• 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.

Anyone tried Plitron transformers?

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I have built amps with these. The big problem with them is their sensitivity to both DC and AC imbalance. This is especially a problem when applying GNFB around the amp as it can exacerbate core saturation during large transients.

The best sound that I was able to get from these involved PP class-A DHT output stages with no feedback. In that application they had superb transparency and low level resolution (assuming you had a clean B+).

While that is no doubt true for any OPT, I found it to be especially the case with the Plitrons.
 
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I have. They have very low winding DCRs so they work very well for making an amp without global feedback that still has a low Zout. My amp's Zout is just over 1 Ohm, no feedback around the output transformer. It is a Unity-Coupled amp so it has very heavy local feedback in the output stage that drives the transformer.

I also tried to find the -3dB point on the low end and found that my measurement setup could not get low enough. I was still at full amplitude at 10 Hz. This was small signal bandwidth. The transformer spec was for -3dB @ 500kHz on the high end but my driver stage ended up limiting my amp bandwidth to 120kHz. Probably not a bad thing.

I used the Guido Tent/Van der Veen bias servo to maintain tight DC balance. It works very well but adds a minute or two to the time it takes for the amp to be ready to play music. It starts at full negative bias with a delay until the tubes warm up and then slowly reaches target bias current, about 30 seconds after the delay times out.
 
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