Power transformer & lowering heater voltage?

[crazytooguy]

Hello! I have a power transformer from my old MusicMan HD130, and I'd like to use it for some low-power SE tube amp projects. The HT rectified is 237VDC, which is fairly low. (The MM circuit used a capacitative doubler to raise the HT for the four 6L6's) If I apply 120VAC to the primary at the 100VAC primary windings, I can get 305VDC, which is more like what I want. But, of course, the heater voltage is too high at 7.8VAC. So, two questions:

Is there a way to lower the heater voltage to a safer value? I've seen the diode mod that lowers it by the forward voltage of the diode, but it didn't lower it very much with the diode I tried. Is it supposed to be two diodes back-to-back?

and:

What difference will I find if I use a 237V HT vs 305V HT? Of course, the output is lower with the lower voltage, but does it change the dynamics as well? Will it sound "squashed" at the lower voltage? (I'm planning a Princeton 5F2 type of design)

I'll probably wire it up and experiment, but I thought I'd see if you all had any experiences with this sort of thing. Thanks!!

[R.G.]

(1) 120V on the 100V tap has the possibility of saturating the transformer and overheating it.
(2) Since this is a made-up-from-scratch setup, why not use a separate 120Vac => 6.3Vac transformer just for the heaters and ignore the heater windings on the main transformer?
(3) Yes, it's supposed to be two diodes back to back. Each diode pair you add removes about 0.5V RMS from the available voltage. To get 7.8Vac back to 6.3Vac, that's 7.8-6.3 = 1.5Vac, so you need three pairs of back-to-back diodes in series to wipe off 1.5V. Notice that the diodes have to be rated for the high currents in the heaters. If you stick with the goosed-up voltage on the primary (bad idea, see #1) it might make more sense to use a power resistor to eat 1.5V at heater currents, or to rectify and filter the 7.8V to 11Vdc, then regulate to 6.3Vdc and use DC heaters.

[crazytooguy]

Thanks, R.G. I was considering a separate heater transformer. I didn't think about saturating the PT - glad you pointed that out. Probably easiest to use a separate tranny.

So how about a 5C2 Fender design with 237V HT? Will that sound weak or does the HT really only scale the power output?

[Chuck H]

Also, keep in mind that the 305V is unloaded. Your actual Vp will be much lower. That, combined with the filament voltage issue, could mean this just may not be a suitable transformer for this project.

[loudthud]

That extra tap on the primary is for the Hi-Lo power switch. It can handle 120VAC. Apparently they ran the heaters overvoltage in the hi power mode. Note that you need some kind of load on the heaters to evaluate the diode dropping circuit. Use diodes rated for twice the expected heater current to handle the startup surge.