Odd Distortion Noise

[Tom Phillips]

Here's a recording I did of the distortion. It doesn't record very well. You can hear it as the chord trails off. Yes, I'm not the only one that heard it..... http://www.soundclick.com/player/sin...&q=hi&newref=1

I don't hear the chord notes distorting. All I hear is what sounds like low level continuous background noise the becomes more apparent as the amplitude of the chord trails off. Then it abruptly cuts off when the recording ends.
Tom

[g1]

Yeah, sorry to say I couldn't hear it either. Maybe I'll try later with headphones.

[Daver]

Yeah, I know. it didn't record well. There is background noise from the local radio station. It sometimes gets picked up depending on the day, the weather, the moon and me using a long cord to get me near the computer. sigh.......

[nickb]

Here's a recording I did of the distortion. It doesn't record very well. You can hear it as the chord trails off. Yes, I'm not the only one that heard it..... http://www.soundclick.com/player/sin...&q=hi&newref=1

I can hear it clipping a little as each chord is hit but the rest sounds fine. The background noise could be masking it.

I'd ideally like a wave file with the 500Hz sine wave recorded too.

[Daver]

Just because, I wired up the power tubes in faux triode connecting the screens to the plates through 470 ohm resistors. That configuration almost completely eliminates the distortion. OK. Why? Any ideas or theories?

[TimmyP1955]

Just because, I wired up the power tubes in faux triode connecting the screens to the plates through 470 ohm resistors. That configuration almost completely eliminates the distortion. OK. Why? Any ideas or theories?

One the schematic, the screens are at full B+. Adding screen resistors and grid stopper resistors is something I think I'd have done even if the amp was working OK. Anyone think these mods to be a bad idea?

[Daver]

I have already done that. I realize this thread is long......

[Daver]

Well I'm back working on this thing after a break. I tried replacing the power tubes sockets. It didn't remove the distortion, but oddly enough the amp is a bit louder now. Any other thoughts on why faux triode mode seems to get rid of the distortion? Anyone know how to remove a curse?

[cyclone]

You should start swapping out resistors, start with the bias supply

[Daver]

There is no bias supply, it's cathode biased. And, yes, I did replace the cathode resistor. I have swapped out almost all the resistors including the power supple. The distortion is in the power section. I know this is a long thread with lots of questions and answers, but I need more than just general recommendations. Thanks.

[g1]

I think we may have touched on this, but have you tried biasing real hot like class A or 100%?
And are you running old spec 6L6 (23W) or modern 30W 6L6's ?

[Daver]

It's pretty close to class A now. I have tried biasing it colder and it didn't get any worse. It currently has GE 6L6GC's in it. I tried some TAD short bottle 6L6's and some NOS Tung Sol 5881's. The thing is, it worked before with its current set of tubes. Thanks.

[Daver]

FIXED!

After a lot of head scratching and putting this thing aside for a while I finally have it fixed. The answer came in a flash. Literally. While playing it a couple of weeks ago I saw one of the power tubes flash at the bottom. The amp didn't make any unusual sounds when this happened. These tubes had tested fine before. So I powered down and tested them again. Now that tube tested positive for shorts. Hmmmm. I had tried several sets of tubes before and subbed out the OT. But I never did both together! Shouldn't make a difference, right? Well it sure as hell did. A different OT I had laying around and a different pair of 6L6's made it work fine.

My guess is that the primary is shorted to some much lower impedance. Now did the shorting tube wreck it or vice versa? I don't know. Coincidence? I don't know. I knew the problem was in the output section. The owner ordered a reproduction OT from Mercury and an NOS set of GE 6L6GC's and I installed it all last night. No distortion! I'm still not sure why I couldn't see anything definitive on the scope or why it took both the OT and the tubes to cure it, but I am damn happy this thing is working like it should.

I'm sure I chased my tail a bit and wanted several times to throw in the towel, but I didn't want it to beat me. Thanks for reading!

Dave