Bias Master Probe Question

[patlaw]

Does anyone know what's inside a Bias Master probe? I thought it would be a 1Ω resistor in series with pin 8, but the resistance across the leads to the meter are 1.28Ω each. I don't have the meter -- just the probes. The meter can be calibrated for whatever it needs to be calibrated.

https://www.tubeampdoctor.com/en/bia...pair-for-tubes

[Enzo]

When you measure that resistance, before taking the reading, touch your meter probes together and measure that. That is the resistance of your meter probes. That amount then must be subtracted from the reading of the Bias Master resistance. In other words, it your meter probes have a resistance of 0.28 ohms themselves, when you try to read a 1 ohm resistor, it will appear to be 1.28 ohms.

Now if the Bias Master probe has its own wires, you can still do this, but make your reading right on pins 8 of the adaptor, not out the wires.

The readings the adaptor makes are across the resistor - which is the current path of the tube. Additional resistance in the wire leads won't affect this since they are not in the current path.

[patlaw]

When you measure that resistance, before taking the reading, touch your meter probes together and measure that. That is the resistance of your meter probes. That amount then must be subtracted from the reading of the Bias Master resistance. In other words, it your meter probes have a resistance of 0.28 ohms themselves, when you try to read a 1 ohm resistor, it will appear to be 1.28 ohms.

Now if the Bias Master probe has its own wires, you can still do this, but make your reading right on pins 8 of the adaptor, not out the wires.

The readings the adaptor makes are across the resistor - which is the current path of the tube. Additional resistance in the wire leads won't affect this since they are not in the current path.

Enzo, I'm totally on board with everything you're saying. FYI, I thought I made the measurement as you suggested, but I just did it again. Measured at the socket, the resistance is 1.006Ω, so we're good to go. By the way, I'm using the meter below, properly zeroed. I've been repairing electronics since 1970, and I just recent acquired my first milliohm meter. It's surprising how useful this thing is. Thanks, Enzo. I appreciate you straightening me out on this.

[Helmholtz]

^^That.

I also use the TAD Bias Master with my DMM.
Resistance measured at probe leads is similar to your results..
But as Enzo explained, only the value of the built-in resistor matters. It is actually a 4-wire arrangement.

The resistor itself can be measured between the adapter's cathode contact and associated adapter pin.
I found it to be very precise.

[tedmich]

When you measure that resistance, before taking the reading, touch your meter probes together and measure that. That is the resistance of your meter probes. That amount then must be subtracted from the reading of the Bias Master resistance. In other words, it your meter probes have a resistance of 0.28 ohms themselves, when you try to read a 1 ohm resistor, it will appear to be 1.28 ohms.

Now if the Bias Master probe has its own wires, you can still do this, but make your reading right on pins 8 of the adaptor, not out the wires.

The readings the adaptor makes are across the resistor - which is the current path of the tube. Additional resistance in the wire leads won't affect this since they are not in the current path.

Enzo's last post I believe, remembering him fondly in 2024