Stuck at power section

[Russ]

The voltages were relative to ground, but measured relative to the junction of the 470R cathode resistor and the 1m grid resistors I get -.015v at the input grid and .022v at the other grid. And when I test voltages, I get a hum through speakers at input grid, but not at the other grid. This was measured w/ NFB disconnected.

Yes, I've got a .01 PI input cap, and a 10k tail resistor

Why would these voltage be so small and even negative on the one side?

[DirtyGeorge]

Measuring the grids WRT the 470R / 1M junction should be 0v. You are just measuring across the 1M grid resistors and there should not be any current flowing.
What do you measure across the 470R? That is your bias voltage and should be 1v to 2v positive at the cathode WRT the 470R / 1M junction .
Did you measure the 470R to confirm correct value?
Did you try another PI tube?

DG

[Russ]

I get 1.23v across the 470R cathode resistor (which checked out to 470R).
Tried 2 different PI tubes (12ax7), & a different PI input cap, same problem.
The speaker's hooked up to 8ohm tap. NFB was hooked to this same tap.

When I rehook up NFB the 'splat' distortion sound gets worse with a kind of ringing sound to it, the presense doesn't work, and the level doesn't reduce.

Could the problem be in the output section?
My screen current might be off - I measured .35v across each 1k screen resistor which, I think, is 35ma current. This seems wrong. My bias is 32 ma/tube

[DirtyGeorge]

The voltage across the 470R looks OK. If you measure the cathodes and grids WRT ground you should see this 1.23v difference. So if you have 39v at the cathodes then the grids should be 37.77. The measurements in your earlier post showed a much larger difference than that. Is this still the case?

Also I think you should switch the OT primary wires because it sounds like what little feedback you have is positive.

DG

[Russ]

WRT ground, the base of the cathode res. is 28.0v and the cathodes are at 29.3v so the diff. is 1.3v - ok. The grids measure 20.2v WRT ground, but about 0.0v WRT the base of the cathode res. - shouldn't the grids be about 28v, the same as base of cath. res.?

Did the screen valtage/current seem right to you?

I tried reversing the OT prim leads and the ringing part of the 'splat' distortion sound seems to have gone away, but everything else stayed the same. I've had the OT prim leads reversed on other builds, where I got a big howl through the speakers - this wasn't like that at all, so I don't think the OT lead reversal is the prob.

[DirtyGeorge]

The screen current would be 0.35mA. I would expect a couple of mA but I don't think the low reading is a problem.

I can't explain how the grid reading can be different from the reading at the base of the cathode resistor since the reading between those points is 0v. Have you checked for Vac readings with the circuit idle? Maybe it's an oscillation problem.

Otherwise I am out of ideas.

DG

[Steve Conner]

I'm starting to think you've made a wiring error that's making your EL34s bias very cold. This might explain the splatting, it could be extreme crossover distortion.

The base of the cathode resistor should be grounded, so at 0V wrt. ground. The grids should also measure 0V since they're grounded through their grid leak resistors. The cathodes should measure +30 or so, ie, 30ish volts dropped across the cathode resistor, which should be getting hot, and the tubes should be egg frying hot too. You mentioned only 1.3V across your cathode resistor, which implies that the tubes are hardly turned on at all.

[Russ]

OK!
Steve you pegged it with the cold bias problem. A thousand Thank Yous!!!!

Wadda knuckle head! Turns out I had 10 ohm instead of 1 ohm for the bias checking resistors on the cathodes of the power tubes. So my .032v reading was only 3.2ma, not 32ma. Adjusted the bias and now I've got .32v and everything sounds right!
I'll stick in 1 ohmers later.
Man I though I was cursed or something.

Many thanks again!!!
Russ

PS
The 1.3v thing was across the PI cathode resistor.

[Bruce / Mission Amps]

Good call Steve.... your the man!