transformer?

[Mark Amundson]

Ryan Schmirler wrote:
> I am a small amp builder in B.C. and
> have received a small box, of what are labelled as, 'Output
> Transformers'. These only have a stamp 'VM-4' and underneath a 86-SS.
> One side has red, red, black, and two very small gauge browns. The other
> side has seven wires, all same gauge: wht, brn, red, blu, org, gry, and
> yellow. They are all 'Z' mount and have a fairly good size core on them.
> Do you know what kind of transformer this might be? I was told they were
> replacement output transformers for tube amps, but as far as I can tell
> they look more like power tranx for a solid state amp. For fear of
> ruining them I have not tried them out on a variac and there are no
> manufactures stamps on them so I can not trace the make. Any help would
> be greatly appreciated. thanks Ryan

Hi Ryan,

First of all, join the rest of diy tube/SS amp builders on the AMPAGE
chatroom to exchange thoughts and opinions with others like yourself. We
have a regularly scheduled meet at 17:00GMT among the enthusiasts.

You probably have a universal output transformer (OPT in chatroom
parlance) with ultra-linear taps for screen-grid connections (likely the
brown leads). The multi-colored leads on one side are likely the
secondaries for multiple output impedance selection. In any case, ohm
out all the leads to verify all the windings. Choose the two leads with
the heaviest wire (secondary) and lowest winding resistance. Connect
them to known source of low voltage of 60Hz AC (like a 6.3VAC filament
winding). Measure the resulting AC voltages across the other windings to
determine turns ratios. You can assume your secondary is either a 2 or 4
ohm tap and calculate the other primary and secondary impedances from
the basic transformer equations.

If you have experiance with E-I core transformers, the relative size
should give you a good judgement of power handling for Class A or Class
AB push-pull.

Mark Amundson,