>On Wed, 10 Dec 2003, Joe Isabella wrote:
>
>> In my amp, when I transmit (espeically in the typical long-winded 350W
>> carrier AM transmission), my RF Parts 3-500ZGs get quite red (almost
>> orange). After 1 year of pretty consistent use (abuse??) like this,
>> they still run full output.
>
>What's the grid current at that point - 80mA or thereabouts? Plate
>current?
>
>I had a couple QSOs early this morning on CW and noticed that the plates
>begin to show a faint bit of color near the bottom of the plates - the
>tops near the cap stay gray.
>
>
>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.
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. Naturally, when this is happening,
P-out drops off. Thus, for a cathode-driven amplifier, tune for P-max
and relax.
>
>> I think the BIG difference between these and the sweep-tubes you
>> reference it is that they're just not transmitter tubes and aren't built
>> handle it.
>
>Must be - I should swap the filter capacitors out for new ones in the AC-3
>and get that thing out of mothballs. Then again, I like my Ten-Tec
>Jupiter... i'll let the Drake age a little more.
>
>
>
>Thanks,
>Jason
>
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