Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on June 19, 2011, 04:29:45 am

Title: Champ Silverface
Post by: Leevi on June 19, 2011, 04:29:45 am

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

Title: Re: Champ Silverface
Post by: plexi50 on June 19, 2011, 06:33:53 am

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

Title: Re: Champ Silverface
Post by: Leevi on June 19, 2011, 07:15:34 am

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

Title: Re: Champ Silverface
Post by: Iannone on June 19, 2011, 11:37:48 am

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

Title: Re: Champ Silverface
Post by: Leevi on June 19, 2011, 12:47:28 pm

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

Title: Re: Champ Silverface
Post by: punkykatt on June 19, 2011, 07:40:16 pm

Did you try another speaker?

Title: Re: Champ Silverface
Post by: Leevi on June 20, 2011, 02:08:29 pm

Quote

Did you try another speaker?

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

Title: Re: Champ Silverface
Post by: PRR on June 20, 2011, 11:44:03 pm

> It sounds like a very fast tremolo

Turn off the ceiling fan.

Measure tube voltages.

Title: Re: Champ Silverface
Post by: LooseChange on June 21, 2011, 05:24:28 am

Quote from: PRR on June 20, 2011, 11:44:03 pm

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

No kidding! Been there.

Title: Re: Champ Silverface
Post by: jojokeo on June 22, 2011, 08:45:26 pm

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

Title: Re: Champ Silverface
Post by: bigdaddy on June 23, 2011, 03:16:09 am

That's the best way to get that Robin Trower sound, like "bridge of sighs", put a fan in front of your amp. :l2: