Noob question about transformers.

[jimmybjj]

Say if you have 2a available for the heaters and 2a for the rectifier. if you don't use the rectifier winding does that leave more available for the heater? Thanks.

[JayB]

I doubt it. Most of the time, there separate windings. If they were derived from the same winding, that may be true.

[PRR]

> if you don't use the rectifier winding does that leave more available for the heater?

Kinda. NOT the full rating of the unused winding though. Maybe another Amp. Enuff for a couple 12AX7.

> I doubt it.

Good safe advice. But with multiple windings you can cheat a little.

There's two limits: per winding and total PT. Mostly the limit is long-term heat. If all windings are loaded exactly to rating, they all run at 50 deg temperature rise (or whatever the PT is designed for). If one winding is NOT loaded, it makes no heat, the PT generally runs cooler. You can suck a little more from the other windings and still stay safe on overall temperature. However the "overloaded" winding will be hot in its center, even though the overall temperature is below the limit. So you can fudge a little but not the full 10VA rating of the unused rectifier filament winding.

This is of course "dirty deeds and cheap tricks". If you have to KNOW the system is fully within ratings, go get another PT rated what you need: that's the best plan.

[JayB]

> if you don't use the rectifier winding does that leave more available for the heater?

Kinda. NOT the full rating of the unused winding though. Maybe another Amp. Enuff for a couple 12AX7.

> I doubt it.

Good safe advice. But with multiple windings you can cheat a little.

There's two limits: per winding and total PT. Mostly the limit is long-term heat. If all windings are loaded exactly to rating, they all run at 50 deg temperature rise (or whatever the PT is designed for). If one winding is NOT loaded, it makes no heat, the PT generally runs cooler. You can suck a little more from the other windings and still stay safe on overall temperature. However the "overloaded" winding will be hot in its center, even though the overall temperature is below the limit. So you can fudge a little but not the full 10VA rating of the unused rectifier filament winding.

This is of course "dirty deeds and cheap tricks". If you have to KNOW the system is fully within ratings, go get another PT rated what you need: that's the best plan.

Couldn't they derive both voltages from one winding? I never taken one apart before to find out.

[PRR]

> both voltages from one winding?

The classic "rectifier winding" lives at +350V DC, the rectifier cathode potential.

The other tubes' heaters must stay near their cathode potential, 1V to 100V DC.

They need to be two separate well-insulated windings.

There's also rectifiers with insulated heater/cathode construction, but rarely found above about 1 Watt amplifier output. These don't need a separate winding, you feed them with the same winding as your other tubes.