Vintage Blackface Twin Reverb biasing

[RedGuitar]

Greetings,
I bought a vintage BF Twin Reverb few months ago.
I performed a cap job on all electrolytic caps.
Recently I noticed a loss in sound volume and crackles from time to time.
The amp still has '65 original tubes, so I decided to change tubes.
I also noticed an hole on the back (sort of jack-hole), where you can access the bias pot taper from the outside of the amp.
Is it for an outside-biasing?
Can you link me a procedure for a correct biasing of my BF TR?
(I have electronics knowledge)
Many thanks!
Cheers

[Enzo]

Look inside the cab on the wall at the tube chart. What model is at the top? It will look like AB763 or AA769 or something like that. Some bias controls were bias level controls that adjusted to current, but others were bias balance control, and you adjust those by ear for least hum. Does it say tube balance by the hole?

A stock Fender adjustment pot would have a standard 3/8" bushing and nut like the other controls, except there is no shaft, just a screwdriver slot inside the bushing. Is that what you have? If you have just a hole to access some pot inside, that sounds more like a modification someone made.

[Tom Phillips]

...I bought a vintage BF Twin Reverb few months ago...The amp still has '65 original tubes, so I decided to change tubes. I also noticed an hole on the back (sort of jack-hole), where you can access the bias pot taper from the outside of the amp...

Can you post photos? From your description it seems that the amp has been modified from stock condition.