New build squealing

[TarheelTechinTraining]

I recently finished a JCM 800ish build and now its squealing. I thought the tubes had gone bad. So I put in a pair of Svetlanas. Still doing the same squeal. Any ideas? Thanks, Greg.

[Chuck H]

The most obvious cause would be the NFB loop phase is incorrect. You can check this by disabling the NFB loop. Just disconnect it or use an alligator clip lead to ground the PI end of the series resistor. If the squeal stops, reverse the OT primary leads on your power tubes and re enable the NFB loop.

[TarheelTechinTraining]

OK I disabled the NFB, no squealing. Should the NFB be 100k or 47k?

[Chuck H]

Most of the 800 schems show a 100k from the 4 ohm secondary for both the 100W and 50W models. This doesn't seem right to me because this means the 100W is employing a lot more NFB. There may be a difference in the actual amps. Maybe not. I've only been inside one 800 and I don't remember what was actually in there, but, I've played through many 800's and as I recall the 50W'ers do sound more hashy and have less presence range. So perhaps this is the case. Since your amp is "800ish" you can fudge all you like. Put a 250k pot there wired as a variable resistor and twiddle it while you play. Be sure to check both clean and dirty tones. Go with what sounds best.

[TarheelTechinTraining]

I had 120k for the NFB. And I agree that the presence doesn't have much range. So less NFB makes it cleaner?

[Chuck H]

A higher value resistor results in less NFB. Less NFB will limit the range on the presence control and (typically) result in more signal distortion. A popular circuit is a variable NFB loop. I usually find that with a variable NFB level there's a setting that seems to sound best over a range of tones and I never touch the thing after that. So it becomes another silly mod knob on the front of an amp. YMMV.

If 120k sounds good, it IS good. If you'd like to see what other values sound like, go ahead. No harm in it. Along the lines of my experience above, WRT guitar amps I usually find that there's a "sweet spot" for the NFB level that depends on the overall circuit (OT, plate voltage, speakers, etc.). Probably different for me than for you because tone is subjective. But it's worth a little experimenting IMO.

[TarheelTechinTraining]

Something to drive the wife crazy with, goody. So a 250k pot , linear?

[TarheelTechinTraining]

So I find the "sweet spot" , read the ohmage and replace with a resistor?

[The Dude]

So I find the "sweet spot" , read the ohmage and replace with a resistor?

You can, yes. There are those who actually use a potentiometer for NFB so that it can remain variable. Another option.

[Chuck H]

And pot taper doesn't matter. All the same resistances are there with either taper. If you think you may leave it variable a linear taper might be the most intuitive feeling.

[TarheelTechinTraining]

Thanks fellas. Quick noon question about bias. mV x resistor= mA? B+ x mA= wattage

[Jazz P Bass]

OK I disabled the NFB, no squealing. Should the NFB be 100k or 47k?

You missed the next step. "If the squeal stops, reverse the OT primary leads on your power tubes and re enable the NFB loop.".

[loudthud]

Thanks fellas. Quick noon question about bias. mV x resistor= mA? B+ x mA= wattage

That should be mV / resistor = mA

[TarheelTechinTraining]

Thanks for that. The cathode resistor I have is 1.1 ohm so much for precision.

[Chuck H]

Thanks for that. The cathode resistor I have is 1.1 ohm so much for precision.

Check your meter null. Touch the probes together and see what the default reading is. Then subtract that from your resistor measurement.