>With respect to 3CX800A7's, and the like, Tom, W8JI, writes:
>
>>Be sure to add a grid over-current protection circuit to your amp, if
>>it does not have one, or you will eat up tubes.
>>
>>A fuse is NOT enough, nor is a warning light. You need something that
>>quickly knocks the [name your amp] off line if the Ig ever gets over
>>[name your maximum comfort level] mA... even for a brief instant.
>
>Agreed.
>
>My copy of Bill Orr's Radio Handbook shows one version of a grid
>over-current protection circuit designed, as I recall, by W8ZR.
>Some years ago, a fellow (might have been ZR) was selling printed-circuit
>boards for his own flavor of circuit.
>
>What's the latest wisdom on creating a suitable circuit? ...snip...
It might be a good idea to take a look at the insides of a kaput,
out-of-warranty 3CX800A7 beforehand. To do this, clamp the kaput tube's
anode in a vice. Use a 32-tooth/inch (sheet metal cutting) coping saw
blade to cut through the copper just above the anode/grid ceramic
insulator. Be careful not to cut deeper than a few mm or you will saw
the grid. Rotate the tube occasionally. When you have circumnavigated
the anode with the saw, the anode cooler will lift off, exposing the
tender parts. Inspect the grid with a 10x or better magnifier. If you
see gold blisters and/or patches of missing gold plating on the grid, the
tube likely died from an intermittent VHF parasitic-oscillation. If most
of the gold blisters are nearest the grounded end of the grid (the grid
collet), and few or no blisters are in the upper half of the grid, the
tube may have died from an intermittent UHF parasitic-oscillation. You
may be able to see tiny gold-melt balls stuck to the cathode and to the
ceramic anode insulator. To confirm this, measure the resistance of the
VHF parasitic-suppressor resistor in the amplifier. If the resistance is
much higher than it should be, a parasitic oscillation is indicated.
- IMO, it takes a pretty sophisticated grid current protection circuit
to protect the grid during a full-blown parasitic-oscillation. Also, It
might be a good idea to put 20 or so ohms of glitch R in series with the
anode supply to limit the peak fault current that's available from the
filter capacitors.
good luck
Rich---
R. L. Measures, 805-386-3734, AG6K
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