Swapping out 6L6s

EddieVN

I've got a 60w Engl Fireball with a pair of 6L6s. With one band I play in I have the master volume around 5 and it sounds sweet. Running it through my Mesa 4x12 the low end clarity is amazing. In another band, for drummer-and-bassist-related reasons, I'm running it at around 9 on the master volume. At this volume I'm obviously getting more of the character of the power tubes coming through, and the best way I can describe it is it sounds a bit squelchy - like the nice mid frequencies are less present. Hitting the front end with a tube screamer definitely helps, but it sounds to me like it's the 6L6s running flat out that are giving it a character I can't quite dial out. So I was thinking about swapping the tubes. I don't think you can swap them out for EL34s and I don't like the low end response on those for the kind of music we do anyway, but what about other options? Are 5881s so close to 6L6s it wouldn't make a difference? What about KT66s?

ICBM

6550s will do it. Bigger, bolder, more even-sounding. They should also be almost a drop-in replacement, the bias requirement is very similar and should be within the range of the trimmer.

EddieVN

Excellent, I've just been having a youtube comparison binge and am now thinking exactly the same thing - 6550s

ICBM

Forgot to say - bias them the same as the 6L6s, not up to 70% of the plate dissipation for 6550s. If you have a scope you can do it by looking for the crossover notch.

You may not need to adjust the bias voltage at all - I've fitted them in many Fender Bassman 70s and 135s when they're going to be used for bass, and those only have a bias balance control.

EddieVN

Great, thanks