• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

480vct at 50VA for less than $12 shipped

ericj

Member
2008-12-08 10:24 pm
Apologies if this post belongs elsewhere, i just figured that high voltage transformers were of particular interest when building valve amplifiers.

A whole bunch of the same power transformer seem to have hit the surplus market. Like this listing:

New Rib Functional Devices Transformer TR50VA018 T 249 | eBay

It's an industrial control transformer with 120v secondary and COM-208-240-277-480 primary winding.

Or, looked at another way, 120v primary and 240-32-0-37-240 secondary winding.

A little weird that the extra winding is asymmetrical but it could still be useful for bias voltages?

In backstrapped use the voltage might be slightly lower due to the nature of industrial control transformer specifications.

All windings have color coded wire leads. The end bells have the 120v winding sticking out the top and the rest sticking out the bottom, which has a flange for a standard 2-gang electrical box. There is enough space between the windings and the outer core to comfortably tuck the 120v leads to the bottom side.

Mine arrived fairly quickly with some minor bending to the flange and a busted conduit fitting on top. Brand new in box. I haven't load tested it yet - probably saturday.

Anyway, I figured it could be useful for a line stage, headphone amp, small guitar amp, etc. Provided you have filament transformers knocking around already, which i do.

Right now there are in excess of 100 of them on epay.
 

ericj

Member
2008-12-08 10:24 pm
some testing today - no tap ends up in the center unfortunately.

With a string of 6 60w light bulbs across the full formerly primary winding and the former 120v secondary into mains power, I'm showing 0-146-168-193-316.

some quick math however suggests that I'm overloading the transformer. if i can think of a 60w 120v bulb as a 240ohm resistor, 1440 ohms across 316 volts is more than 68VA.

And I've just realized that the meter i was using was not 'true RMS' so, I'll try again in a bit.
 
I have a few of these control transformers also except mine are rated at 750VA and 1KVA. Backstrapped they make fine voltage through a 5U4 and cap filter although I cannot remember what exactly the final DC output was but I think it was around 620.

Yes these are monsters at 750VA and 1KVA but I always liked overkill in the power supply department. Besides, I am saving them for when I finally build "The Big One".
 

Junm

Member
2008-12-23 5:52 am
Dau
I also got this things from a local surplus store...50va (pr-380v/sec 220v), 50 va 415v pr and 110v sec at 3usd each and two 500va (380 primary and 220v secondary, 50hz) at 15usd each...
 

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Odds are that the 222V winding sections have the same number of turns but different DC resistance - either due to winding diameter (outside turns are longer) or due to having 50VA rating for any primary (the 222V section would need to have 1/4 the resistance of the 444V full winding to have the same loss, since current is doubled).

So there may be some imbalance if it's used as 444VCT, less as load is reduced. There may be some reduction in capacity if 208 or 240 or 277V segment is used with a full-wave bridge, but gives some more voltage choices.