[Amps] 3-500Z plate durability

[R.Measures]

>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 -

**  Grid-I is usually defined as average indicated current on a typical 
meter.   A system for measuring actual dissipation would be considerably 
more complex.  In any case, grid-dissipation is not a problem when an 
amplifier is tuned for max-P out instead of max grid-I. 

> I would
>think dissipation and current measurements could be easily interchanged
>for display on a meter.

**  Hardly.  Grid-I looks nothing like a sinewave because it only flows 
when the grid is positive, which is less than half of the driving cycle, 
and even then grid-I is not at all proportional to  voltage.
 > ...