Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on December 08, 2016, 11:56:35 pm

Title: Signal goes through even the volume is all way down
Post by: Leevi on December 08, 2016, 11:56:35 pm: Is that a normal behavior in Fender amps like Deluxe, Bandmaster...?

http://www.hdtvrevolution.com/media/www.thevintagesound.com/ffg/schem/deluxe_ab763_schem.gif (http://www.hdtvrevolution.com/media/www.thevintagesound.com/ffg/schem/deluxe_ab763_schem.gif)

I have checked the volume pot with oscilloscope and noticed that it does not leak.

My assumption is that the signal goes through the plate resistors to the PI?

/Leevi
Title: Re: Signal goes through even the volume is all way down
Post by: sluckey on December 09, 2016, 08:35:21 am: Quote
Is that a normal behavior in Fender amps like Deluxe, Bandmaster...?
no
Title: Re: Signal goes through even the volume is all way down
Post by: Leevi on December 09, 2016, 09:06:25 am: What might be the reason? I have met this couple of times before.
/Leevi
Title: Re: Signal goes through even the volume is all way down
Post by: Leevi on December 09, 2016, 10:52:40 am: I double checked the tone stack wiring and found a wrong wiring there.
After I had corrected it the signal almost disappeared, By measuring from the
output the signal is still there but no longer so audible.

/Leevi
Title: Re: Signal goes through even the volume is all way down
Post by: pompeiisneaks on December 09, 2016, 01:59:47 pm: I would think either a. the volume pot is going weak, so the resistance to the signal chain is weaker than it should be, or the ground on that pot is going so resistance to ground is higher than it should be.
Title: Re: Signal goes through even the volume is all way down
Post by: Leevi on December 10, 2016, 12:33:49 am: Quote
I have checked the volume pot with oscilloscope and noticed that it does not leak.

I have even changed the pot. I think the main reason for the audible signal was a faulty wiring in the tone stack
causing that one part of the signal was somehow bypassing the stack. Something goes still through which is visible
with meter on the output but as I said that does not disturb anymore since the signal is not audible.

/Leevi
Title: Re: Signal goes through even the volume is all way down
Post by: pompeiisneaks on December 11, 2016, 06:12:28 pm: the volume should always be a limiting factor. If the tone is getting through it, it's at the volume pot. Look at the schematic. The only path to the next stage is through either the volume pot or in the vibrato channel that bypass cap above the volume. Both otherwise are sent to ground via the volume pot. The tone stack is between stages of the first input tube and the volume comes after it. If the volume is correctly shunting all signal to ground there the only way in is through that one cap and only on the vibrato channel.

~Phil
Title: Re: Signal goes through even the volume is all way down
Post by: sluckey on December 11, 2016, 06:38:24 pm: Tack solder a fresh filter cap across the node B filter cap. Any better?
Title: Re: Signal goes through even the volume is all way down
Post by: Leevi on December 11, 2016, 10:26:57 pm: Quote
Tack solder a fresh filter cap across the node B filter cap. Any better?

I already did that and I think it helped a bit.

/Leevi