What to do with a pair of Magnequest EXO-36?

[Uncle Scary]

Hi everyone,

I've been far too busy for far too long, but I should have some free time to build something again. So I looked through my parts box and came across a pair of Magnequest EXO-36 transformers. I don't remember what project I had in mind for them, so I thought I'd put it out to the community. What would you build with the EXO-36s? I'd kind of like a headphone amp. Would they be suitable for one?

Thanks,
Gary

 

[Paul Birkeland]

Those are really best suited for the 2A3 family of tubes operating by the book.  You could definitely make a headphone amp like this, it would just need some special attention to be sure the noise floor is nice and low.

 

[Uncle Scary]

Thanks Paul. Would they work in the Stereomour II amp?

 

[Paul Birkeland]

The Stereomour circuit originates from the original Paramour amplifiers, which used the Triad C-7X choke as a plate choke and the Speco T-7010 matching transformers as an output transformer.  The T-7010 had multiple taps on the primary, and the most sensible choice for operation with the 2A3 was the 4K tap.  Consequently, the operating point in the Stereomour runs higher plate to cathode voltage at less current to better match the 4K load.  When the Speco transformer was discontinued, we kept the operating point and dramatically improved the iron quality with our own parts. 

This is a long winded way of saying that the Stereomour operating point isn't correct for a 2.5K load, but you could certainly drop a little extra voltage in the power supply and run a little extra current, then use your output transformers in that configuration.  The one sticky issue will be the performance of our plate choke running 60mA vs. 50mA, PJ would have to weigh in on that one and you may end up having to settle for 55mA or some other middle ground.  (You could also just ignore this and try the 2.5K transformer in the stock circuit, there's no harm in doing that)

 

[Paul Joppa]

The PC-3 choke cannot handle more than 50mA. The Hammond 158L looks like a good candidate - 15H, 75mADC - it has the same mounting dimensions anyhow. But the Stereomour power supply puts out about 80 volts too much, which is difficult to burn off and calls for changes to the driver voltages as well. You really need a different power transformer.

The best bet is probably a new design, or adapting one of the existing DIY designs to use parafeed - there are several floating around the internet.

I feel your pain  :^)  I have a similar pair of orphan EXO-36s laying around, and have not yet figured out what to do with them.

 

[Uncle Scary]

Thanks Paul and Paul,

I would rather not try to shoehorn the EXO-36s into a design they were not intended for. It's a shame since they are the cobalt version.
I'll look into buying one of the Bottlehead headphone kits to satisfy my build urge.

Gary