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DIY Stax step up transformer

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I'm looking to build something along the lines of the Stax SRD-7/ Woo Wee but with better iron in a lower turn ratio (maybe 1:10/1:15/1:20). Does anyone have recomendations for iron? Spritzer, do you offer your bias boards still? I'll keep my eye out for s SRD-7 with pro connection but in the mean time this could be a fun project.


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The lowest I've gone was 1:12 and it was just fine with more powerful amps. The 1:25 standard (some were even 1:30) heralds from the 60's so this ratio is utter overkill now.

I should have a few boards left but I'll have to check. I found one which is going into this SRD-7 that I'm converting but there should be a few more sitting in a box somewhere.

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Thanks guys. Spritzer, I'm not able to PM you it seems (too much of a NOOB around here). I would be interested two of the boards if you can part with them? I intend to pick up a SRD-7 at some point also (if you are parting with the one you are workin on, let me know).

Power wise, I have amps ranging from 14-60 watts. Any recommendations on which transformers to try?

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Hmm, it seems I didn't research the right places. From your past posts Spritzer, you have worked with the LL1630 and 1627. There is a guy on HF that had good luck with the Z565.

I'm still fuzzy on the math with this but I have a couple of sets of PP trannies on the shelf. Not sure on the turn ratio for either of them though. From my weak understanding, these are wired in reverse? So this would mean positive to (4/8/16 Ohm) tap, neg to neg (amp inputs), and on the other end, the split phases to the Stax plug. What do you do with the B+ center tap? I'm assuming this will go to your bias board? In the case of a transformer with dual phase inputs (for pentodes), do you tie them together at the stax plug?

Is there any other safety built into the transformer circuit or is it directly wired from speaker post-transformer-stax plug?

Here is what I've got on hand:

AS-250 (Magnequest version of the Acrosound TO-250), ultralinear, 5k, 4/8/16 Ohm taps http://www.clarisonus.com/Archives/Trans/Acro55.pdf

Heathkit 51-29, ultralinear, 8k, 4/8/16 Ohm taps

I forgot to ask, along with those boards would you have a couple of Stax pro plugs you could part with also?

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I've used pretty much every transformers I could lay my hands on over the past years but only the Lundahl's and some old Koss transformers have ended up as finished boxes. All of these were output transformers so you wire them up backwards, the B+ becomes the ground for the bias supply and the 8ohm taps are the input. You leave the transformers as if they were intended for triodes so all other taps are left unused.

Of those two transformers the AS-250 has a ratio of 1:25 on the 8ohm tap and the Heathkit has a ratio of 1:31 on the 8 ohm tap. You can lower the ratio by using the 16 ohm taps.

The way I build my transformers is just speaker connectors directly into the transformer and the output from them is taken directly to the Stax socket. Nothing else in there and I try to keep wires as short as is possible.

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