Bucking down PSU voltages with 5v filament winding

[Greg Smith]

I am in the process of building a couple of cathode biased eiclones and wanted to share a discovery I made wrt reducing power transformer secondary voltages by using the normally unused 5vac rectifier filament winding to "buck" down the primary voltage.

My household mains voltage is typically around 121 vac. Couple this to the typical Hammond PT with primary rated for 115 vac, and I get filament voltages at around 6.7 vac, and plate voltage in the 500 to 505 vdc range. The filament voltage is not too worrisome, but the plate voltage had the output tubes idling at around 28 watts, with plates glowing a dull red in a darkened room.

Remembering something I'd read about bucking transformers, I tried connecting the unused rectifier filament winding in series but out of phase with the PT primary. The resulting filament voltage was reduced to 6.3 vac, and the plate voltage now sits at around 479 to 483 vdc. The outbut tube plates are dissipating 25 watts - still at the max rated plate dissipation, but they don't red-plate anymore.

The simplest way to know the phase of the windings is trial-and-error:

1. Connect the transformer normally and record the 'green' filament (green leads) voltage.

2. Disconnect one of the primary (black) leads from the mains and connect one of the rectifier filament leads (yellow) to the black lead. Connect the other yellow lead to the mains. (you now have the primary and retifier filament winding connected in series across the mains voltage). Measure the 'green' filament voltage again. If the voltage is lower than before, you have the rectifier filament connected out of phase (as desired) and the HV secondary will also be reduced. If the main filament voltage is higher than before, you need to reverse the yellow wires.

Cheers,
Greg Smith

[Shannon Parks]

Interesting, Greg. Very interesting.

[muschy]

Greg,

I'm almost finished building a set of Eiclones. What Hammond PTs are you using? I've got 274BX's. My mains voltage also runs around 120VAC. Is the bucking configuration working out for you so far? Seems like a good solution for my situation as well. What kind of tubes are you using?

Nick

[Greg Smith]

I am using the 274BX as well. My line voltage usually sits at around 120-121. Shortly after my last post, I had to put the project on hold so I can't give you any long-term status on the bucking arrangement. However, I did give the amp a reasonably long stress test at high volume (while using the bucking arrangement) and saw no adverse results.

I am using JJ EL34s. I am not terribly happy with them because they come from the era of undersize pins from a few years back. If the sockets weren't brand-new, I'm sure they would be in danger of losing contact. I am looking to replace them with EH or SED.

Greg