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OPT for PP and SE

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I have purchased a pair of secondhand OPTs from Audion, as used on the Silver Knight MKII.

According to the notes about strapping the primary windings, the transformers can be used for both SE amps and Push Pull. I knew that an SE OPT has to have an air gap to restrict the flow of DC to prevent saturation, so assumed that an SE OPT would only be used for SE applications.

Is it so that an SE OPT works well for PP? If so, these OPTs should give me a lot of flexibility in their use. They have 4 primary windings, and a UL tap is one possible combination.
1. For SE you do need an Air Gap to get good results.
All the E's on one side, and all the I's on the other side, with an air gap between the E's and I's.
Usually the 'air' gap is just a non magnetic spacer, such as paper, kapton tape, etc.).

Of course if you want to use a surplus 12kV to 440V power mains transformer from the power company, you do not need an air gap. The hundreds of pounds of laminations can take lots of DC from your favorite output tube (non RF transmitter extreme high power tubes, and non Radar extreme high power tubes). Your amplifier DC is not going to saturate the interleaved laminations. You can use it for an SE amp 5950 Ohms to 8 Ohms.
Silly, huh?

2. For Push Pull, the transformer can work with an Air Gap.
The main tradeoff of having an Air Gap in a push pull transformer, is the primary inductance will be less. Less versus with the same coils, and the same laminations but that have the interleaved E's and I's of most push pull transformers.

3. I built an SE amp with a 12AT7 driver, an EL34 output driving a push pull transformer that I re-arranged the laminations:
All the E's on one side, Kapton Tape spacer ('gap'), and all the I's on the other side.
The EL34 cathode was connected to the 16 Ohm tap, and Common tap to ground. The primary was connected to B+ and Plate, so that the phase of the EL34 cathode feedback was Negative feedback. The cathode feedback was very moderate . . . 3dB.
The amp frequency response was -3dB from 15Hz to 60kHz.
Not bad for a modified push pull transformer that was modified for SE.
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If you use the whole primary (one end at B+, and the plate at the other end), then you could use the center tap as a 50% UL tap.

However, a center tap almost always means the transformer is for push pull, and almost always means there is no significant air gap.
So . . . you will have to re-stack the laminations into E's and I's with an air gap between them, or it will not work very well.
I have done exactly all the above.
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