Center tapped AC heater supply: A Question.

[DRBILL]

In the amplifiers that I regularly see, there are three ways to deal with a center tapped heater winding on power transformers. In some, the center tap goes directly to ground. Others connect to ground via a small value disk capacitor. Yet others bridge the heater feed with a wire wound pot identified as "hum balance" with the center contact and center tap finding the way to ground by various means.

What are the advantages/disadvantages of each method. Is there a "best way"?

Your input will be appreciated.

THANKS

DRBILL

[WMcD]

Dr. Bill,

This is somewhat of a guess, and maybe what you already know. I'm an old ham, now AB9BE.

The overall problem is to put the heater coil at ground potential, overall. But there is an a.c. voltage imposed on it. So grounding the center tap on the transformer establishes that either lead to the heater is going, say, plus 6 and minus 6 volts relative to chassis, assuming it is a 12 volt heater. The average voltage across the heater is zero.

It may be that the center tap on the transformer is not exactly at the center of the windings. So even with the ground, there is not an exact balance and overall, the heater is getting some a.c.

So the wire wound pot allows some adjustment in loading of the transformer so the high side of the "balanced" transformer is dragged down a lot and the low side is not dragged down a lot. The result is that we're back to exactly opposed voltage across the heater. I'd say this is the "better way" because it allows for compensation for a less than perfect transformer winding.

The cap I'm not so sure about. It is going to ensure that at a.c. voltages, the heater is at ground, but let it be at some other d.c. voltage.

My only thought on that is that the grid is usually at a lower voltage than the cathode in the signal range where the tube is operating. It may be part of a biasing scheme. Here, I'm out of my element. I'm sure that things are different when there is an indirectly heated cathode.

Gil

[DRBILL]

GIL,

I appreciate your thoughts. This question didn't arise out of a vacuum. I had a Heathkit w-4 (Williamson) amplifier on the bench recently that was plagued by hum harmonics. After exhausting the usual suspects I was stumped. Then I noticed the Y-G lead from the power transformer which, on the schematic was marked "not used". I stripped the end and with a little trepedation, touched it to chassis ground. Hum gone. I'm sure the Heath engineers had a good reason not to use this lead, but I can't imagine what.

I pulled all the power amplifier schematics I had out of my files to see how others treated this question. Dynaco favors a .02 disk. Eico has no center tap but uses the wire wound pot with the contactor to ground.

My conclusion is that some people drip coffee while others perk it!

I never sat for the Novice exam for fear of the five WPM code test! My dad was W5ESA and his rig was in one corner of my bedroom --home made: 807's in parallel push-pull. It could be exciting back there in a thunderstorm. I've had rosin in my system since I was in grammar school. In high school I had a bench at a local radio/TV shop after school.

DRBILL