Using tube rectifier to as slow start in HT over 500V

[januaryabc]

It seems that the simple addition of a protection ss diode on the 6SN7 grid to cathode, and on the 300B would suppress any concern about high positive bias levels at power turn-on due to sequencing of the different B+ rails, and conduction delay differences of the 3 valves. With that setup, I'd think the worst case would be a relatively slow 6SN7 turn on, such that the 300B bias sat around 0V for some seconds, although increased 300B conduction would be alleviated by a rising cathode voltage so perhaps not too significant.

The 300B has > 800V across it during normal operation due to signal swing. I don't see that as a stress per se.

Any other stresses?

I not sure the actual position of the “protection ss diode” you mentioned and the way it works. Do you mind explaining further on that?

 

[trobbins]

Connect an ss diode such as a UF4007 from input grid to cathode. The ss diode is oriented to conduct if grid goes positive with respect to cathode, and so clamps that voltage difference between the terminals to circa 0.7V during initial power up if the B+ rails rise before the amp valves conduct. The ss diode is reverse biased for normal idle and signal swings (ie. it is fine for class A and AB1 operation, but would not be appropriate for AB2 operation). The ss diode junction capacitance varies with reverse voltage but is nominally low enough not to noticeably influence the high frequency corner of the stage (ie. driving resistance from previous stage and capacitance of the valve including miller effects).

 

[kodabmx]

A picture is worth a thousand words Like D11 in this photo.

 

[Ketje]

That diode is only usefull if the HT is applied with a cold tube.
Without it the full HT is between grid and cathode.For many tubes Vgk is max 50V.
Mona