Help with persistent squeal

[Moose]

OK, I'm encountering something that's got me puzzled. Any advice or tips are appreciated.

I've got a 36watter (TMB) that sounds terrific as an 18watter, but squeals and oscillates like mad as a 36watter.

In other words, if I pull two power tubes (doesn't matter if it's the inner or outer pair, any two work) the thing sounds perfect. If I put the second pair back in, it's noisy as all damnation. Squeals like mad and the power supply shows so much noise on my scope it looks like it's hardly even filtered.

With two tubes in, or just the preamp tubes in, the power supply scopes perfectly with no unexpected noises or other such silliness.

Another thing to note, when I first powered up I was getting almost no voltage drop across the cathode resistor on one side (I had one for each pair) and normal across the other. I don't know if this is related. I've gone to the extent of putting in all new tube sockets, and I've checked the wiring a gazillion times, just to be sure.

So, any ideas? Anything at all? Even if it's a simple dumb question, someone throw me a bone so I can hopefully trigger some NEW insight.

[2BDH]

Reverse your plate wires?

Brent

[Moose]

Tried it -- even though it doesn't matter. The circuit has no negative feedback and the PI is exactly symmetrical at the moment.

Alas, it didn't work.

[2BDH]

that's a wierd one . Seems it has something to do with an increased load on the output transformer. Good luck, hope you figure it out.

Brent

[Plexi]

If this is a 36 watter..,the wires that jumper the tubes..cathodes and plates ,screens,....these need to be against the chassis or seperated away from the grid wireing or resistors.... if these jumper wires are sticking up in the air off the tubes,try pushing the wires along the chassis.. It can/will make a difference here..

Richie

[Moose]

Interesting -- put in new screen Rs (one for each pair instead of one shared) and it solved a bit of the problem. A little grid lead hygene cleaned up the results.

Very strange symptoms, but I should have expected something like this from cramming this poweramp into a box that should have been a tad longer.

[Plexi]

This is the one thing on the 36 watter that needs to be watched out for..again..lead dress and layout etc,can make or break an amp..

Richie

[Moose]

well, that's true of any amp, actually. In this case, the lead dress WAS good -- but it wasn't right. If that makes any sense.

Oh well. It's my first 4xEL84 amp, so it was something of a learning experience. For this wattage range, I've done something like an Express, so I KNOW lead dress. But that's obviously a two octal poweramp.

I'd have happily done another trainwreck style, or a 2x6L6 poweramp for this guy, but he was pissing his pants over my purple amp, wouldn't even play the others first, and wanted the EL84s in a "100 watt head" version so he could drive a 4x12 on stage OK. I convinced him that I could "Double the wattage of the combo while still sounding similar" and didn't bother to tell him I was only going from 18 to 36 watts. I broke the wattage news to him slowly, as we discussed the deal further, beginning with the old adage that "A 10 watt amp is half as loud as a 100 watt" and by not correcting his comments about "tube watts being more than regular watts." Since 95% of his gigs are mic'd, anyway, he should do just fine with less than 50Watts.

All that said, this thing is damned loud. Just what the doctor ordered -- my purple amp with more headroom. And pink trim. OK, maybe just what the naughty nurse ordered. Heh.

[Plexi]

""""All that said, this thing is damned loud""""

HAHAHA thats what i keep telling people. I think with a 4x12,it keeps up well with any of the 50 watters. But still retains great tone,and maybe even picks up some along the way..

Richie