One output transformers for all octall power tubes?

[leadfootdriver]

What effect on one does miss matching transformer impedance have on power tubes? The most common example is running EL34's in an amp designed for 6L6's. Is it just the freq response of the tube that's affected?

There's a few amps out there that have bias pots on every tube socket, which allows you to bias up any octal power tube type. But really how much tone are you missing out on by having one output transformer for all these different tubes?

[Chuck H]

Bias as it relates to an OT primary impedance is something I don't quite have a grip on. But I CAN tell you that all the octal power tubes for guitar amps operate very happily into the very common 6.6k primary impedance sold by most transformer manufacturers. I've actually developed a preference for this pri Z tonally too. If it's because it's what I'm use to now I can't say. People tend to develop a preference for what they're familiar with of course. I've built a few amps that use a 6.6k primary and have often swapped between 6l6 and el34 types. No reason you couldn't also run 6V6's (voltage permitting), kt66's, kt88's, 6550's, etc. at 6.6k. Any not quite ideal loss of efficiency won't be enough to trouble the tubes and you shouldn't lose more than a couple of watts at the worst. It's not as if we're all hung up on making an amp with the most power possible or the broadest/cleanest frequency response at this point. That's really what all the idealizing is all about and it just doesn't apply to most guitar amps. The 6.6k OT has been an industry standard because it IS somewhat universal.

Tonal differences? Not enough to mention. If anything you may notice a little lower dynamics and some added compression on clipped tones (IMHE). Any difference in clean tones is completely insignificant for electric guitar.

Probably more important is to make certain your bias circuit has a universal range and your plate voltage won't threaten more tender octals.

[leadfootdriver]

Great answer, thanks. I read from another amp designer that a or proper OT needs to be chosen for a particular tube to make a fair comparison. But it must not be a huge deal if other makers advertise their amps to be able to use any octal power tube.

[GainFreak]

But I CAN tell you that all the octal power tubes for guitar amps operate very happily into the very common 6.6k primary impedance sold by most transformer manufacturers.

Most 50W guitar OTs operate quite happily into the 3,4-4,2k. 6k6 is not very common but is usable.

[Chuck H]

Most 50W guitar OTs operate quite happily into the 3,4-4,2k. 6k6 is not very common but is usable.

Not very common for guitar amps. Which, in fairness, may well constitute most tube amps today. But for tube specs and NON guitar amp OT's the 6.6k will be more common, In fact I can't even think one 'not guitar' amp designers would consider reasonable that uses a 4k load for a pair of 6l6's. But even for guitar amps concessions are made for tubes like kt66's and 6v6's for a higher primary. And since 6.6k is indeed very common I chose to offer that as a good, universal option.

[Alan0354]

I basically use one output transformer for all my amps. The Classic Tone Bandmaster/Vibrolux/Pro Reverb OT. It's a 40W 4K primary with 4,8 and 16ohm output taps. I just use 4 ohm tap for 6V6 to drive the 8 ohm speaker, 8 ohm tap for 6L6 and EL34. I definitely get good performance from all the tubes mentioned, can't speak for the other tubes. I sure got the power expected too. I lower the +B where using EL34 to keep the power below 40W to stay within the limit of the OT.

My ears might not be the best, I couldn't hear any difference with speaker impedance mismatched. Basically I loss power when mismatch......and it's still debatable whether it's that important.