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Monday, May 03, 2010

DIY Bias Probe

Like many of you I don't bias enough amps to buy an expensive bias probe. All I need is a breakout box to use a multimeter on. "So," I think, "best I make one".



Now the first thing to do is get together the necessary parts:

Octal tube socket
Octal plug
1 ohm resistor (>= 1/2W) 1% tolerance is best, we want to be accurate now don't we?
wire
enclosure
banana plug binding turrets

Most bias probes go between the valve and the socket like a spacer but fabricating that is a pita. There is an easier way. Also, if you don't have a lot of room between the top of the valve and the amp case you need to remove the chassis to use the probe. On most amps you need to remove the chassis anyway but some amps have adjuster pots accessible from outside the case. Even if you don't have external adjusters with this arrangement you can test how close the biasing is without removing the chassis even on those tight fits.
Octal sockets are easy to obtain but where to get an Octal plug? A lot of people use the base of an old valve but I managed to find the plugs from an old Leak/Radford amp on ebay.

Solder wires to each pin of the octal plug (if you like you can omit pin 6) and an extra wire with a alligator clip on the plug end. I have used shrink wrap to keep it all neat. Electrician's tape would do fine.
Mount banana plug turrets (being careful not to short it out to the case)

Mount the Octal tube socket in the enclosure and connect all the wires to it except pin 8. Pin 1 to 1, 2 to 2, etc.

Pin 8 needs to have the 1 ohm resistor in line with it so take the wire from that pin straight to the negative Plate Current Turret. (You can also place the resistor in the anode circuit (pin 3), either is acceptable.)

Connect the 1 ohm resistor between the two turrets for plate current and a wire from the positive Plate Current Turret to Pin 8 of the octal socket. 
Connect pin 3 to the positive Plate Voltage Turret and the extra wire (the one with the alligator clip on the plug end of the lead) to the negative Plate Voltage turret.




Close up the enclosure and you are ready to use it. Just plug the lead into the valve socket, the valve into the socket on your probe enclosure and the alligator clip to earth on the amp.
Now with a multimeter you can measure the plate voltage directly on the Plate Voltage turrets - this will be in the order of 400V so treat it with due respect
By measuring the voltage on the plate current turrets will tell you the plate current. 1 mV = 1 mA


Job done.

PS: You might want to mak you lead a bit longer than mine. 12" should be sufficient but you can go longer

Schematic:


IMPORTANT SAFETY PRECAUTIONS
Be careful that you don't short out any wires or tags to the enclosure and always be aware that some of those the banana plug turrets can kill you if you touch them while it's working. Wearing gloves is not a bad idea. I take no responsibilty for your carelessness. Only do this if you are comfortable working with things holding high voltages and know how to solder.

6:56 am - 3 comments - 6 Kudos