power supplyvacuum-tube
I want to build a simple single-ended valve amplifier from an old TV.
I am facing a problem, that I don't know how to produce the 13V, 300mA supply for the heater.(PCL86)
I am using the mains transformer directly from the old TV. But while other amplifiers I see have a transformer with separate windings for a heater, this one only has one primary and one secondary winding.
The TV obviously had solved this problem somehow, so how could I solve this most easily?
I agree with @Dave about the modelling. You only need to worry about the resistance at operating temperature. The lower resistance at room temperature might increase the warm-up time, but unless you're worried about that, it should have no significant effect.
An option would be to create an ~8V DC supply and use a boost converter to create a regulated 300mA DC supply. A simple circuit using the LM2577 (or one of the ubiquitous modules) should work fine. You can probably make the circuit smaller and cheaper, and easier on the transformer.
If you do decide to use the capacitor voltage multiplier, make sure that the ripple current rating (RMS ripple current) of the capacitors is observed. Cheap electrolytics may not last long in this service, particularly if they see high temperatures from your tube ('thermionic valve', sorry), on top of the self-heating from the ripple current.
Consider this circuit of a smps circuit:
You can clearly see three windings. Right one is secondary which is producing the output. Top left is primary and bottom left is auxiliary.
Auxiliary is nothing but an additional secondary. Usually in a switching circuit, we need some low voltage power source for the working circuit. You can't power them using 220V or 110V line directly. So you are left with two options -
1) Somehow use the low voltage output which is being generated which will be a very bad idea because the sole purpose of using such a schematic is to provide galvanic isolation to the secondary side and by connecting output side to switching IC, you have defeated the purpose.
2) Make a separate low power winding just to provide enough power for the chip to run which is what we do by making an auxiliary winding. When you cut open a transformer, you will find that secondary winding will be usually made using thicker wire because it has to handle a larger current whereas auxiliary will be made using thin wire because it has to carry just small amount of current to keep the chip functioning.
Both secondary and auxiliary windings get their energy from primary winding. You can add more windings to the transformer and call them aux2, aux3, sec2, sec3 etc in case you need such an arrangement.
Best Answer
It's possible that the TV put all of the heaters in series directly across the AC line input. Very common in cheap "hot chassis" designs.
There's no reason you couldn't add a second power transformer yourself. 12.6 VAC transformers are very readily available.
Or you could use a separate DC 12V supply for testing purposes — Heaters don't care whether they're getting AC or DC.