Strange sound disappearing

[Aleksi]

Yep, that's me

I'll try to post the schematics later today when I get home.

BR,
Aleksi

[Aleksi]

Hi,

here are the schematics. The Glorified OD reverb basically powers the Belton through a DC blocked voltage doubler. Please check that part of the schematic, to my eyes it seems it's missing a diode from the voltage doubler's negative part.

Edit: in the Glorified OD the entire voltage doubler circuit is grounded at the cathode GND of the triode that amplifies the Belton's output signal. As you know the Belton signal and power GND are connected internally, therefore I had to ground somewhere where the output is amplified, otherwise I would get a horrible buzz.

The second one is a amp I'm working on still, but most likely everything should be correct in the schematic. I have to turn the FX output pot to really minimum when using the reverb, basically it will need even more attenuation to not go over the 1.5VAC limit for the Belton.

Any questions, fire away!

BR,
Aleksi

[iulker]

Ok, with my man, I was able to remove temporary the reverb block from the circuit ... and no more interruptions! So, indeed, the Belton reverb is the problem, but I still don't know what in wrong, I guess the place where I put him is not proper ...
Now I put a 1M resistor in parallel with the reverb block, like Fender does with them 3,3M resistor, and the sound is disappearing again!
I have no idea how to move input and output of the reverb, so he act normally.

[iulker]

The d-verb form ggg site is intended to be a pedal, therefor I suppose it could be used in front of the amp, at guitar levels ... so is not a problem of a low level coming at the input of the reverb block, but I guess in a form of a pedal can be inserted very well in a fx loop, where for sure it's a much more level then it is coming from a guitar.
That's why I don't know where is the best place to put this thing ...

[rock_mumbles]

You don't need to move the reverb, But you do need the resistor for the dry signal 'leak' (I saw someone use a 470k w/ parallel treble cap) that will let more dry signal through.

The reverb tap from the 470k input used a ~600pf cap (maybe even smaller to limit lows in reverb) then it needs a voltage divider ... a 1M (trim) pot.

The (trim) pot wiper fed the input of the brick, on the output in the circuit is the level pot ...

[iulker]

I must return to this topic, because the problem is still present ... and it's driving me crazy ...
All the things I've done so far:
- different tubes, 3 pairs - nothing
- VVR circuit out for good - nothing
- belton d-verb circuit, entirely removed - nothing
- NFB removed - nothing
- volume pot replaced with a new one - nothing
- ALL electrolytic capacitors replaced with different ones - nothing
- tried different output transformer - same shit
So, after done all the changes above, the damn amp is still suddenly fade out!
The mojo part is that I can't make any measurement of voltages when he is without sound, because when I touch anywhere, in any point, with the positive lead of my multimeter, the amp comes alive instantly!! like the simple shock of touching the circuit "open" the path for the sound again!
It's like twilight zone .......................

[Tillydog]

- belton d-verb circuit, entirely removed - nothing

[snip]

So, after done all the changes above, the damn amp is still suddenly fade out!

Just to clarify: the amp still fades out, even if the reverb is taken out?

The mojo part is that I can't make any measurement of voltages when he is without sound, because when I touch anywhere, in any point, with the positive lead of my multimeter, the amp comes alive instantly!

It sounds like you have a missing ground connection or a missing grid leak resistor somewhere - where have you connected the B+ ground and the Belton ground? Is the input grounded to the chassis? Is there also a connection from the chassis to the B+ ground?

Photos??

Andy

[Firestorm]

You have to remember that electrons come from ground. Bad connection, nothing works. Make sure all the ground connections are solid.

[iulker]

Yes, everything is grounded properly, and when he is work, is sounding very good.
Last night I've spotted an arch somewhere under the tagboard ... my suspicion is that somehow high potential spots are arching with the chassis ... is that possible, even if I have about 8mm between tagboard and the chassis, with spacers ?!
Anyway, tonight I gonna remove the board, and even make another one from scratch, possible with new components, I have plenty. I'll post the result here tomorrow, maybe will be useful to see what happens if something goes wrong and it's not noticeable. You loose many evenings, nights, like I did, and make many modifications and replacements, and in the end the result is the same from beginning, an frustrating amp ........

[Structo]

Yes sometimes you can get a solder blob hanging down under the board if you have switched out component while tuning the amp.

Or maybe a wire lead is hanging down too far underneath.

Do you have any kind of inspection mirror? Like dentists use?

You can see under the board with one of those to see if anything is hanging down and touching the chassis.

Good luck!

[iulker]

Ok, many thanks for the advises so far.
I've done replacing the tagboarg completely, as I mentioned above, now everything sound good until ... it's doesn't
So, the sound still disappear, but not so often as before I guess, and the positive thing is that now I can put the oscilloscope to see the signal without making the amp to comes alive , which is a big step forward.
Conclusion: when the amp is without sound coming from speaker, the signal is present at the grid of 6V6, but not at the speaker jack, or is a little amount of sound, but it is at very low level, so is inaudible.
Regarding the fact that I already replaced the power valve with several others 6v6's without improvement signs, what remains to suspect ? The output transformer!
But I did'n hear so far about such a behavior of an output transformer, like mine does, to cut the sound suddenly, and then, with higher levels of signal coming from the input, to start sounding again ... i'ts possible to be that the problem ?!
The transformer is not an original one, is a transformer from an old tube radio, ancient, may the fact that he does not match the impedance with the 6V6 tube, to act like this ? I know the theory with the mismatch impedance, but I never experienced such dramatic phenomenons ...

[martin manning]

Now that you can see that there is signal at the power tube grid, you have isolated the loss of signal to the palte circuit (OT primary) or OT secondary. When the sound cuts out, do you have DC voltage at the OT (B+ side) and on the plate and cathode of the 6V6? Is there any chance that the signal is getting grounded out on the secondary side (leads, jacks, etc)?

I would suspect the OT at this point but I believe you said earlier that you tried another OT and the problem was still there?

[Tillydog]

Also check the voltage on the cathode of the 6V6 under fault conditions- your bias resistor (or its connections) might be intermittent.

Andy

[iulker]

Now that you can see that there is signal at the power tube grid, you have isolated the loss of signal to the palte circuit (OT primary) or OT secondary. When the sound cuts out, do you have DC voltage at the OT (B+ side) and on the plate and cathode of the 6V6? Is there any chance that the signal is getting grounded out on the secondary side (leads, jacks, etc)?

I would suspect the OT at this point but I believe you said earlier that you tried another OT and the problem was still there?

You are a genius, that's for sure! and I'm not, because I didn't expect a fuckin' output jack to not making ground with the chassis when he is supposed to do, being in contact with the chassis with the metal part all the time, at least I assume so ....
Well, I' not 100% sure (I'm not sure of anything anymore), but I guess this is the motherfucker that kill my serenity and took many hours from me ... the damn speaker jack, somehow, and intermittently, as I described in this topic, cut the ground to the chassis, when he felt to, and then, in strange conditions, involving higher levels of signals, started to make ground again ... I can't explain this even now, but it's the last call, since I even replaced 6V6 with EL84 and a different output transformer, so it's almost nothing in this amp as was at the beginning ...
Anyway, tomorrow I'll go with the final verdict, crying in my fists if a simple 6.35 mm jack made me change completely an entire amplifier, from input jack to output transformer - except the speaker jack, obviously ... LOL

[Structo]

Glad you found it.

Don't be too hard on yourself, you learned something and you will likely never make that same mistake again.