Champ Silverface

[Leevi]

I'm repairing a Champ aa764 from 70's and I wonder the raspy sound (at least IMO).
I have changed all the coupling caps that improved the sound little bit.
My question is what to do next. Do you think it would help if I start to change the old
Carbon resistors to newer ones or should I change the filter caps? The amp doesn't hum at all.
Or is that type of sound typical for Champs from that time?
/Leevi

[plexi50]

Shouldnt have hiss or crackle if thats what you mean by raspy. Plate resistors,power supply caps or check bias and preamp tube solder points. Preamp tubes as well can make noise when going bad

[Leevi]

Maybe the raspy is not the correct expression for the problem. It's not hiss either.
It sounds like a very fast tremolo i.e. the sound is not solid.
/Leevi

[Iannone]

Sounds like the filter caps may be failing.  The "not solid" sound you describe sounds like motor-boating.

[Leevi]

I tried with new filter caps but I didn't hear any effect.
In my opinion the sound is still not clean, even not with low volume.
The tube tester gives good value for the rectifier tube. Power and
preamp tube are new. The amp has still an U.S. PT and I'm driving it
with and an external transformer (230V/115V).
/Leevi

[punkykatt]

Did you try another speaker?

[Leevi]

Did you try another speaker?

Yes, I have tried. I'll change next the European PT to the amp and
do some measurements.
/Leevi

[PRR]

> It sounds like a very fast tremolo

Turn off the ceiling fan.

Measure tube voltages.

[LooseChange]

> It sounds like a very fast tremolo
Turn off the ceiling fan.

No kidding! Been there.

[jojokeo]

me too when it gets hot, I want the fan on but I hate the out of tune notes and sound.

[bigdaddy]

That's the best way to get that Robin Trower sound, like "bridge of sighs", put a fan in front of your amp.