Date: Sat, 17 Aug 2024 20:51:43 +0000
From: "n4is@comcast.net" <n4is@comcast.net>
To: "amps@contesting.com" <amps@contesting.com>, Dennis W0JX
<w0jx@yahoo.com>
Subject: Re: [Amps] 3CX800A7 Grid Current
<I switched to a pair of 3CPX800 20 years ago and they are full power
today. The <3CX800 <is sensitive to plate load. The way to tune is to
increase the load ( open the <capacitor) <and drop the grid current, peak
again the plate capacitor to max output and <repeat the <process, increase
load to reduce grid current and peak plate capacitor, you <should find a
<load good for max power with lower grid current, max 50 mA per tube, <my
tubes run at 20 <mA grid current each for full legal power.
<My two cents,
<73'
<JC
<N4iS
Smart move. The 3CPX-800A7 is the pulse rated version, and will handle a
lot more B+ vs the plane version. (3500 vdc vs 2250 vdc).
Years ago, Eimac made cavity amplifiers for FM broadcast, using a single
3CX-800A7 (750 w PO)....and also a 2 x tube version (1.5 kw PO).
Later on, Eimac issued a bulletin for end users of the above 2 x amps to
switch to the pulse rated version. Reason was.... the tubes were being
destroyed by anode to grid arcs....due to transients / spikes / surges /
hiccups etc...coming in off the street via the 230/240 vac 50/60 hz
incoming power.
Both versions of that tube only have floozy 4 watt grids in em. Any anode
to grid arc will take out the tiny grid in an instant, even with a hd 50
ohm glitch resistor and fast HV fuse in the B+.
Jim VE7RF
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