Potentiometer in HV winding

[XgamerGt03]

I've been talking to a guy about an odd thing he is seeing in his valve amp.

He has a potentiometer, set up like a humdinger, in the HV winding of the amp. The Heater winding does not have this. Both are center tapped, the center tap of the HV is connected to the pot and ground. The center tap of the heaters is connected to ground.

The HV line goes into a simple full wave rectifier, and nothing else is connected to the HV winding from the transformer.

He says that if he removes the pot, but keeps the center taps connected to ground, the 50Hz hum increases.

Any ideas as to what the heck the pot could be doing in the HV line?

[Firestorm]

the center tap of the HV is connected to the pot and ground.

I am not sure what you mean by this. If the center tap is grounded, anything else connected to it would be shunted out.

[XgamerGt03]

the center tap of the HV is connected to the pot and ground.

I am not sure what you mean by this. If the center tap is grounded, anything else connected to it would be shunted out.

The center tap of the HV and the center of the pot are both connected and grounded. The Pot is wired like a Humdinger.

[Structo]

Not sure why that would be that way but sounds like a fire waiting to happen.

Not sure of the current that is present at the CT but I wouldn't think that would serve any purpose.

Is it a home build or a commercial amplifier?

[David Root]

I don't get it xgamer. Draw us a schematic, your descriptions are ambiguous. How is it all wired up?

[XgamerGt03]

This is the schematic he drew up of the HV winding.

http://www.element14.com/community/serv ... 70/cct.bmp

The heater winding as a center tap, and that center tap is grounded.

[Cliff Schecht]

I would never do that, it's going to roast the pot eventually. Ignoring the current through the thing, the V^2/R power is going to be over what the pot is rated for most likely. Taking a standard TW 600VCT power transformer and a 1M pot for example, you get about 425V peak into 1M (425^2/1M) giving close to 0.2W of dissipation (pots are 0.25W max or 2W for the AB style mil-spec guys).

Surely the source of noise is coming from somewhere else. I've never had to balance the AC windings using anything other than the standard centertap (definitely good enough balance-wise). Pics and/or a full schematic might help. My money is on something else being wrong.