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:

Billm Bias Board

Click for larger image.

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:

Installing the bias board

Click for larger image.

15 Comments

  1. Jeff says:

    This is a great addition to the octal conversion + TP24 / TO26 transformer mods. Very simple installation and works great. I will be ordering a matched pair of SED 6L6GC output tubes to complete the mod.

    I also added a terminal strip and 1ohm, 1W, 1% resistors to measure the actual idle currents.

    • Jeff says:

      Blown away by the amp with the SED 6L6GC output tubes.

      Cathode current (including plate and screen grid) is set to 49mA at 370V plate voltage. This is a conservative setting and I could certainly increase the operating point.

      Also, I have been experimenting with 15″ speakers and am so enamored by the sound I ordered a custom oversized Blues Jr cabinet.

  2. devindewitt says:

    I plan on ordering the bias board and just want to make sure it won’t be a problem to install since I have already installed the bias pot in the cream board upgrade kit.

  3. devindewitt says:

    I have the octal conversion with the matched 6v6’s and I can’t get the voltage down as low as you recommend. Before I installed the tubes the voltage read -28 volts when I got a click off the bias pot. With the tubes in they are running at 5.6 volts.

    • bill says:

      Do you also have the TP24 power transformer? Different pairs of tubes run at different operating points. To get the tubes running cooler, you should probably opt for the bias board.

  4. devindewitt says:

    I have both the tp24 power transformer and the bias board. With the bias turned all te way down ( I think, pretty sur I heard a click although could have been my imagination) the voltage was right around 5.6 volts on the jj 6v6 tubes.

    • bill says:

      What’s the voltage on the top leads of R31/R32? You should have more than enough voltage bias to idle the 6V6s properly. -22 volts ought to do it, but increase it to whatever you need to idle the tubes. Something like -25V may be needed, depending on your tubes.

  5. devindewitt says:

    The voltage at the top lead of r32 was about -28 volts when I believe I heard the first click on the bias pot. I turned the pot back a little but it didn’t click again though I didn’t want to chance breaking it. The installation went smoothly and I only questioned myself when attaching the filament leads from v4 to the tube board.

  6. leslie says:

    Hi Bill, I just put the new bias board in today…When I was putting the bord back in place I broke my mid control pot will the pots you have work for the mids?If not where is the best place to get one? Also you said somewhere on here that the treble pot could be switched with the mid pot am I right? By the way I have done all your mods on both my Blues Jr’s…..:)…..Thank you for any help you can give me!!!

  7. durhadj says:

    Hi bill. I intend to exclusively run 6v6’s in my Blues Jr. Am i correct in assuming that i don’t have to install a TP24 or the bias board circuit in order to run 6v6’s? Also, if i were to install those components would there be any major tonal improvements over a basic 6v6 loaded Blues Jr.?

    Also, would there be any drawbacks to installing those components if i intend to exclusively run 6v6′s?
    (other than extra time and energy spent on installation, obviously)

    • bill says:

      You can just install the octal conversion kit and one of the upgraded output transformers to run 6V6s.

      The TP24 will give you more clean headroom and more power. The amp sounds brighter, too, and has more authority in the bottom end because there’s more power on tap. As noted, it requires the bias board. There are no drawbacks to running the TP24 and bias board.

  8. JerryK says:

    Bill,
    Do i need the Bias board if using the TO20 with the TO24, green board and EL84’s ? Basic mod and hi voltage preamp and a few others have been installed.

    • bill says:

      No, you never need the bias board for EL84s. They don’t need as much bias voltage to control them, and they are within the range of adjustment of the basic mods bias pot, even with the TP24.

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