Yes, Dynamotors were big, heavy and (P-in/P-out) inefficient. They were also durable and very tough to kill! They could survive severe overloads and keep on purring. This photo is what I believe to be (haven't seen one since 1967) the transistorized Multivibrator replacement. The Dynamotor and the MV version plugged into the main unit and held fast with sliding latches.
Transistors at that time line were Germanium junction. Few of us (not even the senior techs) had much experience with them. It was a Vacuum Tube world back then. How the hell these things ever passed MIL SPEC eludes me. Durability was not the benchmark of Germanium junctions. They'd blow their brains out with the briefest overload!
To answer this question:
CDdriver... That's very interesting... that crazy dynamotor is freaking big and heavy! And what kind of solid state? does it replace with? solid state relay?
The primary windings of a Step - Up Transformer formed part of the multivibrator circuit. The Secondary of the Transformer produced HV AC and was rectified and filtered.
Now that that little bit of history is out of the way let's get back to your topic.
As I stated earlier TRIAC's & SCR's (Thyristors) don't turn off merely because the gate signal has been shut off. An SCR (in your case) will require either of 3 things to occur. Anode to Cathode current is either cut off, bypassed or the polarity is reversed. Oh, there's one more thing that will shut it off. That would be that the Anode - Cathode current has fallen below the SCR's "Holding Current" rating. This value varies depending upon the model.
If I were you I'd forget using an SCR. For that matter I still reiterate ALL the relay benefits that Ed so clearly pointed out.
Chris