I have a GMC-300+ and it says V7 on the board inside.
I bought this with the hopes of using other tubes in it, but it seems like there is no pot to adjust the voltage any more?
Is there a way to still change the voltage by soldering resistors maybe? I was hoping to put an SBT-9 in it ...
Exactly this problem I have with a GMC 320 + V5. On the board is also V7 and no potentiometer for the high voltage. It would also be interesting for me how you could realize the conversion to a SBT-9 counter tube? Pictures would be very nice too?