Filament Power, for larger builds

[systemtruck]

Hello,
I’m taking steps towards a build that will power up around 20 or so tubes in one unit.
It’s all 12.6V heaters.
I have a toroidal “tube transformer” that offers 4A on its 12.6 VCT secondary and I am considering using 3.6A of that 4A.
The HT secondary of same transformer offers 550mA, but I’ll only ever use around 150-200mA of that.

Would be it smart to get a separate 12.6V supply? Maybe something like a Hammond that provides 10A?

I’m sure I can “get function” with using the onboard 4A as mentioned, but will that constant near capacity current draw from one secondary be a real slog on the other secondary that’s providing the HT?

And will it also reduce the life of the transformer?

I might want to run things as cool as possible. So maybe a 10A filament supply is the better choice.

 

[Rusan]

If it's designed and rated for 4A, then a 3.6A load is not only perfectly fine, but actually desirable. Secondaries are designed to produce their advertised voltage at full, manufacturer-rated load. AAMOF, in some cases a severely underloaded filament winding can produce an excessively high voltage that's out of spec for the tube. This could be possible with a 10A transformer. The only time you'll run into problems is when you inadvertently exceed the published ratings by a certain margin. One such possible scenario would be failing to account for the actual RMS current draw on the high voltage secondary, when huge filter cap values are used right after the rectifiers, in a Class AB amp that has higher current draw at full power than you might think.

Ignoring its current rating, a transformer's heat rise is also very dependent on what the core material is. A given transformer stacked with M19 laminations will run as hot when moderately loaded as an otherwise identical trafo using M6 lams that's presented with a heavy load, due to M19's greater hysteresis losses.

All other things being equal, your existing transformer will be fine, especially since toroidals generally tend to run cooler anyway. And, because you're significantly underloading the high voltage secondary, that'll be even more magnetic and thermal headroom to spare.

 

[systemtruck]

If it's designed and rated for 4A, then a 3.6A load is not only perfectly fine, but desirable. Filament windings are designed to produce their advertised voltage at full rated load. In some cases, an severely underloaded winding can produce an excessively high voltage that's out of spec for the filament.

The fact you're underloading the high voltage secondary just gives it that much more magnetic and thermal headroom.

Well that’s pretty cool!
I suppose the opposite is also true.. that if I go over the limit a bit, it’s really not a major issue especially if I’m not using all the HT?
In one alternate scenario, I’ll draw 4.2A of the spec’d 4A. In a third scenario, I’ll draw 4.8A. And that will be the max scenario.
I just worry about affecting performance of the high voltage in some manner.

 

[emrr]

Likewise, if the B+ winding can deliver 2-3x what you will use, B+ will be high and need big dropping resistances or regulation.