Chasing Noise in a Blues Jr / Adjustable Bias

[tdvt]

I bought a used Blues Jr more than 15 years ago to use as backline at a monthly music event I used to host & it later became a house amp at a theater where I ran tech.
It was used by many, many people, was always reliable & many players commented that it was a particularly nice sounding example of that model.

Over the last year the amp developed a fairly loud hum along with some static, which I assumed was probably filter caps, so I ordered new ones.
Recently, when I finally got around to working on it, the caps seemed OK, but since I already had new ones on-hand, I changed them anyway.


While the new caps helped some, hum & occasional static was still apparent, worsening as the amp got warm.


I traced much of the remaining noise to the PI ribbon cable & shielding that cable with copper foil (for guitar control cavities) I got the noise down to an almost tolerable level, but not eliminated.


After too much YouTube, I came across a video of a BJr with a very similar noise issue that the tech traced back to traces on the stock Fender tube board leaking between runs as the board got hot.


He replaced the board with an after-market unit that eliminated the noise issues & the new board included adjustable bias built-in. I decided to try the same one & also decided to convert the ribbon cables to discrete wires with shielded grid feeds while I was at it.


The end result was worth the trouble as the amp is now nearly silent at idle with all extraneous noise eliminated.


While I was installing the new tube board, I really didn't give the modified bias circuit much thought & it works as intended. But it later occurred to me that since it is mounted on the tube board & not the main board (w/the other bias components), that the new circuit must be bleeding off the excess bias voltage right at the power tube grids.


The marked-up schematic section below is my best guess, as I didn't really look that closely before I installed it.