• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Soft Start for Tubes


2006-10-10 8:44 pm

I'm considering the P39 Soft Start circuit from Elliot Sound.

What is a reasonable delay time to use?

Please clarify for me the sequence. Should both the +B voltage and filament voltage increase slowly? or one should turn on before the other?

I could have 2 seperate soft starts circuits but that might be over doing it.

Thanks and have a fruitful New Year! :D


2006-12-28 12:19 am
you might want a soft start for the heaters if you're trying to minimize inrush current, but the B+ doesn't need a soft start, since tubes are by their nature "soft start" devices. the heaters draw a lot of current when cold, but the resistance rises as they heat up. all a soft start will do is limit the cold-heater inrush current.
what about cathode stripping when the HV hits the still cold plates? I understood "slow start", i.e. delayed start on B+, is a good thing.

That is true. Amps that use tube rectification don't need a soft start B+ because the rectifier takes time to warm up and conduct. If you’re using solid state rectification it is a good idea to have a soft start circuit to both minimize cathode stripping and to avoid excess current peaks.

I'm not familiar with the P39 but this is usually what I do.

If the first filter cap on your B+ filter chain is small-med sized
1) Turn heater on
2) Delay 30 seconds for B+

If the first filter cap on your B+ filter chain is large sized
1) Turn heater on
2) Delay 30 Seconds for B+ with a resistor in series between rectifier and filter capacitor. (Is to avoid loading down the transformer, and exceeding peak current ratings of other devices)
3) Delay relay #2 another 10-15 seconds for direct (short out the series resistor). This will directly connect the rectifier to filter capacitor.

If you have an extra relay not being utilized, replace step 1 of the above sequences with the following two steps.
1) Turn heater on with resistor in series (small in value, just large enough to help suppress the initial current spike)
2) Delay 5-ish Seconds and have the relay short out the series resistor.