Power tube matching

[bluetweed]

Is it possible to use a single ended amp like a 5F1 and read current draw and that match tubes together that have the same current draw/ seems possible or am I totally off base here?

Also on cathode bias amps, do the tubes really need to be matched?

[John Phillips]

Is it possible to use a single ended amp like a 5F1 and read current draw and that match tubes together that have the same current draw/ seems possible or am I totally off base here?Not really with a cathode-biased amp because the bias is essentially self-adjusting and so it will read about the same even with fairly different tubes. If you very carefully measure the cathode voltage you may be able to tell though.

Also on cathode bias amps, do the tubes really need to be matched?It depends.

On ones that share a common cathode resistor, yes absolutely - more than on other types of amp, since a 'hot' tube will raise the bias voltage for all of them, and make the others conduct even less, so it will 'hog' the current disproportionately.

But if there are separate cathode resistors for each tube, the least of any type of amp since each tube will now bias itself independently.

[Blue Strat]

They should be reasonable matched for cathode biased amps.

A single ended amp will get you in the ballpark. The problem is that these amps "auto bias" to some degree, making tubes with different current draws seem closer than they really are. It's actually more accurate to use a fixed bias (the type that CAN be biased) amp.