I just received a pair of used UTC push pull output transformers with a tapped primary. They are designed for 5K or 3K primary impedance and judging by the remaining wires they were used as 5K primary in their former life. The odd (to me) thing I noticed is that they have a pair of 100V zeners wired across the 3k taps back to back or front to front (can’t tell which way). Any ideas why that was done?
Thanks,
Marty
Thanks,
Marty
It could be a protection against no-load operation, when a high-voltage, high-frequency oscillation occurs. This condition can lead to arcing, insulation breakdown, etc.
During normal operation, these diodes should never come close to conduction as they would introduce huge non-linearities
During normal operation, these diodes should never come close to conduction as they would introduce huge non-linearities
Not bad idea protecting OTs from arcing when unloaded but I am very surprised at the very low Zener value.
100V peak is "nothing" when we are talking 5k and/or Tube plates.
Which will have way more than that just as raw supply voltage, go figure.
Weird.
100V peak is "nothing" when we are talking 5k and/or Tube plates.
Which will have way more than that just as raw supply voltage, go figure.
Weird.
Well, they came to me in good shape so glad they did it for whatever reason 🙂
They are HA 135, I’ve been looking for a pair at a reasonable price for a few years and finally found them.
Oh, I'm very curious about those! I saw a batch for sale but decided I had enough stuff in the shed. ;-) The specs look great. I've built triode EL34 amps with LS-57s. Beautiful amps! What are you planning to do with the HA 135s?
Some of the manufacturers use protection diodes to prevent output transformer damage when the speaker is disconnected.
Attached is a data sheet of a typical diode used for such protection. Low voltage Zeners are never used .
Regards.
Attached is a data sheet of a typical diode used for such protection. Low voltage Zeners are never used .
Regards.
These will be used for push pull 6b4g amp probably along the lines of a brook 12A. The UTC transformers have worked well for me in the past especially the HA series. The power output choices are limited compared to LS series but the chance to try the high permeability cores in output transformers sent me on my quest.
They are "flyback" clamping diodes. Normally you just see high voltage diodes from the plates of P-P pentodes to ground so that high negative voltages on the plate pin-3 generated by the output transformer do not arc to the screen grid pin-4 terminal. This dumps current back to the PS and the diodes do not have to dissipate the energy.