Do the grids of the phase inverter look like this? And what's creating a 1.0000 kHz tone?

Image
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

If you could supply a schematic version including all the changes you've done, that would help too.
preamp changes 20210107a.png
power amp changes 20210107a.png
I'll post voltages later on today
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Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

TriodeLuvr wrote:
Thu 01/07/21 11:07 am
This looks like it might contain strong higher-order harmonics. Is this clipping occurring at the output tubes, or is it in one of the preamp stages? Do the grids of the phase inverter look like this? And what's creating a 1.0000 kHz tone?

Image
For that waveform to be really useful, he needs to drop the amplitude significantly (assuming that's what's driving the amplitude read by the scope). The signal generator amplitude should be 125mV. He should also stretch the time base a little so we can see the rise and fall better, in which case the crossover distortion will likely be more visible (if present). At its current amplitude, he could be introducing problems that aren't relevant to actual circuit voltage levels (assuming his source is set too high).

Bieworm, what's the signal generator amplitude set to? Where is the signal being inserted?
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

Have you removed the Zener at the output cathodes? I think that's needed in order to be sure bias destabilization isn't playing a role in this. I don't see how it's possible to have a softer breakdown If the cathodes have 30V DC when pushed.

Jack
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