Mismatched power tubes

[Drummer4gc]

Hi all,

I just finished working on a Fender Blues Deluxe Reissue (new filter caps, rebiasing to factory spec). I'm sending a 1khz sine wave into the power amp input, and this is what I get on the scope:



One side of the waveform is clipping much sooner than the other. If I flip the power tubes, the top of the wave clips early instead of the bottom.

Is this a case of mismatched tubes? Is this expected in tube amps? Is this considered normal operation, or is there something else I should be looking at?

Thank you!
Matt

[eschertron]

If the early clipping follows the tube, then the tube is at fault. I do not expect this behavior to be considered normal.
What audio power do you get when adjusted right at the point it begins to clip?

[Chuck H]

To expand on this...

You absolutely can expect asymmetry in most tube amps when clipping, but typically only when clipping hard. Hard as in pretty much a square wave. I mention it just so tubes won't be your first suspect when you see it under this circumstance. In this case, particularly since the asymmetry follows the tube, I'd agree with eschertron and call out the tubes. Is it normal? Nope. That's way too much mismatch. Even with "unmatched" tubes you'll never see that much difference. You've got a very weak tube there.

[Drummer4gc]

Thanks folks. Threw in a pair of new 6l6s I had waiting for another project and it looks nice and powerful and symmetrical. Much appreciated!

[alexradium]

Hi all,

I just finished working on a Fender Blues Deluxe Reissue (new filter caps, rebiasing to factory spec). I'm sending a 1khz sine wave into the power amp input, and this is what I get on the scope:

[ATTACH=CONFIG]54230[/ATTACH]

One side of the waveform is clipping much sooner than the other. If I flip the power tubes, the top of the wave clips early instead of the bottom.

Is this a case of mismatched tubes? Is this expected in tube amps? Is this considered normal operation, or is there something else I should be looking at?

Thank you!
Matt

That is the waveform of a single tube,the other is not conducting at all

[SoulFetish]

That is the waveform of a single tube,the other is not conducting at all

^^^^^ exactly right. The audible symptoms are usually very low output volume and heavily distorted. Swap them out with some pulls and see if it fixes it. If so, then one of your tubes is shite.
If it still looks like this, then dollars to donuts that one of the plate resistors in your phase inverter is open.

[g1]

I think he wrapped it up in post #4.
Good info anyway though.