Schematic help

[Wasnex]

I believe the function of the diode protection scheme is not for the diodes to start conducting if the voltage becomes excessive - it's to prevent a situation in which the voltage goes negative because of impedance mismatch.

Consider what is going on in the output tubes and transformer when it is trying to develop high power into too high an output impedance. Current through one output tube is decreasing, the other is at high current. The voltage at the transformer primary is high on the first tube (with low current flowing) and low at the second tube conducting heavily. The mutual inductance of the two halves of the primary forces the voltages to be equal magnitude but opposite polarity relative to the center tap. For AC purposes, the center tap is grounded (the DC voltage is high, but constant).

Eventually the decreased current draw on the first tube will force the opposite side of the primary (i.e. the voltage at the plate of the tube conducting heavily) to go negative. At this point, the tube that was conducting heavily shuts off suddenly (it's acting like a diode). The energy stored in the magnetic field produced by that current has to go somewhere! The voltage across the primary spikes to try to keep current flowing - this is the "flyback" condition that causes arcing. The protection diodes give that energy a place to go by allowing the current to continue to flow (shorting the spike to ground).

If the impedance is matched better, the first tube cuts off while the second is still conducting. The second tube's plate voltage remains positive, that tube continues to conduct, and negative feedback in the power amp forces that tube to do all the work. With the impedance matched properly, this all happens while the second output tube is still n a range where it is capable of operating linearly. This is class AB. And it's why impedance matching is so important for tube amps.

Edit: minor clarification.

This is the explanation if I have read as well. With the diodes paltry 50uA max reverse current rating I am sure they would fry quickly if they were pushed into reverse current, and they probably do. Perhaps this is why the Ashdown amps use 3 diodes in series instead of 2 like the older Trace Elliot designs.

 

[agedhorse]

Different schemes protect the transformer but with different fault current paths. The TVS across the primary limits the differential mode voltage across the primary in this case.

 

[Bertr]

I don't know which one could have saved the Major:
https://www.thegearpage.net/board/index.php?threads/suggest-kt88-amps-to-me-in-here.1093675/page-2
A bit OT and not sure about its validity, but it´s a good story.

 

[rickdog]

Rick you dog, you’re making my head ache!! LOL

Lots of words trying to make it clear to people who haven't worked with tubes like you have

Different schemes protect the transformer but with different fault current paths. The TVS across the primary limits the differential mode voltage across the primary in this case.

Right. It doesn't matter where the fault current goes but it has to go somewhere.

 

[BogeyBass]

nah, tubes are fuses

there is no need for protection diodes.
you just do it like Ampeg and destroy the tube instead. duh

or just under rate the resistors and let them pop instead