70s Bassman 100 bias mod question

[Randall]

Customer wants to add a bias level adjustment to the CBS bias balance in this amp. What would be the way to go about this? Would replacing the 3.3K with a 5K or 10K pot do it?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[The Dude]

It would work, but I wouldn't do it that way. If the pot goes bad, you lose bias all together. I'd change the 15K for a pot and resistor. It's much safer. See this:

https://robrobinette.com/Silverface_...t_Bias_Balance

Edit: I should add, I wouldn't do it at all. There's nothing wrong with the circuit as is and there are plenty of old Fenders still out there running without bias modification. But if you must,.........

[Justin Thomas]

I don't know if it's Mister Thud or Mister P. Bass who had the plan for both, assuming your chassis already has other holes added.

Justin

[Chuck H]

It would work, but I wouldn't do it that way. If the pot goes bad, you lose bias all together. I'd change the 15K for a pot and resistor. It's much safer. See this:

https://robrobinette.com/Silverface_...t_Bias_Balance

Edit: I should add, I wouldn't do it at all. There's nothing wrong with the circuit as is and there are plenty of old Fenders still out there running without bias modification. But if you must,.........

If you wired the pot as a rheostat at least some bias voltage wouldn't be dependent on the wiper connection. Just thinking out loud.