Heaters in series and regulation

The transformer has two heater windings that give 6.4 VAC RMS when loaded. I need to power the heaters of four EL34 tubes, so 6 amps total. If I want to do DC heaters, however, I can't seem to find any rectifier with low enough dropout to give me 6.3 VDC. If I put two heaters in series, and the transformer windings in series, then the rectifier dropout effect is cut by half and I do have sufficient voltage; however, in testing the heaters are not matched well enough and that worries me -- putting two heaters in series with the AC voltage I get 6.4 across one heater and 6.8 V across the other. Now, is it possible to do low dropout regulation that would force the voltages to be divided (so two regulators on top of each other, one parallel with each heater)?

The irony is I wound the transformer myself and didn't think to put an extra turn... as these are the bottom windings there's no way I'm taking it apart again.

"E" series valve heaters should always be run in parallel; similarly "U" and "P" series should always be run in series.

I would recommend you run the two pairs of EL34 heaters off separate windings. If you want d.c. heating, use discrete Schottky rectifiers such as the IR 50SQ060G connected in a bridge. Use heavy smoothing to reduce recified waveform harmonics.

Like I wrote, even with low forward voltage rectifiers, I simply don't have enough voltage to get even 6 VDC after the rectifier losses. But if I put the windings in series, then I do manage to get at least 12 VDC.

Why not run AC heating, now you have made the power transformer for that?

Jakob E.

It's an ultra-low noise amp. I don't want any possible source of noise.

Here's what I came up with that does make enough voltage (barely) if I put the windings in series, giving 13 VDC. Works in simulation, but I've yet to build it. Each of the two pairs of heaters gets 6.3 V, the way the regulators are on top of each other to split up the 13 V.

[quote author="Boswell"]"E" series valve heaters should always be run in parallel; similarly "U" and "P" series should always be run in series.[/quote]
Boswell, I'm trying to understand the reason behind this. Can you provide a reference? I've been unable to find any info this far.