by Bob Latino Mon Dec 28, 2020 11:55 am
That amp is an original Dynaco ST-70 which was upgraded with a VTA driver board from before 2012. Yes - This is one of the older VTA driver boards with an AC balance control. That AC balance control should have been placed on the BOTTOM of the driver board because once set, this pot should not be readjusted. If that pot is on the underside of the board, the owner is less likely to tinker with the pot once the board is installed into the amp. What you are supposed to do before you install this 10K pot is to measure from the center pin to the two outer pins and adjust the pot until both sides read equally. Ideally these should read 5.00K from center pin on each side but many times you get something like 4.92K per side when you adjust the pot so that the two measurements are equal.
The newer VTA boards (2012 to present) have a small LM334 integrated circuit that replaces this 10K pot on the older VTA boards. The LM334 will keep the current the same on both sides of the phase splitter part of the circuit automatically.
Bob
