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Alex Pogossov Alex Pogossov is offline
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Default Overvoltage protection


"Phil Allison" wrote in message
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"Alex Pogossov"
"Phil Allison"
"Alex Pogossov"


In any case, an abrupt interruption of the plate current will cause a
voltage spike on the OPT primary. The stored magnetisation energy of
Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a
decent amp. If a load is connected, then the energy will be dissipated
in the load giving you a loud crack in the speaker. A residual smaller
energy stored in the leakage inductance Ls*(Ia^2)/2 will most likely be
safely dissipated in a snubber of say 2200pF+5K usually connected in
parallel to the OPT primary.

But what is the load is not connected? Then the huge magnetisation
energy


** Why huge ???

If Lp is say 100H and Ia = 0.1A then how many Joules is that ?


Apparently 0.5J.



** IOW - **** all.

Even the smallest Varistor you can buy will absorb it with ease.

Like this one for $ 1.58 + gst

http://au.element14.com/epcos/b72205...9?Ntt=100+4399

Agree, it is no problem to clamp the surges, but the point is: why I have
not seen varistors across the OPT primaries in various schematics? Is this
sort of protection not needed? Or is the assumption that no one will be
changing tubes on a working amp? Or occasional arcing and sparking is not a
problem?