Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: punkykatt on November 12, 2024, 12:49:11 pm

Title: Scratchy volume pot Mesa Boogie Blue Angle.
Post by: punkykatt on November 12, 2024, 12:49:11 pm

Hey Guy`s, Got a Mesa Boogie Blue Angle on the bench. https://el34world.com/charts/Schematics/files/Mesa_boogie/Boogie_blueangel.pdf
There is no DC voltage on the volume pot or any of the other pots. I replaced the volume pot with two new pots thinking the first new one I replaced could be bad. Still got the scratch. What gives??? :BangHead: .  Any help is much appreciated.

Title: Re: Scratchy volume pot Mesa Boogie Blue Angle.
Post by: Deric on November 12, 2024, 01:09:12 pm

Swap V2?

Title: Re: Scratchy volume pot Mesa Boogie Blue Angle.
Post by: punkykatt on November 12, 2024, 01:59:25 pm

Deric,  Holy crap, It worked.  :worthy1:   I just serviced this amp and put in all new tubes? Is this something common with this model? Go figure. Is there an explanation why this happens? Thank you sooooo much.

Title: Re: Scratchy volume pot Mesa Boogie Blue Angle.
Post by: Deric on November 12, 2024, 02:56:22 pm

The schematic shows ~117vdc on the plate of V2A.  I've come across several 12ax7 that will leak a tiny bit of DC out of the grid when the plate voltage is that low.  It's small enough that it's hard to measure but just enough to make the pot scratchy.


Glad it worked!   :occasion14:

Title: Re: Scratchy volume pot Mesa Boogie Blue Angle.
Post by: punkykatt on November 12, 2024, 04:17:40 pm

Thanks. Good to know about the downstream grid possibly leaking DC to the pot. Learn something every day.