Wiring Heater Windings in Parallel--or Separately?

[David Root]

Not run across this choice before. I'm planning two amps, both will use 6V tube rectifiers (EZ81/6CA4 or 6X4). Neither of these have filamentary cathodes.
I bought two PTs the same. Each has two 6.3V windings, one is 3A, the other 2A. If I parallel them I have 6.3V 5A and would connect them conventionally, starting with the rectifier and moving on thru the preamp tubes.

However, in both cases I have the option of using the windings separately. In one case I could put four 6BM8s on the 3A winding (calls for 3.12A), and the EZ81 and first stage preamp tube on the 2A winding.
If I do this, is it OK to have the first stage on the same winding as the rectifier, directly next in line? IOW can there be any crossover of rectification nasties into the first stage tube thru the heater connection?

The other amp is similar except four 6AQ5 w/two 6X4 in parallel, total heater current req'd 3.0 A, obviously the 3A winding is a natural. The 2A winding would then supply the PI and two preamp stages, total heater draw 1.35 A.
So in this case the rectifier and preamp stages are not on the same winding.

Is there a possible issue with the 6BM8 amp or should I just not worry & be happy?

[R.G.]

Do not parallel them. Load them up with separate tubes up to, but not beyond their rated current.

They'll both be grounded in the middle, of course, for lower noise, but do not parallel the loads.

There is a big problem with paralleling voltage sources, which these things are close to. Any tiny differences in voltage generate currents that are limited only by the winding resistance, not the loads on the windings. The results can be needlessly exciting.

And transformer "6V" windings may be practically any voltage close to the nominal, and will only by rare accident or careful design be close enough to parallel.

[David Root]

Thank you RG. The DCR of these windings are 0.10 and 0.14 ohm, so I think I see the logic. It doesn't take much of a voltage mismatch to generate big current in a parallel connection of 0.10/0.14--that is 0.058 ohm.

Neither of these windings has a center tap, so when you say "they'll both be grounded in the middle, of course...", are you referring to an internal winding ground I don't know about, or to the pair of 100 ohm referencing resistors I'll have to use on each winding in lieu of a CT? I'm not expert in transformer design.

[Phil_S]

There are only about 19 total turns on a 6.3v winding. It is all too easy to see a variance between the two windings.

[pdf64]

Neither of these windings has a center tap, so when you say "they'll both be grounded in the middle, of course...", are you referring to an internal winding ground I don't know about, or to the pair of 100 ohm referencing resistors I'll have to use on each winding in lieu of a CT? I'm not expert in transformer design.

I think that RG meant the middle in regard of the voltage level halfway between the max and min points of the wave, ie the ground reference you will use for the heaters, be that a winding CT or balancing resistors.
So in your case, each heater winding will need its own set of balancing resistors.

[David Root]

OK, thanx, I guess I just misinterpreted what R.G. was saying. I checked again and the 3A winding does have a CT so only one pair of referencing resistors needed.

[pdf64]

I suggest that the winding with the CT is used for the power tubes, and that the B+ winding be suitably fused.

[pdf64]

To help cope with power tube shorts.

[gingertube]

A power tranny has just a total VA (Volt Amps) rating for all windings based upon the allowable temperature rise. CAUTION: not all manufacturers rate for the same temperature rise. Dodgy manufacturers will rate for a higher temperature rise and so their trannies will run hotter given the same load.

Within limits (say +25%) it is OK to overload one winding so long as one or more of the other windings is under loaded by the same VA amount such that the total VA draw from the tranny remains the same.

Cheers,
Ian

[stretch2011]

You should wire them separately, not in parallel.

Whichever one is used for the preamps make sure the 1st stage is last.

The fact you are using filament transformers separately from the hv means there will be less hum.

If you can connect the center tap to the bias for the power amp it will decrease hum.
-I just read firestorms post on another thread, reread Merlin blemcos book, and everything I have stated was in his book.