Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Planobilly on December 15, 2016, 09:40:13 am

Title: Hoffman AB763 2 channel reverb hum issue
Post by: Planobilly on December 15, 2016, 09:40:13 am

Hi,

Hoffman AB763 2 channel built to the schematic with no mods except 25uf cathode caps.

The amp is pretty quiet except for the reverb. With  the reverb pot turned down there is no noise. The noise/hum increases as the pot is turned up.

I can't seem to measure the frequency of the hum with a scope or DMM. It is low freq that I hear. I have been over the schematic and no apparent wiring issue. The plate voltage is 430VDC. Moving the reverb driver has no effect.

What should I check next?

Thanks,

Billy

Title: Re: Hoffman AB763 2 channel reverb hum issue
Post by: Willabe on December 15, 2016, 10:24:10 am

The output signal from the verb tank is less than a guitars PUP signal into the amp.

So the verb tanks output end has to be away from the PT and AC cord.

Try flipping/moving the verb tank until you find a quiet place for it.

Also you might have to shield the open side of the tank.

Verb tank cables have to be shielded wire as does the verb on/off foot switch cable.

Title: Re: Hoffman AB763 2 channel reverb hum issue
Post by: Planobilly on December 15, 2016, 03:41:05 pm

It looks like the noise/hum is being picked up from the environment. Pulled V!,V2, V3 and still had the hum.

Disconnected grid 1 on V4 and the hum goes away...goes away also if i disconnect grid 2 on V4.

I am not sure what has changed where I live. I have not had this issue before.

I also don't understand why the amp is quite other than the reverb circuit.

Cheers,

Billy