Recommendations for EL34 for 520V on the plate

[jrfrond]

JJ E34L is probably your best bet of modern production.

I've used these in Laney Super Groups, 600V plate and screen, with 2.2k screen grids and they seem to survive OK.

Be careful of the nomenclature here. While I DO agree that JJ's are the hardiest of new production EL34's, there IS a difference between the JJ EL34 and E34L. The E34L is meant for use in place of 6L6's, without having to mod the bias supply (though you still have to lift one side of the grid resistor on Fender amps, where they use Pin 8 as a tie point). The end result will be VERY cold biasing.

Cold biasing..... this brings me back to the original post: Even with JJ El34's, you want to keep the SG current down and bias slightly on the cold side. I recommend that you increase the SG resistors a bit more to 2.2K/5W. For proper cold-biasing, you really need a scope, resistive and signal generator to employ the "crossover" method, and you need a CLEAN signal. In some amps, like the Marshall DSL, etc., you'll need to go in through the effects loop to bypass the preamp, which will NEVER let you achieve a clean sine wave output. With the amp driven JUST to clipping, you need to adjust the bias until you see an ever-so-slight hump in the sine wave at the zero-crossing line. That is a "crossover" notch, and occurs when the idle current of the tube is turned down sufficiently where it is not conducting over the full 180 degrees of the sine wave half. JUST eliminating this allows full conduction angle of both sine wave halves, operating the amp closer to the "B" side of "AB" operation. The slight distortion component that occurs actually cancels in the reactive load of the output transformer. IF you were to measure operating current at this point, it would be BELOW spec, but ThAT is where you want it. This is where you can greatly extend tube life, and because of this, I've kept failure rates to a minimum.

[pdf64]

Do you worry about / remove the global negative feedback when using the scope method (not that there is much global NFB on these amps to start with)?
Pete