Biasing amps with PCB mounted power tube sockets....

[Joshuatsp]

My normal method of biasing is the cathode resistor method.

This includes installing a 1 Ohm resistor from Pin 8 to ground....

....how does one do this on tube sockets mounted to the PCB?

[defaced]

I use a bias probe. You can find them set up for either the cathode resistor method or the cathode current method. I built my own using a dead tube for the base and bought a socket from the local liquidation store. You can also buy the base and socket from places online.

There are also other ways to bias your tubes, including the scope method and the transformer shunt method, neither of which I'm particularly familiar with.

[Joshuatsp]

I use a bias probe. You can find them set up for either the cathode resistor method or the cathode current method. I built my own using a dead tube for the base and bought a socket from the local liquidation store. You can also buy the base and socket from places online.

There are also other ways to bias your tubes, including the scope method and the transformer shunt method, neither of which I'm particularly familiar with.

Elaborate? How does cathode current differ from the cathode resistor method?

I have a scope (but no signal generator or power supply)....but not sure how to use it yet.

[Steve A.]

I use a bias probe. You can find them set up for either the cathode resistor method or the cathode current method. I built my own using a dead tube for the base and bought a socket from the local liquidation store. You can also buy the base and socket from places online.

There are also other ways to bias your tubes, including the scope method and the transformer shunt method, neither of which I'm particularly familiar with.

For a push-pull amp the transformer shunt method basically runs one side of the output transformer primary through your meter set to the appropriate DC current range (10A works but is not as accurate as the lower ranges). With the high voltages involved you would definitely want to use insulated test clips on the end of your probes. And you do have to study the schematic and layout drawings to determine the proper location for the test clips- with most DMM's the DC current range is a dead short across the two probes so you can create a lot of damage to you or the amp if you are not careful. DISCLAIMER: this is a very general description off the top of my head so anyone who wishes to pursue it any further really needs to research the topic in much greater depth.

A bias probe makes a lot more sense when you can't add the 1 ohm resistor to the cathode circuit, but the transformer shunt method could be used for output tubes with pinouts different from the typical octal and nine pin tubes.

FWIW I did find the transformer shunt method to be handy in my Peavey Classic 50 once I figured out where to put the probes (since you can remove the back for access to the PCB when the amp is powered up- unlike the Classic 30).

Is WeberVST still selling the bias probes? I thought that his pricing was reasonable because he offered you the option of using your own DMM as I recall...

Steve Ahola

[hasserl]

Simplest and most accurate method (for a two tube, push-pull type amp):

1. Measure the resistance between OT center tap and each leg to the tube plates, write it down.
2. Set your volt meter to measure dc volts at a low range of just a few volts. connect the black lead to the OT center tap with some type of clip-on probe, turn the amp on and let it warm up, take it off standby, touch the red lead probe to each of the tube plates and write down the voltage.
3. Divide the voltage you recorded in step 2 by the resistance you measured in step 1, voilla! You've got the current flow thru the tubes.
4. Measure the voltage at the plates (wrt ground).
5. Now do the math to determine whether the quiscient current flow is correct or needs adjusting, and adjust as needed. Remember to recheck the voltage at the plates as it can/will change as you change the bias point of the tubes.

[Enzo]

The cathode resistor method measures VOLTAGE across the resistor to infer what the current through it must be by Ohm's Law.

The cathode current method inserts a current meter in between cathode and ground and measures CURRENT directly. Not a common approach except where tube fuses exist.

The bias probe is a handy little thing. It has that 1 ohm resistor right inside, and is very easy to use. PLus in many cases, you can check bias current without demounting the chassis.

[defaced]

I have a scope (but no signal generator or power supply)....but not sure how to use it yet.

If you've got a computer, you can use it as a signal generator. Get a file of a sine wave and play it thought your sound card plugged into the amp's input. This is what I do because I don't have a signal generator. If you need a sound file of a sine wave, let me know.

Eurotubes is a registered Webber dealer, so you can get the probes through them. The probe alone is 25 bucks.

[Chuck H]

The last time I had to check bias on an amp with pc mount sockets I was abroad with the bare min of equipement. I did have a DMM and a 1 ohm resistor. So I just scratched through the trace for one of the power tube cathodes and bridged the 1 ohm resistor between the nearest two solder joints. Once I was done I repaired the board by soldering a jumper in where the resistor was. For the record, this was not a critical project. But I got the job done.

Chuck

[pila]

I've done it like Chuck H did it, only I didn't remove the resistor. The board trace needs to be built up a bit with a piece of wire to make the trace survive.