Parasitic oscillation problem untraceable

[JMPGuitars]

As for the scope... I just didn't know how to use it I guess. I bought it a couple of weeks ago... but I intended to bring it back. I kinda got the hang of it today... should I change it for the Siglent 1102 ?

Yes, get rid of it! Do it now while you can. "Buy cheap, buy twice."

[Bieworm]

Here's the voltage chart.
V1 triodes are swapped compared to the schematic due to a layout thing

20210105_222540.jpg

Isnt the waveform on the first pic oscillation? Or what does it mean?

20210105_204247.jpg

[JMPGuitars]

Don't have target voltages. I wish I had em... top secret tone king gear...

Why do you think It's the clean channel? They only share the 1st triode and PI etc..and that's fine. It situates between the 1st triode and the 3rd preamp triode of the OD channel according to the scope? Or am I reading the wavefirms wrong?

You stated previously that the problem exists on both "channels."

I haven't really looked at your waveforms. But an ugly waveform isn't necessarily going to be obvious. If you have a symmetrical waveform, even if it's ugly, that doesn't mean there's crossover distortion (and it sounds like there is), that likely just means that you're at a higher gain area in the circuit. When it clips, a sine wave turns more towards a square wave (distortion time). What you actually see on the waveform itself gives more clues as to what's going on. For example, you have to look at where the wave form crosses over. Is it symmetrical, or is it distorted?

They only share the 1st triode and PI etc..and that's fine.

This sounds like a misunderstanding of your circuit. Look at your schematic more closely. They share the signal path any time the relay is activated. The relay section, that I assume noticeably increases your gain, can't exist without the "other" channel.

Signal path = input jack to V1b to tone stack to V2a to X (PI)
Signal path with relay active: The same as above, with the addition of V1a into V2b (parallel with V2a) to X (PI)

There is no independent channel switching. If a problem exists even on your "clean channel" then it can exist on both "channels." This is why you need to test without the relay active so you can try and eliminate the issue before increasing the signal levels with the relay. If it starts that early in the circuit, and you increase the signal causing the trouble, you're increasing the troubled signal.

If the signal level is too high, you could try adding a 470K resistor to ground after R20, and add another 470K resistor to ground after R21. This will cut the signal voltage hitting the PI in half, and possibly fix the problem. This is similar to the Gain knob on the Trem TMB. That knob bleeds signal voltage to ground to reduce the signal level. Without it, that circuit would have the exact same problem you're having.



Side note: I'm guessing this is the amp you wanted to add a trem channel to? It won't work how you think. You can add a tremolo circuit to the relay, and then you'd be adding or removing a tremolo effect from the original channel, but it's not a separate channel. If you want it to be two separate channels, you would need to significantly modify the way the amp works. For example, use the relay to actually switch between the two halves of V1, 1 side for normal channel, 1 side for trem channel. Then either channel would need to go to an LTP PI, each one using only half of the PI so that they're actually independent channels. Doing that is sort of a complicated way of recreating the Trem TMB poorly. This amp isn't a good candidate for that. Maybe if you used a different type of Trem circuit it would be more relevant in the relay, but still questionable.

[Bieworm]

Thanks. I'll look into that tomorrow. Time for bed..
But I already addeda voltage devider before the PI. It's a 500k pot with 470k to ground.
Something is off.. we'll find it (with the scope?)
ya tomorrow

[JMPGuitars]

Thanks. I'll look into that tomorrow. Time for bed..
But I already addeda voltage devider before the PI. It's a 500k pot with 470k to ground.
Something is off.. we'll find it (with the scope?)
ya tomorrow

That voltage divider is backwards. Leave the 470K R20 in place, and add the 500K pot to ground after it.

If you want to test this way, then do the same thing for R21.

[crgfrench]

We were discussing a single frequency sinewave (per your illustration). Any distortion through a circuit that does not introduce an additional signal source at a different frequency can only consist of harmonic energy.

I apologize, I'm really not trying to be disagreeable here, but that is false. Harmonics reside in the frequency domain. An unbalanced phase inverter changes amplitude, not frequency. The balance of the phase inverter has nothing to do with frequency nor harmonics.

[TriodeLuvr]

We were discussing a single frequency sinewave (per your illustration). Any distortion through a circuit that does not introduce an additional signal source at a different frequency can only consist of harmonic energy.

I apologize, I'm really not trying to be disagreeable here, but that is false. Harmonics reside in the frequency domain. An unbalanced phase inverter changes amplitude, not frequency. The balance of the phase inverter has nothing to do with frequency nor harmonics.

It's just a technical discussion, I'm OK with it. You're right about the phase inverter. It's Class A, and when it's unbalanced, that only means one anode has a different amplitude than the other. The problem occurs when the unbalanced signals drive the AB1 output stage. The output stage relies on the two drive signals to be equal and opposite in order to reproduce a correct sinewave. Each tube only amplifies the positive-going portion of the signal impressed on its grid, and the two positive-going waveforms from the two tubes (negative-going at the anodes) are applied alternately to the output transformer in push-pull.

So, at the output of the amplifier, after being combined in the transformer, one tube's output becomes the positive-going half of the sinewave, and the other becomes the negative-goint half. If the two halves aren't equal in amplitude, the resulting composite waveform will be distorted and will contain harmonics not present in the original waveform.

Jack

[TriodeLuvr]

I thought you didn't have a scope? What does it look like at the output (speaker)?

Not really like crossover distortion 20210105_221143.jpg

As for the scope... I just didn't know how to use it I guess. I bought it a couple of weeks ago... but I intended to bring it back. I kinda got the hang of it today... should I change it for the Siglent 1102 ?

My preference would be a good used scope made in the West, such as HP (Agilent), Tektronix, LeCroy, etc.

About the waveform, is that pic showing the output of the amp when you hear the fizz? It looks like hard clipping, a waveform not too different from a solid state clipper. I agree, there's no crossover distortion of any significance.

Jack

[Bieworm]

I thought you didn't have a scope? What does it look like at the output (speaker)?

Not really like crossover distortion 20210105_221143.jpg

As for the scope... I just didn't know how to use it I guess. I bought it a couple of weeks ago... but I intended to bring it back. I kinda got the hang of it today... should I change it for the Siglent 1102 ?

My preference would be a good used scope made in the West, such as HP (Agilent), Tektronix, LeCroy, etc.

About the waveform, is that pic showing the output of the amp when you hear the fizz? It looks like hard clipping, a waveform not too different from a solid state clipper. I agree, there's no crossover distortion of any significance.

Jack

It's with a dummy load on the output jack. But it should be seriously fizzing at the actual settings. Vol on 6 (max headroom is about vol 3) , tone on 5 and midbite on max
What do you think about the sawtooth measured at input grid of stage 3 OD channel? I think bad stuff is happening between stage 1 and stage 3. I will scope it out later today to see where the first signs of this phenomenon starts...

[JMPGuitars]

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

[TriodeLuvr]

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.

[Bieworm]

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??

[JMPGuitars]

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).

[Bieworm]

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?

[crgfrench]

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)?

[Bieworm]

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!!!

[TriodeLuvr]

Dear friends ...
I kindly ask to take this discussion on a dedicated thread.

My fault, sorry. Won't happen again.

[JMPGuitars]

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

[Bieworm]

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.

[JMPGuitars]

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.