• 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.

SSR switching HT transformer?

Hello, I have been thinking about using an SSR to switch the primary of my GM70 projects HT transformer. I have just noticed in MJs latest book that he says he is not sure about this because of problems during the zero crossing (page 415 "the switch-on surge")

Has anyone had any problems using an SSR or other triac/dual SCR type switching device in an amp?

Also I always thought that switch on surge was greatest if you switch on at the zero point because the waveform is rising in which ever direction and the current very rapidly increases to the peak dictated by wiring impedance etc. Morgan Jones states it is at the peak on either side of zero but I can see the current only going down from this point.


Last edited:


2005-08-21 10:46 pm
If you switch at zero voltage, the voltage across the primary and the primary current rise with the sinusoidal waveform of the mains.

Switching on at any other instant applies a large voltage across the primary *instantly*, leading to a switch-on surge.

So switching at the zero crossing of the mains is best. You can get SSR that automatically detect the zero crossing and switch then, usually they are only slightly more expensive than the randomly switching ones. Worth the money.