>> I would still like to see a resistor between gate and cathode to
>> lower the sensitivity to stray hum and EM stuff triggering the SCR
>
>What would you suggest ? I see quotes that some SCR in-built
>resistors are as low as 20 ohms. Others suggest 100 ohms, eg

Hmm, in view of the method of driving, I would use somewhere between 1k & 
10k, and do away with the 1n4007 entirely by shorting it out. 

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