The normal filament current is about 14 amperes at 5 volts. The grid/filament
short generally draws less than 100 ma of grid current and less than 300 ma of
plate currnet. An abnormal current of less than 0.5 ampere is not a problem
for the major floating secondary winding of a filament transformer.

In the TL-922, the filament transformer would burn after a short while because
on that same transformer there is another secondary (80 Volts) which gets nearly
shorted to ground when a filament-to-grid short occurs in a tube.

73

Tony I0JX

"Antonio Vernucci" wrote in message
...
The normal filament current is about 14 amperes at 5 volts. The
grid/filament short generally draws less than 100 ma of grid current and
less than 300 ma of plate currnet. An abnormal current of less than 0.5
ampere is not a problem for the major floating secondary winding of a
filament transformer.

In the TL-922, the filament transformer would burn after a short while
because on that same transformer there is another secondary (80 Volts)
which gets nearly shorted to ground when a filament-to-grid short occurs
in a tube.

73

Tony I0JX


Dear All,

Update:-

Had the case open and indeed 1 of the 3-500zs was showing a grid/fil short,
however a little inadvertent rough handling cured this and when reassembled
the amp worked fine for an hour or so. Then it was switched off for lunch
and after lunch the problem had returned and this time is persisting despite
removal of the valve. I haven't yet tried the mains blast across the pins
as I'm not that confident!

good news is that the filament transformer seems fine and hasn't been
apparently damaged by the few very short excursions into short territory.

A new valve will be ordered. May try the whisker zap if feeling brave at
some point

Learnt a bit today, I now have better understanding of the innards.
Interstingly the previous owner had done some but not all of the Measures
mods, e.g. cathode and low Q VHF parasitic suppresors.

thanks for all the advice

Jim G7NKS

"bob.martin" wrote in message
...

" and after lunch the problem had returned and this time is persisting
despite removal of the valve. I haven't yet tried the mains blast across
the pins as I'm not that confident!

e advice

Jim G7NKS


You can use a charged cap to "blast" the short, might be less
dramatic than the unlimited current behind the mains.. This
sometimes vaporizes part of the grid structure and affects
tube operation..

Pete

"Uncle Peter" wrote in message
...

"bob.martin" wrote in message
...

" and after lunch the problem had returned and this time is persisting
despite removal of the valve. I haven't yet tried the mains blast across
the pins as I'm not that confident!

e advice

Jim G7NKS


You can use a charged cap to "blast" the short, might be less
dramatic than the unlimited current behind the mains.. This
sometimes vaporizes part of the grid structure and affects
tube operation..

Pete
Well done to you all,it's nice to see real amateur radio for once on this
board.More of it please
Keith.