> Thanks for the suggestions, DD and Tom; I got into the
> input circuitry; Tom you were right on -- found the grids
> 'floating' through some bypass caps; RF was being fed
> through 10ohm resistors to the filament leads (probaby not
> to bad); the filament 'bus' grounded through 1 ohm/ 25pf
> series network... removing all of these extra parts and
> bringing it back to the schematic yields evenly-heating
> tubes on keydown, and 1300 watts output with 80w drive.
Brian,
As I suspected, your amplifier had some useless
modifications. These mods add needless parts that don't fix
a thing and really just potentially cause problems.
If you want a glitch resistor, use a high voltage pulse
rated resistor of some reasonable value. 10-20 ohms would be
the minimum useful value, and the resistor belongs in the HV
supply line just after the filter caps. The small glitch
resistors in your amp don't do a darned thing except cause
problems. The supply already has several ohms of internal
resistance, so a couple ohms in a resistor rated at only a
few hundred volts really doesn't change glitch current any
notable amount.
By all means just ground the grid pins. Ground every grid
pin with very short leads to the chassis.
Then you can get rid of all the problems waiting to happen,
and the next time the amplifier arcs stress on one tube
can't happen.
73 Tom
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