Un-Matched Tubes = Best Tone?

[Robboman]

I'm no tube guru so maybe you guys can comment on this...

I've been messing with tubes in my Tweed Bassman RI for some time. I have four 'sets' of 6l6s on hand, Sovtek, Mesa, GT and some old GE from the 50s. All have plenty of volume and sound good, (so I assume the tubes are all good). For the best tone overall, the CLEAR winner to my ears were the Sovteks.

Now, i understand that just swapping tubes in and out without re-biasing doesn't tell the whole story, one set may be running 'cold'er than another. Recently (with help) I installed a vintage point-wired board-kit w/bias pot (the stock pcb had no bias adjustment). I also got one of those Bias-Rite testers and learned how to use it.

After MORE tube swapping, bias adjusting and serious listening I came to 2 interesting conclusions:

1). The same Sovtek 6l6 set still sounds the best by far, despite optimized bias on the other sets. Even the GEs.

2). The Sovteks were significantly UN-matched (apart by roughly 9 mA!). All other sets tested close to matched!

Of course, what 'sounds best' is subjective, but I know what I like when I hear it. I won't just automatically install 'matched sets' from now on. I'm gonna mix and match until I like what I hear the most.

BTW, the other three sets were all 'matched' pairs that I bought, the unmatched Sovteks were in the amp when I bought it second hand.

[VacuumVoodoo]

Theoretically, in an ideal push-pull amplifier the even harmonic distortion products from each tube will cancell each other leaving only odd harmonics. This mechanism assumes perfect symmetry of drive signals from the phase inverter, symmetrical an identical primary windings of the output transformer and perfectly matched tubes.

In reality none of these symmetry requirements is satisfied and the even harmonics cancellation mechanism can be severely weakened.

By intentionally mismatching the output tubes you can vary the ratio between even and odd harmonics in the output signal.

This effect is easily measurable and will have a varying effect on the perceived tone quality. When you run the amp close to clipping the difference in tone between different degrees of mismatching will be more pronounced.

How much mismatching you want or tolerate is of course a matter of personal taste.

[KLB]

When one tube is running hot compared to the other, it's input grid is more sensitive and it will tend to breakup sooner, which creates an asymetrical clip more like a single ended amp. As long as the "mismatch" isn't so much that the level of hum is objectionable, the resulting tone can be very nice.

You will tend to get the most output power with tubes matched and biased to the symmetry of the circuit and transformer set, which can be seen using a scope and looking at the waveform as it begins to distort on each side (push-pull).

Because of assymetry possible from causes other than mismatched tubes, try swapping the power tubes in the sockets to see if you like the sound even better.

Some amps have an individual bias control for each tube, which lets you play around with this. You can try different types of tubes together, say an EL34 and 6L6.

- Ken

[Robboman]

Thanks very much for the input guys. With these tubes on my amp, there's next to no hum, and it's the quality of tone right around the point of breakup that I really zero in on when I'm fiddling and listening (this amp never really sounds super-clean to me). I'm not concerned with maximizing output power, as it's plenty loud!