Hi Folks,
During my rebuild, one of the things I wanted to do was replace that very sucky and dangerous bias arrangement in the ST-70.
Currently, if the wipers on the pots lift, the tubes die. Perhaps some iron too.
We need a method that if any wipers lift, the tubes just simply cutoff.
I also wanted thte ability to use non perfectly matched tubes.
Here's the circuit that so far has proven itself quite useful for doing this:
I've just used regular pots for the controls, one could even use the existing 10K in there for the balance control. But when a PCB is made, I will likely use the multiturn Bourns 3296 trimmers that are often used in pro gear for their reliability.
Cheers!