Twin Reverb output tube balancing

[Randall]

I have a 65 Twin Reverb in for a recap and tube check. It came with two Sovtek 6L6GBs in the inside position and two Sovtek 5881WGC in the outside position. Upon measuring idle current at 471Vp, I range from 18mA to 27mA. If I put the highest and lowest drawing tubes on one side and the middle ranging tubes on the other side, I get 45mA and 48mA for each pair, close enough of a match for me. But this results in the two 6L6s (22mA and 26mA) being on the same side, and the two 5881s (27mA and 18mA) on the other. Is there any harm in this, or should I pitch my customer a new quad?

[nevetslab]

While I've never been a fan of mixing two different type tubes together like that, or even same type but an equal share of a different mfgr's tube in a set (pair of Sovtek, pair of Svetlana 6L6GC's for example)....this type of mixing seems better. But, what I've never investigated is JUST HOW the different tubes as in your set of 5881's on the one side, share current under drive conditions. At idle, one obviously does more work. Does that trend continue under drive. One of those questions I've yet to stop and look at, myself.

Is this just us being finiky and not thinking about these just being guitar amps, where you can actually get away with gross mismatches? Enzo?

[TimmyP1955]

While I've never been a fan of mixing two different type tubes together like that, or even same type but an equal share of a different mfgr's tube in a set (pair of Sovtek, pair of Svetlana 6L6GC's for example)....this type of mixing seems better. But, what I've never investigated is JUST HOW the different tubes as in your set of 5881's on the one side, share current under drive conditions. At idle, one obviously does more work. Does that trend continue under drive. One of those questions I've yet to stop and look at, myself.

Is this just us being finiky and not thinking about these just being guitar amps, where you can actually get away with gross mismatches? Enzo?

I usually set 6L6s in Twins to about 32ma per tube. Fender tends to set them fair bit colder, but I don't recall what their norm is. So long as it sounds good with your mismatch, I'd say there's nothing to worry about. That being said, 18ma is pretty low, so maybe bump the bias current up a bit? Oh yea, that amp just has balance instead of level. Back to 'if it sounds good it is good'.

[Randall]

No, this amp is blackface, so it has a bias level control, no balance. I will bump it up once I get a concensus on the original question.

[Reader]

- - -So long as it sounds good with your mismatch, I'd say there's nothing to worry about.- - -

I agree there is nothing serious to worry about. The "balance" you achieved will keep the amp from excessive humming in idle condition. The sound check will let your (or customer's) ears decide if the amp is good to play. This is assuming that the customer just doesn't want to buy all new tubes as a precaution. I'd guess probably not since the customer had the mismatched set in there already.

[g1]

If he was running 2 different tube types for pairs, I doubt he cares much about matching.
Even if they are matched at idle, they will not track very 'matchedly'.
In any case, I think at most you would only have to replace 1 pair, isn't one pair still matched?

[Enzo]

HArm? When the amp was made, they didn't install matched tubes.

And remember, you are only matching idle current, you are not matching tube gain or other dynamic characteristics.

In other words, this really is not a problem.

[pdf64]

I wouldn't worry about it.
Electrons aren't affected the printing on the glass, what matters to them are the tube characteristics. As 5881 and 6L6 ostensibly have the same/closely similar characteristics, and given normal tube variance from bogey, there's probably a 50% likelihood that the tube idle currents would match in the way you describe.
I suggest to check that the waveform of high power / near clipping signal is ok, and that the power output is reasonable; if so then it's good to go.

...Even if they are matched at idle, they will not track very 'matchedly'. ...

I don't think that's necessarily the case, they may all track together sufficiently well for the application, the proof of the pudding is in the eating

[Pedro Vecino]

Today 5881 nomenclature and GB suffix mean little or nothing. Being Sovtek they could even be the same.

[Justin Thomas]

If they look like THAT on the inside, then I'd treat them like the Sovtek 5881WXT on the left, and run the crap out of them. Those are known for being dang near indestructible. If they DON'T look like those on the inside, all bets are off.

Justin

[Reader]

Another thing to keep in mind is that some amp makers use different tubes in the output set on purpose. Then they sell it as a feature and claim they have invented a unique circuit.

Just another bit of input to support that it's OK to use the output tube set currently in the amp.

[Randall]

One set is coin base in a smaller glass, the other is normal base, but the internals look quite similar. I have run it by the owner, and he chooses to leave it be as is. Thanks for the input, as always!