Quick heater voltage question (voltage too high!).

[viceroyal]

I built a small power supply to use for a few 12A_7 preamp projects and I'm having trouble with the heater voltage. The heater winding is supposed to be 12.6VAC at .3A. Unfortunately, I'm getting closer to 16V under load. I tried rectifying and using a regulator but I didn't have enough current left. I tried putting small resistors (560R 2W) in series with each heater winding which brought it down to 11.9V. This worked fine but the resistors were bulkier than I would like.

Looking for more options, I read on Valve Wizard's site that you could also put 2 diodes in opposite directions on one of the heater windings to lower the voltage. I used 2 1N4007's and it brought it down to 13.4 which is in range but this leaves me with one question. If the diodes are only on one side of the winding will it only lower the voltage for half a phase and will I still 16V for the other half?

Thoughts? Advice? Criticism?

Thanks for reading!

[J M Fahey]

I tried putting small resistors (560R 2W) in series with each heater winding which brought it down to 11.9V.

Check your resistors/check your math ..... a 560r resistor will limit your current to 16000/560=28mA into a short; even less into a filament.
Those 12A_7 need 150 mA each.
You'll need a 22 ohm, 1W resistor in series with the 12.6V filament at each socket.

[viceroyal]

Check your resistors/check your math ..... a 560r resistor will limit your current to 16000/560=28mA into a short; even less into a filament.
Those 12A_7 need 150 mA each.
You'll need a 22 ohm, 1W resistor in series with the 12.6V filament at each socket.

I don't doubt that this is the case but I tried 22K and it killed the voltage. 560R worked just fine but it was mainly a temporary solution.

[viceroyal]

If any one is interested I started a thread at DIY Audio and they helped me with most of my B+ problems but my heater problem seems to have been overlooked. I should have come here first anyway. I always forget they're all hifi types!

[exclamationmark]

I think you should be using 22 or 23 ohm resistors. 1 Watt would be sufficient. Basically what Fahey said.

Assuming you're at 16v and you want to drop around 3.4v to get down to 12.6v at 150mA. R = V/I, ie, R=3.4/0.15, Then R is equal to approx 22.6 Ohms. Power dissipation is simply 3.4*0.15 (or V*I), which equals a piddly 0.5 watts. You could probably get away with half watt resistors if you wanted.

[viceroyal]

Thanks for the help with the math guys. So would you say that resistors are a better idea than diodes?