The Bias Board
The adjustable bias board extends the bias voltage range so it covers the hottest-running 5881 and 6L6 tubes, as well as 6V6. It’s necessary when you boost the plate voltage with the TP24 power transformer. Along with the TO26 output transformer and the octal conversion kit, it allows up to 30 watts of output. But the higher plate voltage requires higher bias voltage, especially if you want the most clean headroom.
We’ve engineered the bias board for exceptionally easy installation in the Blues Junior. Two leads fit into existing resistor holes and a third one attaches to a component lead:
You need the bias board if you are running octal tubes (6V6, 5881, 6L6) in conjunction with the TP24. It is not recommended if you are using 6V6s or EL84s with the stock power transformer.
With the adjustable bias in the basic mods, the voltage can be adjusted from a few volts up to -26 volts. With the stock power transformer, a typical voltage range for EL84s is -11 to -14V. 6V6s typically require -22 to -26V.
With the higher plate voltage from the TP24 power transformer, 6V6s need about -32V, 5881s and 6L6s require -37 volts and up. The bias board picks up where the basic mods adjustable bias leaves off and can supply up to -54V.
The bias board is a custom Billm-designed circuit board specifically for the cream board Blues Junior. It uses premium components: Fairchild diodes, Panasonic high-temperature capacitors, metal film resistors and a Bourns 25-turn adjustment trimpot. Here you can see how the pins from the bias board match existing holes in the main circuit board: