• 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.

Don't be stupid, you can't possibly do that with output transformers?

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Can I parallel two push pull ul output trannies, each with one primary side o/c?

The idea is that both secondaries are full parallel, the primary centre taps are connected and only the working half of each primary is wired to the valves.
They are matching transformers (same make and model) pulled from old record players, that commonly blow one side of the primary.
Plan B is to be a total pervert and run DC bias current through the secondaries, to cancel the magnetizing effect of the DC current through the primaries.

That would work best if the output tubes are arranged as a LTP, fed by a constant current source.

edit: E.g. The secondaries could be wired up like this:


  • bias perv.GIF
    bias perv.GIF
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Thanks for the replies. To give a little more information, I have a couple of old pye black box amps that have both blown their OPT. They are only simple EL42 push pull UL jobbies. I'm still learning about valve amps, having only completed one kit type 4x EL84 PP guitar amp so far. So for my next project I thought I'd go for a stereo pair of these as there are a few of these about on ebay.

Thomas - Thanks I read a bit about parafeed, and as I suspected the chokes would be more expensive than the transformer.

Flatheadmurre - I read a bit about SEPP, very interesting stuff, but I think the drop in performance would not be worth it from the EL42's. Plus, it would mean pushing more power through the surviving primary and I wouldn't hold out much hope of that surviving, unless output was to be halved (definatley not worth it for the EL42's!).

Godfrey - Nice idea, I'm sure I could get that to work, but with the extra cost of parts including decent, very high value caps to block the secondary DC, I don't think the circuit would have any real fedelity unless I spent more than the cost of a new transformer on the parts.

I have decided to keep it simple, and buy some Hammond OPT's. I figure this will also provide me with better specked OPTs, and hopfuly a better output fedelity. I will of course keep the iron, and maybe one day, I'll have a play with a couple of the suggestions.

My last question is, with the EL42's prefering 9K Ohms primary load and my speakers asking for 6 Ohms output load, I'm thinking the 8K to 4/8R 10W model is the best fit for my needs (I know 10W is well over spec, but thought for the price difference it would be worth it for the extra headroom).

Thanks again.
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