[AMPS] Re: 3CX800 failures

[W8JITom@aol.com]

When I was prototyping some 3CX800 amplifiers, I ran them without grid trip
circuits.
I found I could waste a tube in  a matter of a second or less if the load was
shorted and full drive applied. Even mistuning the PA's during tuning would
cause accumulated damage after a period of a few weeks. 

Remember in a grounded grid PA the input and output circuits are in series.
That generates negative feedback, which couples most of the drive power
through to the output. Only a small amount of drive power is applied to the
grid in normal operation.

The amount of negative feedback (and driver feedthrough power) is controlled
by the ratio of anode load impedance to cathode driving impedance. As the
anode load impedance is lowered negative feedback is increased and it takes
more drive to produce the same grid current. If the anode impedance is
raised, it takes less drive to produce the same grid current.

Grid damage is a function of time and the amount of overload, and a 100 watt
exciter can wipe the 3CX800 grids out in less than one second if the anode
impedance soars. The grid circuit MUST have a fast electronic disconnect to
prevent damage.

I would never try to operate a PA using ten watts of total grid dissipation
with a 100 watt exciter unless the PA had electronic overload protection that
would knock the PA off line if the grid was overdissipated for more than a
few milliseconds.

Even if the failure isn't instant, the accumulated damage would eventually
ruin the gold plating on the grids after a series of quick accidental
overloads.

73 Tom

--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Sponsored by:             Akorn Access, Inc. & N4VJ / K4AAA