[NT]

On Jul 9, 3:08*am, "Alex Pogossov" wrote:
If you are experimenting on a running tube amp, you might be at some stage
pulling a hot output tube from its socket while the amp is running.
Alternatively you might accidentally send a high negative pilse to a grid of
the tube while working on the circuit.

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
will either cause:
- arcing in the tube;
- breakdown in the OPT winding insulation;
- breakdown of the subber capacitor (say 2200pF);
- arcing elsewhere in the wiring.or in the tube socket.

Would it be a good idea to placa a varistor rated slightly above the +B
across the primary? Or a spark gap of some sort, or gas discharge surge
arrester tube (the later have miniscule capacitance and very reliable). I
remember in vertical deflection stages of old TVs such varistors were a
must, since the abrupt cutoff of the current (during flyback) in EL84 often
used for this purpose was the mode of operation.

However I have never seen any discussions on the varistor protection issue
on this site.
What is your opinion?

Regards,
Alex

I've seen spark gaps on the primary on amps occasionally. But not
often.


NT