I think my particular problem was a flashover in a pesky new production, GZ34 indirectly heated rectifier.simon wrote: ↑Mon Jan 08, 2018 11:21 pm It's problems like this that make me reluctant to move from an indirectly heated rectifier - the slow start hides a multitude of my sins. Even speccing a bleed resistor to draw enough current to ensure the PS always stays in choke input, and making sure the choke has enough capacity for the DC and AC components, I still can't bring myself to try SS rectifiers (which some say are superior). Just one of my hang ups I guess.
That firework display buggered up the choke, as being directly in the line of fire, it took the full blast of the flashover. One of the two SS diodes in the hybrid bridge was also destroyed.
What I'm saying is that it wasn't the type of rectifier that was to blame, it was the fact that the valve was faulty in the first place.
More subtly, the aftermath of the flashover also resulted in a sparky choke developing, which in turn, sparked up an old Bentley 5U4G, provoking that into another firework display, destroying it and half of the hybrid bridge again.
Upping the ante with a tough Russian harp filament 5U4G allowed the amp to start up, but if I hadn't discovered the sparky choke, then it was only going to be a matter of time before the startup failed again, and blew up something else, so as Mark said earlier, it was fortunate I spotted the secondary culprit and dealt with it.
So I wouldn't worry about playing around with DH rectifiers when you have DHTs as output valves. They are no worse than an IDH rectifier, as the DHT output valves will heat up and start drawing pretty sharpish. IME there is very little HT overshoot because of the similarity in the rectifier filament type and its heat up characteristics, with those of the DHT.
They have about the same voltage drop as a GZ37.