[sdiy] dual primary power transformer and how to connect them for 120-ish VAC

[anthony]

I have a BIG power transformer from an old piece of tube gear. The HV output 
is not center-tapped which is a big bummer since I wanted to use my 6AX5-GT, 
but I suppose I'm going to either have to use some 1N4007's in a bridge or a 
half-wave rectifier of some sort (which I don't think I want to do)

But that's not what I'm really fussed with at the moment. The thing has dual 
primaries for using it either with 240VAC or 120VAC (Telonic VHF Sweep 
Generator - made in England I think - also the source of the 4 vintage 
Mullard EL84's I found in my basement...). I took it out not remembering how 
the damned thing was connected to the switch and the color codes aren't the 
same for other dual primary transformers (like the one in some Fender Bronco 
amps for example). I tested with a lamp and a 9V battery which wires were 
the ends to which windings.The color code for one winding was black & 
red-on-black, the other winding was yellow and red-on-yellow. I remembered 
seeing somewhere that for 120VAC service the windings are connected in 
parallel and I was certain that proper phase orientation was important or I 
would get zero output. So I took a wild stab and connected the solid color 
wires to each other and to one wire coming from a plug in my variac and the 
wires with red in them together (logical I thought) & to the other wire on 
the plug in the variac. Then I brought the voltage up and got a reading of 
about 300VAC where I have the variac dial calibrated for 120VAC wall service 
(it'll do 140VAC like most other Variacs if they're wired that way).

So I figure my guess was right and that had I connected the primaries in the 
incorrect phase, I would have gotten zero output, right? I'm lazy and don't 
feel like trying it the other way just to see. But is there a chance that it 
didn't matter and I could have hooked them up the other way and gotten 
300VAC again? I could hook 'em up in series and get a pseudo 1:1-ish 
isolation transformer, but I can't imagine wanting to.

I wonder what to use this thing for? It powered something like 15 tubes 
including 4 EL84's doing voltage regulator & constant-current-source duties 
(not at the same time, but there wasn't an input for a guitar or a speaker 
on this thing so they weren't pulling audio amp duty either...).

I guess I don't need to lament not being able to use a tube rectifier, well 
not a full-wave one anyway. I can use this for a big brute push-pull, maybe 
something like an AC30 or AC50

I can use my smaller transformer and 6AX5-GT for an AC15 or GA15 type clone 
and dial in some sag on that.

And I'm up to my teets in small single-ended amps though I love 'em so. I 
could use 'em more as effects pedals if I had a big head and a 4x12 cab for 
it.

I miss my Ampeg VT-40 so much, maybe I'll try a clone of THAT... Or 
something in that dead Hammond organ that I still haven't actually BOUGHT...


cheers,
aa