[Amps] 3-500Z plate durability

[jsb at digistar.com]

On Fri, 12 Dec 2003, R. Measures wrote:

> >I have to learn to be more careful though - i've been sloppy keeping
> >the grid current under 260mA when tuning.
> >
> ** 3-500Zs have no grid-I rating.  They have a grid-dissipation rating
> of 20w -- which can be exceeded (as in glowing grid wires) for brief
> periods without damage.  Grounded-grid/cathode-driven amplifiers should
> not be adjusted with the grid-I meter.  Instead, they should be tuned
> with max rated drive, preferably pulsed, and adjusted for max P-out with
> the Tune and Load controls.  With P-out peaked, grid-current will always
> be ok because excessive grid-I will always rob electrons from the
> output.

I wonder why grid current is measured instead of dissipation - I would
think dissipation and current measurements could be easily interchanged
for display on a meter.

>   Also, because grid-I only flows when the anode is not very positive,
> too little grid-I tells us that the anode-V is not swinging low enough
> during the peak in cathode-emission.

Interesting - I thought Ep dropped because of traditional voltage drop and
because typical plate voltage supplies are not regulated to the degree
that filament voltages are regulated - so plate voltage drop is a
necessity?  Or should I read the drop as an indicator of proper electron
flow?

> Naturally, when this is happening, P-out drops off.  Thus, for a
> cathode-driven amplifier, tune for P-max and relax.

... and be sure to stop applying additional drive when P-max is reached?

My usual method for tuning is to apply drive, look for maximum output,
continue increasing drive and tuning for max output until one of the smoke
alarms goes off (meter shows max acceptable plate current and/or grid
current).

I have also been experimenting with touching up the load to lower grid
current with the result of increasing power output.  This seems to yield
an additional 200-400 watts at best over 1000 tuned to peak output.  I've
mostly abandoned this practice for the most part because of the scant
increase in signal strength by the receiving station (most of my operating
is on CW), however it does seem to help on SSB - however I do not know if
the additional 200-400 watts are comprised solely of of splatter with no
beneficial yield to the receiving station on the intended frequency.


thanks,
jason