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mick mick is offline
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Default Overvoltage protection

On Sat, 09 Jul 2011 21:25:39 +1000, Alex Pogossov wrote:

"Phil Allison" wrote in message
...

** Cos the usual RC network suppresses the spike sufficiently, along
with the load.


If a load is connected -- correct. If no load, then as I have mentioned
you need microfarades + several kiloohms to absorb it, not a few
nanofarades usually connected to dump HF ringing of the leakage
inductance.


Or is the assumption that no one will be changing tubes on a working
amp?


** Well, not with no load connected.


What about an amp being fool proof then?

Or occasional arcing and sparking is not a problem?


** The Aussie made 6V6GT used in my first valve amp liked to spark
internally with no load. Never did it, the A&R OP tranny or the socket
any permanent harm. But if you wanna go belt and braces - fine with
me....

BTW - how much HT have you got ??


Only 250V. But I do not want to risk any possibility of insulation
breakdown in an old lousy OPT.
Probably a small varistor rated at 350...400Vdc and with less than 500pF
of capacitance will be OK.

The tread is also of general nature -- why do not we see varistors in
the tube amp circuits. Probably you are right -- no one suffered serios
damage of this particular stress mode.



It's probably traditional. :-) Small, effective varistors are a newer
invention than valves. The two would never have met when the circuits
were being developed. That, and engineers having it hammered into them
that you *never* run a valve amp without a load.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
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