Tube matching question

[Plexihead]

Is it ok to go with just the "mmhos" matching of power tubes or would "mA" matching be a more important figure. I'm thinking about buying a few NOS tubes but it appears the match is via mmhos and not mA.

[John Ward]

I'm no expert, but the person matching the tubes is more important than the particular units. mmhos is a measure of transconductance (mA/V) -- see definition from OTM below. mA is a measure of plate current at idle and will depend on the plate voltage and bias voltage at the grid. It is best to match them at close to the plate voltage you are going to use.

[Old Tele man]

...FWIW they're basically referring to two different events:

1) transconductance (gm, or micromhos, in "amps-per-volt") is an AC-value, ie: how much plate current change will be produced by a changing input grid voltage.

2) "mA" is a DC-value, typically the plate current resulting from a given static DC-bias voltage for a given plate voltage (typically about 250Vp).

...hence, the "mmhos"-value indicates how "close" the tubes will respond to AC-signals (ie: maximum power) while the "mA"-value indicates how "close" they'll respond to DC-signals (ie: at "idle").

[Plexihead]

Great, thanks for the info guys.

So I guess it would be ok to go ahead and order a few without having the mA measured, 'cause I'm not sure what the plate voltage is of the amp, it is self-biasing though.

[Blue Strat]

Idle current (mA) is most important for guitar amps. Idle current AND transconductance (mmhos) is a bonus.

You can wind up with VERY mismatched tubes if they're just matched for mmhos.

[Plexihead]

Thanks Mike, I won't get them if I can't get some sort of mA rating then.