12V heater to raise B+?

[birt]

In an amp that has 12V heaters, is it possible to put the heater winding of the PT in series with the HT winding to get a higher B+?

naturally this would be with a diode bridge rectifier because the CT wouldn't be at half supply anymore.

would this do any harm to the HT supply? introduce more hum?

[FYL]

It's possible, but not worth the bother IMO : you'll get a 17V boost, a very small increase compared to the usual 400V+ B+.

[gar13]

What about in series (and in phase) with the transformer primary winding?  Gar Gillies's book mentions something about this, but I'm not at home right now to look it up.  Out of phase series connect reduces voltage on the secondary, in phase increases it, but I can't remember by how much.  This method would allow you to retain the CT and use full wave rectification.

[birt]

if that works the HT voltage would also be more than the method i suggested?

[gar13]

What are the specs on the transformer in "normal" operation?

[birt]

well it has 5V, 12V for the heaters, 18V and 320-0-320.

i can also use the 18V.

[Tiny_Daddy]

Don't try to boost the primary voltage unless you like the smell of burning varnish. Reason is, power transformers are run near or slightly into the saturation knee.

[darryl]

Problem 1. If you plan to add the heater winding to the HT secondary, and also still use it for powering the heaters, then the maximum heater-cathode voltage will be exceeded.


Problem 2. If you ignore the HT winding's centre tap and use the full secondary with a diode bridge and capacitor input filter, your HT voltage will be approximately (640 + 12) x 1.4 = 913 volts DC. 

[PRR]

> with the transformer primary winding?

Remember there's heaters on this winding. If you do that, these heaters are on the NON-isolated side of the PT. Since you are going to touch the cathodes (near-enough, through small cathode resistors to ground and gitar-plug), your life hangs on the fragile heater-cathode insulation.

And the other issues mentioned. Mostly: that 17V or even doubled 33V is not going to make an ant's worth of difference in a 300V-400V system.

[birt]

i was quite sceptical to add the heaters to the primary because of safety issues. my original idea isn't going to do much either i guess.

however i seem to have 2 entirely different possibilities for the B+. i can use the normal setup with a tube rectifier and get lower B+ or i can use half of the HT winding with a diode bridge and get high B+. in this setup i can use the 18V winding in or out of phase to get about 25V more or 25V less B+.

[gar13]

The Gar Gillies method I mentioned was assuming that you were using an unused heater tap. In his book, the example he shows uses the 5V reticfier filament taps for a gain for an approximate 15v AC gain on the secondary.