Crossover Distortion/ Bias Excursion

[wpaulvogel]

Anyone had any luck with a transparent way to get rid of the crossover notch when these amps really get singing? Or is this something that really just needs to be left alone. I have used clamping diodes and eliminated the notch before but it put a unpleasant fizz on the notes. I couldn’t see it in the scope but it was there when I play. Removing the diodes got the tone back.

[DimenBrun]

What voltage did you clamp at? I have done this on a marshall type circuit with some success. The sound was slightly smoother with the zener clamping.

[frankdrebin]

Yes,you can totally eliminate crossover distortion with the appropriate zener voltage,but I think this is a trick more suited to el84 amps,plus, whenever you change tubes,if they don't bias them at the same negative voltage your zener clamping can be off.

[rooster]

One thing I do after having looked at the Express under load is to always put the colder EL34 in the V5 spot. At higher volumes the V5 side of the PI gets hammered more so than the V4 side. I believe this is responsible for some of the crossover distortion seen/heard in the amp. Typically a matched pair of power tubes might be 3mA out, right? Well, I say take advantage of the slight misbalance if you hear/see a difference.

Also, since this is a typical Marshall Plexi PI setup, I have heard that back in the day ppl used to swap V4 and V5 and then notice there was quite an improvement with a certain placement. And since biasing was kind of a crapshoot thing in the early days, you have to wonder how far the pair might have been out of bias, right? I suggest that this 'improved' result happened when the tubes were positioned as I suggest. I've also heard it said that 10-20mA difference was not uncommon - and - the amps were even reported to sound amazing using this large of a spread.

It's hard to appreciate but in the early days when Kenny started building his Express amp, Groove Tubes wasn't around. It was just starting, the whole power tube 'balance thing'. Is this because the tubes of the day were built to much better tolerances? Maybe, but FWIW I have never read where Kenny was actually concerned about this particular aspect of things, that instead he used his ear to adjust the bias of his amps. And when you think of amp biasing today (and tube matching), as a means to better TONE, we will never know what the mA current draw of the power tube pair in Clapton's BluesBreaker combo, or Peter Green's Dual Showman 6L6 quad at the Boston Tea Party gig, or Mick Taylor's 50 watt Marshall with John Mayall, or David Gilmore's 100 watt HiWatt, or so many others. Just thinking out loud...