Triode Amplifier Bias Boards
For Sale by Tony King, W4ZT

This Bias Board is FULLY ASSEMBLED and TESTED. It is built as a direct two wire replacement for the typical zener diode bias regulator or series string of silicon diodes found in thousands of commercially manufactured and home brew triode amplifiers. The bias voltage is variable over a range of about 3.5 to 39 Volts. An additional 10 to 15 volts is available on the high end by cutting a single jumper (at the expense of the low end) and adding the zener and capacitor shown on the schematic. The board can be mounted and supported by the tab of the transistor (hardware and thermo-pad are included). There is no need to mount the board with additional hardware but a hole is provided for that purpose if you use a heatsink other than the chassis and prefer to mount it that way.

The transistor MUST be bolted directly to a large (preferably aluminum) surface or a good heatsink that receives good air flow to take the heat away from the transistor. You MUST insulate the tab of the transistor from the heat sink using the thermo pad and shoulder washer provided. The B- and cathode leads must be bypassed as shown below to insure that there is no RF reaching the bias board.

There have been a couple of cases where guys did not have the cathode and B- leads bypassed for RF well enough. This caused a failure of the TIP-147 transistor with increased drive. Everything functioned properly in idle with no drive but as drive was added a point was reached where the transistor failed. Be sure that you have good bypassing on the B- and cathode lines. It is also wise to add a .1 uF 200V capacitor across the cutoff resistor. If you do not bypass these lines adequately you may experience a bias board failure. Also be sure to include the fuse in series with your bias board (or bias zener diode or diode string if you aren't using this board) so that should there be any failure of the bias board or system, the fuse will keep the tube from drawing excessive plate current due to the cathode being connected directly to B-.

(click the small pictures to see larger pictures in a new browser window)
This board is built using the TL431 Voltage reference chip and the TIP147 PNP power darlington transistor. This circuit is right from the data sheet on the TL431 and is basically the same circuit used by GM3SEK and WD7S on their triode control boards. I built this board primarily as a retrofit for amps that already have a Zener diode in them or for the builder that isn't interested in a full blown control board. The board measures approximately 1-1/4" by 1-5/8". It comes completely ASSEMBLED and TESTED. I do NOT sell KITS or Boards. It uses only two wires to interface with the amplifier circuit. There is a Metal Oxide Varistor on board to help guard against any high voltages arising from an arc within the tube(s). In some cases additional protection can be enjoyed by adding an NE-2 neon bulb in parallel with the bias board.
The circuits shown above and below are two of the most popular applications using a zener diode for bias.
Transistor Mounting
The transistor MUST be properly mounted to a surface that will adequately remove the heat. This can be an aluminum chassis or a heatsink designed for the purpose but in either case there should be plenty of air moving around the transistor. Mounting MUST be done as shown above. This mounting hardware is provided with each bias board. The screw can come through from the other side and the lock washer and nut moved to the top of the shoulder washer but the Thermo Pad and Shoulder Washer MUST remain in the positions shown above. If you fail to mount the transistor properly, heat may destroy the transistor. NOTE: No thermal grease or heatsink compound is needed if you use the Thermo Pad that I provide you.
The diagram below is the method I recommend for using the bias board. The cut off bias resistor in the amp is chosen so that the tube will be cut off when the keying relay contacts are open. Be sure to install the fuse in series with the bias board and the keying line. The fuse is chosen to have a value that is slightly higher than the maximum current you expect your amplifier to ever draw. An example would be a 1 Amp fuse for an amplifier where you expected your maximum current draw to be 750 mA. This fuse helps to protect the bias circuit and the amp if you exceed the value you set as your maximum current. If there is a malfunction in the bias board due to any unexpected event, the fuse will open and the amp will cut off. The capacitors marked with * should be added to prevent any possibility of RF damage to the bias board. It is very important to add these capacitors.
If you wish to remote the bias adjustment to the front or back panel of your amp, you can do so easily. Simply clip one end of R5, the 100 Ohm 1/4 Watt resistor next to the trimpot R1. Solder the leads to your panel mounted 10K linear pot between the cut end of R5 and the pad it was connected to. Then set trimpot R1 to the MINIMUM bias, least resistance, end and your panel mounted pot will work just fine. I suggest you leave about 100 to 500 ohms resistance in the trimpot which will only effect the low bias end of your adjustment. Having your bias adjustable from the front or rear of the amp is GREAT!
Links to the Bias Board in use:
Note to our friends in the EU:
My Bias Board is built using Kester #44 60/40 solder and is NOT RoHS (Restriction of Hazardous Substances)
compliant. I have no plans to switch to lead free solder in any of my projects.
No longer available from w4zt...Bias Board is available from k4poz.

Outside the continental USA including Mexico and Canada -
The price is US$27.50 plus shipping. Email me at the address below and ask for a quote on shipping to your address.

k4poz accepts PayPal, USPS Money Order or personal check. is preferred.

Make checks and money orders payable to:

Ron Wilson
2905 Dellinger Dr
Matietta, GA 30062

If you have questions you can contact Ron by email:


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