On Sat, 6 Sep 1997 00:44:51 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> From: km1h@juno.com
>
>> Directly grounding the grids in an otherwise unmodified 220 is
>asking for
>> fireworks and possible tube destruction. I also suspect that those
>stock
>> parasitic suppressors are junk by now.
>
>Nonsense Carl.
I guess we are both entitled to our opinions Tom.
Anyway I never said to install a surge resistor in the grid, that would
be a mistake since a true surge resistor should absorb the energy and not
act as a fuse.
What I did say was that the stock choke acts as a surge suppressor due to
its 25 Ohms resistance. That was probably a wrong choice of words for
this group. Due to the wire size the absorbing ability is very short
before it opens.
I STILL prefer the idea of the choke acting as a fuse in the absense of a
true protection circuit.
I know of at least one 2x 3-500Z amp made down in the land of cotton that
directly grounds the grids. When the tube arcs, all sorts of expensive
damage results.
The SB-220 and similar designs generally survive an arc with the tube
intact and require replacement of a few inexpensive parts. ( Assuming the
220 multimeter has back to back protection diodes or was not in the Grid
position when the bang occurred)
>The last thing you want, if the tube faults, is to have the grid fly
>up to 3000 volts.
>If the grid, because of series resistance, reaches anode voltage
>levels during a fault... the next stop for the arc is the filament
>and everything connected to the filament including the exciter.
I have never heard of that happening in any 3-500Z amp that uses grid
chokes! Your comment may be technically correct but a real stretch IMO.
Maybe we should ask Rich what the Grid- Filament breakdown voltage
is.....preferably with a non bent filament.
If your statement were true Tom, then the thousands of Heath, Kenwood,
Drake amp owners have lost thousands of rigs and kept silent about
it.......
>A HV series resistance is the way to go. Sticking a surge resistor
>in the grid path is unwise, to say the least. Anyone doubting that
>should ask Eimac or Svetlana.
73....Carl KM1H
>73, Tom W8JI
>
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