Limiting no-load output voltage of a tube amp power supply

Question

I am designing a power supply for my tube amp. Here's what I have come up with:

Duncan's PSU designer shows that it should work OK (and provide 256V at 320mA). However, should one or both output tubes fail, the voltage would rise to 550V, which the other tubes may not like. Also, since the rectifier is directly heated, it will warm up much faster than the output tubes, probably resulting in too high voltage for a brief time after turn on.

So, I need to somehow limit the output voltage. However, I do not want to use any semiconductors anywhere in the amp. So far, I have come up with one way to do it and want your opinion on it or other suggestions:

^{simulate this circuit – Schematic created using CircuitLab}

R3 is made up of 10x 100Ohm 10W resistors, R2 value calculated so the relay turns on when the voltage is too high and turns off when the voltage gets too low (output tubes warmed up etc).

This would probably oscillate if R2 is not calculated precisely enough or with line voltage variations. Also, it would result in an overload of the power supply when the relay is active and the output tubes start working.

Are there any better ways of doing this?

Answer 1

Use a vacuum tube time delay relay. (Check Ebay)

Hook it up across the mains, with the NO contacts in series with the HV output.

Then, when you turn on the amp, the heater in the relay will close the contacts some time after the tube filaments/heaters warm up and supply HV when everything is ready for it.

Answer 2

I have solved the problem. Because I need a low current stabilized voltage too (for the input selector - the cold cathode thyratrons I am using self-start at 260V), I used a gas stabilizer SG-1P (140-150V) and a 10K resistor. In addition to making the input selector work properly it now makes the voltage at start not go over 460V. If I want to bring it down still, I guess I can just add a bleeder resistor.