Hello all, New to the forum and have some tube questions

[jy]

Maybe a stupid question, but here goes:

I've recently acquired a pair of NOS GE 6L6GC's that have identical manufacturing plant codes, but date codes that are 7 years apart (60 and 67).
The getters are slightly different, as are the colors of the logos (the older one is red the newer is white).

The kicker is, that they bias around 1mA apart in my 1968 Vibrolux Reverb, and sound great.

Should I just roll with them? I've never mixed tubes before, anything I should be concerned about?

-Jim

[xtian]

Welcome, Jim.

No issues I can think of. Many folks have been successful running differently biased power tubes, like 10mA different or more! I understand it can increase harmonic distortion. But it can also increase hum.

[Phil_S]

Welcome to the forum. Those tubes are no good. Please send them to me for proper disposal.

Lucky you are! NOS and bias within 1mA.

[Structo]

No kidding, with just 1 ma difference you are indeed lucky with that pair.

Even when I buy a set from a place that burns them in and matches them, there is still usually a difference up to 4 ma.

On my D amps I build a dual bias system so I can set each side of the push pull as close as I can.

[jy]

Well cool then, I'll put them through the paces this week

Thanks!

[cbass]

To be totally honest I can't hear
the difference between a perfectly matched set and one that's 5 or 6 mA's apart

[Structo]

True and some say that some mismatch is desirable in the tone department.

And if you only bias them up to 60-70% of max, then if a tube is a few ma's higher it shouldn't hurt a thing.

[Cliff Schecht]

True and some say that some mismatch is desirable in the tone department.

And if you only bias them up to 60-70% of max, then if a tube is a few ma's higher it shouldn't hurt a thing.

This. I don't just ignore mismatch, I prefer it. This forces the output stage to amplify (and distort) asymmetrically and incurs additional even harmonics (which will sweeten things up). Too much mismatch and you lose the ability for the push-pull circuit to cancel out common-mode noise, but that's where your experience and a good ear come into play .

[pdf64]

Bear in mind that 2 tubes having a closely similar static plate current does not necessarily equate (or perhaps even make particularly likely) that their plate currents will closely track each other as Vg1-k tends towards 0V, eg even with everything else 'well balanced' it won't guarantee symmetry at high signal levels.
Given a shared bias arrangement, it's probably a good start though!
Pete

[Cliff Schecht]

Great point Pete! Tube matching is a modern phenomenon brought out by the inadequacies of Russian and Chinese manufacturers. People didn't match tubes back in the day unless circuits specifically called for it. The crude circuits we are plugging tubes into are designed to literally be plug and play with whatever you have laying around. This is also why a lot of companies bias amps very cold from the factory, gives the user a margin of safety when they start shoving whatever they have in the sockets.