I Wrote:
> >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.
Carl replied:
> 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.
You are entitled to your opinions, but experience has shown
otherwise.
> Maybe we should ask Rich what the Grid- Filament breakdown voltage
> is.....preferably with a non bent filament.
Carl, think before you assume something needs to "arc".
1.) The cathode is emitting electrons, the grid is suddenly rapidly
slammed to the positive rail. When the grid is at 3000 volts, the
emission currents alone will be at the saturation point of the
filament, up around a dozen amperes.
2.) The tube, if it faults, most likely has some gas (unless the
anode welds broke, and the anode hit the grid). The dc test voltage
of a "cold good tube" NOT under a fault condition has almost NOTHING
to do with the breakdown voltage during an arc.
Finally, the reason a 220 has less problems with arcing than higher
power amps is the HV power supply in the 220 is LESS than AL-80B
standards. It was a 600 watt CW output design, NOT a 1500 watt output
PA.
> 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.......
I'm not sure about "thousands" Carl, but I know for a fact they have
lost "dozens". Usually the damage is small, confined to a protection
diode or solid state device at the receivers input. In rare cases,
the damage is more severe.
Carl, I respect you hands on experience but remember this. NO ONE
gets as much feedback about field failures as the manufacturer
does. Radio Kit is a very small fish in big pond. You have no idea
what field problems really are, because you see and hear only a tiny
tiny bit of what really goes on.
When you become one of the largest end users of 3-500Z tubes, or
start ordering 811A's at a rate approaching 1000 tubes per month, and
sell 200 or more new amplifiers a month your data stream takes on a
whole new level of accuracy. If you ignore any problems, you
simply go out of business (or sometimes in and out).
I doubt YOU, despite running a successful customer driven profitable
small business, can pull up a data base and track the service history
of two thousand new amplifiers a year, or look at the percentage of
returns on thousands of tubes in a year.
73, Tom W8JI
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