oscillation

[Miles Prower]

Originally posted by merkur
It does'nt matter if the signal is a pure sinus or a square wave. It happens when the output stage clip. And when the output clip you have a square wave so my problem it's when the output start to clip(AB2 class kind of). Sorry if my english bad but i'm a french Canadian.

That sounds like a loop instability. Let's have a LQQK at the schematic, and see if you can determine what the oscillation frequency is.

[merkur]

Here the schematic. Some modification don't appear on the schem like the voltage double has change a bit I will update it and post it again.
I also try with and without feedback.

[merkur]

I have post the schematic to have a little bit of help. Now I could see that it been downloaded 8 times but nobody had give me feedback!

So if someone have any idea on what the problem could be please tell me!

Thanks

[merkur]

I know that my problem might be solve by one way are a other but I prefer a straight design, solid and purify. It is sure that my problem come from a lack of knowlege or experience and that I don't argu but I want to learn everything that will make me a better designer so every advice or suggesthing readings are welcome so, to everybody, don't hesitate throw what you want if it is something that I could learn.

Thank you!

[Merlinb]

There is nothing obviously wrong with the design except the standby switch. Disconnecting cathodes while running will damage valves overtime. A standby switch should be in the HT supply before or after the rectifier.

Are you sure the oscillation is actually coming from the output stage? You aren't hearing oscillation from the pre amp?

I would reduce the value of C14 C15 to 33nF to prevent low frequency oscillation. Also check them for leakage, and check your wire layout.

[SY]

C10 and C11 are really in series like that?

Grid stoppers nice and tight against the grid connections?

A strong second to the idea of tracing back exactly what stage is the culprit here. Don't just assume output.

Is that voltage doubler powering the output stage too? If so, it's woefully inadequate for ripple and regulation (44u on a voltage doubler?) and could be folding up on you. Screens especially need very tight regulation. Sag is OK in a guitar application, but if the supply just falls apart at overload...

[merkur]

Here how the power supply is right now. Looks ok to me but I never deal with voltage doubler before and there is not much information on it. But i'm open ear to critic's and recommendation

[merkur]

oops! I forgot to add the file

[merkur]

C10 and C11 are really in series like that cause I have a reverb circuit to goes there but not for now.

The grid resistor are right at the tube , closest that can be!

How much in (%) ripple is acceptable on the plate and on the screen voltage supply when the amplifier is driving at maximum?

also The output transformer put a load of about 4.3k on the output section. I have to live with that transformer only for that project.

[kathodyne]

in the schematic the driver (12ax7) has no grid stoppers.....also the output tubes may come to rest with bigger gridstopper (try between 1 and 10k)