Hi Patrick,

now, that's an extremely useful circuit to protect
the tubes (and ourselves and wallets...). Thank
you for sharing it!

I think most of us forget about such "paraphenalia"
all too often.

I for myself only came up so far with a very basic and
simple bias control circuit using the 4x comparator
IC LM339 (so 4 tubes could be monitored) that shows
"too much bias deviation" by a LED per output tube,
but it has no active protection facilities (for example
cutting B+ or mains).

But finding a suitable relay capable of reliable
switching the B+ rail (and all that stored energy in
the filter caps) at, say, 350V-500V and some hundred
mAs continous current running seems to be no easy
task for the electronics parts shop around the corner...

Tom

--
Falling in love is a lot like dying.
You never get to do it enough to
become good at it.