Zenith OPT question

[no gas]

I've got an old Zenith PP OPT that has an uneven primary. One half measures 56 ohms less than the other and in the original amp there was a 56 ohm resistor on that side. My question is, how do I use this OPT? Can I drive it with the normal phase splitter? Thanks for any guidance.

[kevinkr]

I'd probably start with something else were it me, however if you use it in a manner analogous to the original design with any phase splitter you choose (if I understand you correctly) it should not be an issue. Just how good that OPT actually is might be an issue however.

[no gas]

It's a fairly heafty 15w or so out of a 7K31 series Zenith. Next question is why it would be uneven?

[richwalters]

Primaries not wound equally on core. One half prim innermost to core has lower resistance and other half prim wound outer equals higher resistance due to longer MLT.(mean length per turn) and secs plonked somewhere in the middle. A mass produced NON quality product, but it works.
For use in a cathode bias o/p stages, the unsymmetrical problem is solved automatically and this is where it was intended.

richy

[no gas]

Thank you for a concise answer. Makes sense to me. Well, at least the power transformer is a beast!

[20to20]

Zenith seems to be one oddball engineering company. I have a 7D30 chassis that takes the 8 ohm Channel 1 output and doesn't send it to a speaker. They send the line back over to Channel 2 OPT and run it right into a centertap of the secondary 16 ohm coil, and then use 2 taps on that OPT for output to the speakers. AND never mind the additional secondary coils on each OPT of @50 ohms for a dedicated feedback circuit, not tapped off of the speaker terminal like 90% of other amps.

You ought to get the PDF diagram from Sams and see what's what. You might find you have a couple of trannys with a special design.

[tubelab.com]

I've got an old Zenith PP OPT that has an uneven primary. One half measures 56 ohms less than the other and in the original amp there was a 56 ohm resistor on that side.

That is pretty common in cheap OPT's. I have a big bunch of bottom dollar P-P OPT's designed for guitar amps. There is no interleaving whatsoever. One half-primary is wound first, then the entire secondary, then the other half-primary. Both half-primaries have the same number of turns but the outer one has a larger diameter, so it has more resistance, even though in my case it is wound with a thicker wire.

I use them in the same manner as any other OPT, the resistor is probably not necessary. I have found that in fixed bias amps the lowest distortion is often achieved with slightly unequal tube currents. I find the same thing with high dollar Plitrons though.

[kenpeter]

Is one of those weirdos that send A+B through bass transformer, and A-B
through the smaller cheaper one that would saturate at low frequency?

I'm asking, not sayin'... I've never actually seen real one like that (yet).

Hmmm... Suddenly thinking new abuse for toroid here.