Question about matching tubes - pairs versus quads

[greiswig]

I have a cathode-biased amp with 4 power tubes. I tried doing some "mix and match" with some tubes I have, reasoning that since each power tube has its own cathode resistor, they should find their own operating point. So, for example, one matched pair of JJ 6V6's (one per side of the P/P pair), and one matched pair of EH 6V6's (again, one per side of the P/P pair).

The amp doesn't sound as good as having all 4 tubes matched. Why would that be the case?

[Firestorm]

Maybe the tubes weren't well-matched for current. How close are the cathode voltages at idle and with signal?

[greiswig]

I don't know how closely matched they are. This is the basis for my question, though...

On one side of a Push/Pull cathode-biased amp, do *those* tubes have to be matched relative to one another? Or is it just that the pairs on either side of the P/P set have to be mirrored?

[Phil_S]

You can still place a 1 ohm resistor between Rk and ground to find out the current. You should probably do that to see what's what. Yes, in cathode bias, each tube is finding its own bias point, but they may vary enough to make a difference.

I think it is reasonable to look at the average of each pair (left side vs. right side, not inner/outer) as the pair makes a unit. If they are far apart, it may not be so good.

[David Root]

You can stand a bit of mismatch both ways I think, but much more than 5mA cathode current difference on one side, push or pull pair, does not sound great IMHO.

Similarly for each push-pull pair, inside or outside. I don't know that fixed or cathode bias would make much difference, per Phil's remarks about separate pair cathode bias.

Now if you had all four tubes on the same cathode bias resistor & bypass cap, that would force the cathode currents closer I think.

I only have one amp where the tube pair is 5mA apart and that is a pair of used '50s 6V6GTs in my '62 Deluxe, which is also fixed bias. Sounds great!

[ampman]

"Matching" often comes along as a hard determined fact.

IMHO it is not.

There may be a technical aspect that may call for a specific matching procedure.
There may be a description of "how to meet the specs".

Since guitar amps are about sound it doesn't make too much of a sense
to meet the specs if it doesn't sound right to your ears ...

My suggestion is to stay within the technical limits of the components
or the system and choose operation points according to your personal
desires and preferences ...

Good luck!

[thorens]

Hi everyone!
The bias is one point the matching of tubes is another. With bias adjustment you're looking for the correct working point. Matching means comparing the whole amplifying curves and choosing tubes with similar values (how much is the amplifying factor at Imput 1 Volt, 2 Volts ...)
Tom

[Firestorm]

Now if you had all four tubes on the same cathode bias resistor & bypass cap, that would force the cathode currents closer I think.

You could also try going to two cathode resistors: one for the outboard pair and one for the inboard pair. The feedback this produces would linearize the behavior of each pair.

[Cliff Schecht]

oops