How am i getting 100vAC on the heaters???

[beasleybodyshop]

So im restoring a 76' Ampeg VT40. i am checking all the voltages at the secondary of the PT, and when i check common to ground (chassis, in this case) and the other to the filament wire im getting 100v......wtf? i check across the winding (one lead on one wire, common on the other wire) and i read 6.7v.

huh?

I checked my lead going to ground in multiple spots on the chassis, even lightly sanded a bare spot and checked again. I also checked with another meter, just to make sure mine wasnt crazy.

Filament circuit was originally artificially CT'ed with a 100R hum balance pot, which was bad so i removed it. now it has no CT. It still shouldnt show wacky voltages with the artifical CT lifted right?

Does this mean i have some sort of grounding issue on the primary side? looks like the center tap for the B+ winding is connected to one end of the bridge rectifier and has another connection to the positive end of the bias supply caps.....So im going to assume that this wire is supposed to be grounded to chassis in some way or another. So confused.

[jelle]

Put two 100 ohms resistors in and measure again

[Leo_Gnardo]

Filament circuit was originally artificially CT'ed with a 100R hum balance pot, which was bad so i removed it. now it has no CT. It still shouldnt show wacky voltages with the artifical CT lifted right?

Can't leave your filaments just floating - see what happens? Lucky they don't go on a three hour tour...

Gotta ground, or float on DC, the CT or "artificial" CT made with a couple resistors or hum balance pot. So do what jelle says to start with, cheap, fast & easy.

[jelle]

Sorry, short and sweet from my phone...

[printer2]

Can't leave your filaments just floating - see what happens? Lucky they don't go on a three hour tour...

Thanks, I needed that.


Just to add, say you have some stray capacitive coupling going on between the high voltage winding and the heater winding. There is a very high resistance between the two windings, actually reactince because it is AC but no need to quibble. Since there is such a high resistance, nothing to load down the high impedance of the electric field other than your meter's high imput impedance... ...I hope you are seeing where this is going.

[beasleybodyshop]

Sorry, short and sweet from my phone...

Thanks Jelle and Leo, this solved the problem. Bad hum balance pot was causing them to float - wired in the double 100R's and now we are back in business!

Good lesson learned here. For some reason I thought it was ok to float the heaters - now I know!

[C Moore]

Filament circuit was originally artificially CT'ed with a 100R hum balance pot, which was bad so i removed it. now it has no CT. It still shouldnt show wacky voltages with the artifical CT lifted right?

Can't leave your filaments just floating - see what happens? Lucky they don't go on a three hour tour...

Gotta ground, or float on DC, the CT or "artificial" CT made with a couple resistors or hum balance pot. So do what jelle says to start with, cheap, fast & easy.

So what happens exactly...that cause a 6.5 VAC winding to read 100 VAC at the tube filament.?
Thank You

[Tonegeek]

I don't understand how an isolated transformer winding (in the case of the heater) can develop a voltage to another part of the system. That it does, implies a return path. ALso I have to guess that the ac radiating from the heaters is modulating the DC in the signal, or there is capacitive coupling somewhere. BTW many tweed amps used to just ground one side of the 6.3v winding to the chassis and had no CT. I have an old Tremolux in my shop at the moment done this way and it is surprisingly quiet. I do notice though that many tweed amps produce DC type noise when you adjust your guitar volume knob. I wonder also if this is related to the heater wiring method.

[beasleybodyshop]

After doing some further research, I've discovered that many of these older Ampegs were notorious for having the hum balance pot go south. This makes sense. Now on to figuring out why I don't have any plate voltages...

[sluckey]

So what happens exactly...that cause a 6.5 VAC winding to read 100 VAC at the tube filament.?

The 100VAC reading is a bogus reading. The filament winding no longer has a reference to ground so any voltage reading to ground is bogus. As soon as you connect the filament winding to ground whether by grounding one side, or grounding a real center tap, or by using an artificial center tap such as two resistors or a humdinger pot, the bogus voltage reading will disappear. You could even use a 1MΩ resistor to provide the ground reference and kill the bogie but hum reduction would suffer with such a large resistor.

[C Moore]

So what happens exactly...that cause a 6.5 VAC winding to read 100 VAC at the tube filament.?

The 100VAC reading is a bogus reading. The filament winding no longer has a reference to ground so any voltage reading to ground is bogus. As soon as you connect the filament winding to ground whether by grounding one side, or grounding a real center tap, or by using an artificial center tap such as two resistors or a humdinger pot, the bogus voltage reading will disappear. You could even use a 1MΩ resistor to provide the ground reference and kill the bogie but hum reduction would suffer with such a large resistor.

Good Grief...now I get it
Like measuring a battery with one probe on the negative, and the other probe on your tennis shoe.
Anyway.....Thanks for spelling it out for me.
I am Not Einstein.....

[LOUDthud]

Many transformers are wound like this: First the primary or primaries are wound. If there is a shield between primary and secondary, that is applied. Next the high voltage secondary, starting at one end to the center tap, then the second half starting with the center tap and finishing with other end. Lastly the heater windings on the very outside. The heater windings are usually only a handfull of turns and there is lots of capacitive coupling to the finish end of the HV secondary. The capacitance isn't huge, but the voltage is high so you might measure 100VAC on a heater winding unless you ground it somehow.