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Old 8th February 2016, 07:50 AM   #1
G is offline G  United States
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Default Anyone tried Plitron transformers?

My God! They must be wound with gold wire or something. I can build a entire amplifier with Magnequest transformers for what they are asking just for a pair of output transformers.
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Old 8th February 2016, 08:29 PM   #2
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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.

Last edited by obseedian; 8th February 2016 at 08:32 PM. Reason: Added PP
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Old 8th February 2016, 08:46 PM   #3
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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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