60hz filament hum from high wall voltage?

[R.G.]

In the USA there is supposed to be a ground wire at the power pole connected to that center tap of the 240V. Each "customer" is supposed to also ground the center tap at the incoming breaker panel. In my city, they caught a guy that was going around to power poles cutting off the safety ground (it's only #10 or #8 AWG) with a hatchet to steal the ground wire and sell it for scrap.

Yeah.
I read about a guy found dead at a power pole on a country road. He had been cutting the ground wires on successive poles, tying the wire to the bumper of his pickup, then pulling off as much of it as he could before the wire broke. Loss of the distributed grounding let leakage build up and his last (!) ground wire was hot with a KV or so.

A real issue in older buildings and especially old, beer-soaked electrical supply in moldy bars is gradual corrosion in the distribution box, where the incoming, usually aluminum, center tap from the pole gets high impedance AND the connection to the building ground rod gets high impedance. The voltage at any one outlet can be almost anything between a few tens of volts and nearly 240.

The only good solution for a band working really funky places is a regulating, isolating constant voltage / ferroresonant transformer.

[CraigGa]

A friend of mine is a CSI and they were called to an incident where a guy tried to cut through a 3 phase cable in an electricity substation with a hacksaw.
Suffice it to say that the cable never made it to his van!

Definitely a win for Darwinism

Craig

[lightfoot]

So I’ve found a Couple of things.

1st- At the time of the original post I wasn’t using any negative feedback on this 5F1-ish type topology

2nd- There are no grid-stoppers anywhere either!

3rd- At a certain point in the volume sweep it would naturally go into oscillation

So I added the stock value NFB resistor value and the noise floor dropped as expected but with the make-do speaker I’m using the amp sounds like there’s a blanket over it so…..

I’m considering taking the NFB resistor out and adding grid-stoppers at each stage and add just enough NFB resistance to stabilize the amp if needed.

Any thoughts?
Thanks

[R.G.]

1st- At the time of the original post I wasn’t using any negative feedback on this 5F1-ish type topology

I checked the thread. Is this your new construction, a modification, or a pre-existing amp?

2nd- There are no grid-stoppers anywhere either!

OK. Grid stoppers are not a panacea. They are a tool for specific cases like grids driven above the cathode voltage or RF response reduction.

3rd- At a certain point in the volume sweep it would naturally go into oscillation

That means that in its existing layout and values, it's conditionally stable. Something in there is letting unexpected feedback happen, and at some gain it sings. This is often something like wire dress or ground layout, but it could be component placement too, especially if it has been hot-rodded for higher gain.

So I added the stock value NFB resistor value and the noise floor dropped as expected but with the make-do speaker I’m using the amp sounds like there’s a blanket over it so…..
I’m considering taking the NFB resistor out and adding grid-stoppers at each stage and add just enough NFB resistance to stabilize the amp if needed.

Don't know what to say here. NFB lowers gain and distortion, and stabilizes operating points. Grid stoppers at each stage won't necessarily cure an oscillation problem, and may make it worse, as they add a single R-C phase shift. With multiple stages of phase shift, an existing gain/oscillation problem, it might add another oscillation problem. Gain phase oscillation is best cured by fixing the underlying phase shifts so that there is one dominant pole, or phase compensation where that's not possible. Three RC-phase shifts are enough to cause oscillation, and a transformer on its own is two of those, so there are already limits on how much NFB can be used just because there is a transformer in the gain path.

If it were mine, I'd try to find and fix the oscillation problem before tossing solutions at it.

[lightfoot]

1st- At the time of the original post I wasn’t using any negative feedback on this 5F1-ish type topology

I checked the thread. Is this your new construction, a modification, or a pre-existing amp?

2nd- There are no grid-stoppers anywhere either!

OK. Grid stoppers are not a panacea. They are a tool for specific cases like grids driven above the cathode voltage or RF response reduction.

3rd- At a certain point in the volume sweep it would naturally go into oscillation

That means that in its existing layout and values, it's conditionally stable. Something in there is letting unexpected feedback happen, and at some gain it sings. This is often something like wire dress or ground layout, but it could be component placement too, especially if it has been hot-rodded for higher gain.

So I added the stock value NFB resistor value and the noise floor dropped as expected but with the make-do speaker I’m using the amp sounds like there’s a blanket over it so…..
I’m considering taking the NFB resistor out and adding grid-stoppers at each stage and add just enough NFB resistance to stabilize the amp if needed.

Don't know what to say here. NFB lowers gain and distortion, and stabilizes operating points. Grid stoppers at each stage won't necessarily cure an oscillation problem, and may make it worse, as they add a single R-C phase shift. With multiple stages of phase shift, an existing gain/oscillation problem, it might add another oscillation problem. Gain phase oscillation is best cured by fixing the underlying phase shifts so that there is one dominant pole, or phase compensation where that's not possible. Three RC-phase shifts are enough to cause oscillation, and a transformer on its own is two of those, so there are already limits on how much NFB can be used just because there is a transformer in the gain path.

If it were mine, I'd try to find and fix the oscillation problem before tossing solutions at it.

The amp is finally fixed now. It seems to work just fine with the stock 68K resistors at the input jack for the High/low function. I added 1.5 K grid stoppers at V1B as well as the grid of the output tube with no phase issues. Now I temporarily removed the grid stopper at V1B and it actually created a phase issue so that seems to speak to your point about RC phase shifting so I reinstalled it. I believe the original problem with the high noise floor was that I idiotically forgot to tie the switch terminal of the input jack-1 to ground so lots of noise/hum with no guitar plugged in…Embarrassing for sure but that’s what it was. Now as far as the oscillation issue that did happen on occasion with or without the guitar plugged in and I believe that that was a simple matter of there being too much gain without any negative feedback from the output.

[angelodp]

Is your heater winding Center tapped? Have you tried elevating it to the 6V6 Cathode. On my VC that does the trick pretty well and the amp is very quiet.

[lightfoot]

Is your heater winding Center tapped? Have you tried elevating it to the 6V6 Cathode. On my VC that does the trick pretty well and the amp is very quiet.

I’ve finally managed to address the issues. There was one primary reasons for the excessive noise floor (while unplugged) in this 5F1 type build.

Not tying the input switch to ground!
Not sure how or why I missed it but I did. I just installed a jumper between the switch terminal and ground and no more noise with no instrument plugged in.

Now there was what seemed to be a higher than expected noise floor and hum with an instrument plugged in and I suspect that was due to not having the the NFB loop hooked up which left me with more gain than the amp needed and the extra noise associated with it.
I had never had good luck with the resulting tone with the NFB loop connected but I think there was some kind of phase issue earlier in the pre amp that was responsible (This isn’t the same thing as OPT phase issues though).
It’s possible that the fact that the lack of grid stopper resistors anywhere in the amp in my initial assembly created some kind of phase problem that only became evident with the loop hooked up. This may have explained the poor tone with the loop installed.

I will also add that the installed OPT has an 8 ohm secondary with a 4 ohm tap. I’m hooked up to an 8 ohm speaker but I’m taking the signal for the feedback loop off the 4 ohm tap.

All I can say is that it’s dead quiet with more clean headroom than I expected with a Telecaster and the overall fidelity is very good. Sounds better than it ever has in the past…