Tom Schlangen wrote:

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).

Bias should control itself, with cathode bias, or when fixed bias is
adjusted. But tubes drift, or give trouble sometimes,
so some sort of B+ interupter is needed...



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...

The HT winding is usually up to 350 vrms and since its AC, there is no
problem finding
a relay to switch it. Heavy duty relays for 240v rated for 10 amps are
usually fine.
I have a cap across the contacts to stop arcing.

In my 300 watt amps the HT is 200vrms, less than the mains, since I use
a doubler.

The 240v mains relays are having it easy there.

Maybe a triac could also be used.


Patrick Turner.



Tom

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