Is this "Crossover Distortion"?

[hasserl]

Take a look at the scope screen shot, the distortion in the center of the wave, is that from crossover distortion? This is a cathode biased amp with a pair of EL84 tubes idling at about 9 watts ea.

note: sorry for the poor resolution of the pic, I used my cell phone and I couldn't avoid the glare from the flash.

Thanks

[Enzo]

I normally expect Xover distortion to look more or less the same going up and coming down. If that is mere crossover, something is seriously different between your push and your pull. That is also way more distorted than I would expect from crossover distortion.

Did you meaure idle individually, or is there a common cathode circuit and you divided by two to get the individual current?

[hasserl]

Common cathode circuit, divided by two.

[J M Fahey]

One of your tubes is dead.
Just to check, exchange positions and see what happens.
I guess you use no feedback, or it should mask that somewhat.

[hasserl]

You're right, no nfb. Dead tube, huh? OK, I'm out of town for a few days, I'll swap them when I get back.

Thanks

[Enzo]

The tube could be at fault or some other thing like an open screen resistor might be the deal.

[Bruce / Mission Amps]

I don't know either but it would seem that with a dead tube the bottom half should have virtually no real sine wave... except from the OT unwinding it's energy.
That thing looks like it is suffering from severe negative vdc, grid sticktion.

[hasserl]

I don't know either but it would seem that with a dead tube the bottom half should have virtually no real sine wave... except from the OT unwinding it's energy.
That thing looks like it is suffering from severe negative vdc, grid sticktion.

I've never heard of such a thing. Can you elaborate? The grid should be a ground potential, not sure how it would go negative. Note that the screen shot is with the Volume control cranked up pretty high, probably ~ 3:00 or so. It starts out with just a small distortion in the wave as the volume is increased it gets more extreme.

[Chuck H]

It looks like one half of each tubes cycle is fine but the second half of tube ones cycle cuts off sharp and the first half of tube two is slow to recover from a cutoff state. It's like each tube is one quarter of a duty cycle.

Is this a new build or did it work properly at some time?

Are there any diodes like the "Paul Ruby zener mod" being used?

Chuck

[J M Fahey]

Hi hasserl.
To avoid shooting so much into the dark, as we are all doing now, please post the schematic of what you built, show cathode, screen (if applicable) and plate voltages at least of power & PI tubes, and screen pictures both the one you posted and what's available at PI plates or Power tube grids.
If your scope can't handle such signals, build a 10:1 and a 100:1 attenuator with a .047x600V capacitor for DC isolation, a 470K resistor in series, a 47K resistor to ground (10:1) across which you connect your scope, and an optional 4K7 resistor that can be switched in parallel with it to get 100:1.
Place alligator clips where needed and be *very* careful because you will be messing with high voltage.
Leave the ground alligator *always* connected, and move the "hot" one where needed.
When taking the picture, cover your flash with your finger or if you are a perfectionist, with a piece of NOS , 1947 vintage, black electrical tape, "the same brand that Leo Fender used".
It will sound so much better.
Remember that in our profession, Mojo is more important than a properly equipped Lab.

[loudthud]

How does the wave change when you change the frequency?

[Bruce / Mission Amps]

I've never heard of such a thing. Can you elaborate? The grid should be at ground potential, not sure how it would go negative. ...

Because it is all relative... and your power tube's cathodes are "biased" positive, which means; with respect to the cathodes, the grids are negative.... even though they are at zero vdc with respect to chassis ground.