Does it happen on all bands and into a dummy load? If so I'd suspect a tube.
If it happens on a band that fixed capacitors are switched in then I'd
suspect them.
There could also be something heating up in an antenna such as a trap,
toroid/capacitor network as in Cushcraft verticals or similar.
If the amp has been used a lot on WARC then it may be a plate choke with a
very close resonance.
Carl
KM1H
----- Original Message -----
From: "Martin Sole" <msole@loxinfo.co.th>
To: <amps@contesting.com>
Sent: Saturday, November 03, 2007 9:07 AM
Subject: [Amps] Drifting grid current
> Having read with interest the recent 8874 thread I am prompted to ask a
> question that has been bugging me for a while.
> My Alpha 78 with what I believe are 3 NOS 8874 tubes, 2 from one source
> third from another does about 1500 watts out key down. Plate voltage
> shows about 1800 at that time, about 2150 to 2200 off load. Grid current
> runs about 80 to 90 mA but if I key up for a while then key down again
> the grid current first registers just over 100mA and then over a period
> of a few seconds drops back to about 80mA. Power out does not seem to
> change.
>
> Is it something getting hot? Drifting? Or is it just normal?
>
> Thanks
>
> Martin, HS0ZED
>
>
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