Can you "match tubes" by checking the Cathode curr

[Macshaft]

Can you roughly match tubes by testing the Cathode current?? or is it even worth the bother??

Cam

[schorsch]

Yes you can. Insert a measure-resistor with 1 Ohm/1% between cathode and ground (at fixed bias - stages) or between cathode and common cathode-resistor (at selfbias - stages). Then measure with a voltmeter across the measure-resistor. The ,measured value in mV corresponds to the current in mA.

For example:
measured value: 30mV -> cathode current: 30mA

In the same evironment matched tubes shoud have nearly the same cathode current.

George

[mlp-mx6]

There are several parameters to match on. That technique matches for one of those.

[Macshaft]

What else , if anything can be checked without a tube tester

Thanx
Cam

[Alexo]

What else , if anything can be checked without a tube tester

Thanx
Cam

Height.

[Firestorm]

For a guitar amp, current is the biggie (unless the transconductance is WAY off). Actually, some people suggest using tubes matched for current but UNMATCHED for transconductance as a way to increase harmonic complexity. YMMV.

When you measure cathode current, you are seeing plate current and screen current so you may need to adjust your target slightly, smaller tubes like a 6V6 won't matter, but 6550s, KT88s, etc. may have enough screen current at idle to throw you off.

I've got a Hickok 539C that lets me test all kinds of stuff, but for output tubes, the plate voltage is way too low. I think the AVOs can run the plates higher. Maybe you should get one if you're really into it.

[Phil_S]

What else , if anything can be checked without a tube tester

In a variable fixed bias amp, you can take a series of readings at different grid voltages. This will allow you to compute transconductance. Two tubes with the same t/c are likely to continue to function as matched as operating conditions change while using the amp. Cathode bias won't provide this info.

The thing about a good tube testing rig is that makes it different from an amp is that you can adjust for both plate voltage and grid voltage. This allows you to keep plate voltage constant. In an amp, each time you change the bias voltage, the plate voltage will change.