You can purchase a bias board kit from Dr Tube in parts or fully assembled form. You will need to modify your pin 5 position on the power tube sockets using a 5mm hollow drill. I opted to just buy a new mother board as my original board was aged 25 years. Buying used without knowing the amps history I would recommend a new board replacement. Took me 45 min to install. Just take pics and tag all the connectors. Test for bias stability 1st as some boards hold up likely due to the soldermask holding up against the prolonged heat protecting the 1mm trace design flaw on the boards. Once the soldermask breaks down the material becomes conductive and bias drifts due to the high HT voltage increasing the negative grid voltage. One bias side will be stable, one side won't.
