Sounds like a classic case of low drive power. That tube typically has a
gain of around 14 dB with plate voltage that low.
If the cathode bias is set to a level higher than optimum, the gain could be
even lower.
If you increase the drive level I bet you will see both plate current and
power output rise dramatically. As this happens, the grid current will also
start showing, possibly requiring readjustment of the plate tuning and loading
to keep grid current at the proper level.
The input circuit will require adjustment as the amplifier power increases,
to minimize the input vswr. Cathode impedance is heavily dependant on cathode
current which is the sum of plate and grid currents.
Sounds like you are on the verge of having a well working 2m PA!
73,
Gerald K5GW
In a message dated 5/9/2007 5:32:30 P.M. Central Daylight Time,
hduff@cogeco.ca writes:
Hello all !
I'm testing the homebrew W6PO amp that I just completed and I seem to be
having trouble getting more than 300 watts out of it.
Before I powered it up I tuned the input and output sections for minimum
SWR at 144.1 MHz using an MFJ-259 antenna analyzer.
I have the amp connected to an FT-847 exciter and a large Bird dummy
load. HV is between 2500 and 3000vdc on the plate. I see about 300mA
current on the plate when I'm getting 300 watts out but very little
current on the grid meter, which I find a bit odd. I 'm using a G3SEK
Triode board to provide the bias and control. I do see 12vdc bias on the
cathode during TX.
The 3CPX1500a7 tube was purchased as a medical pull. I have no way to
verify its condition so there is always a chance it's no good. This is
my first big tube amp so I'd like to make sure I'm not doing anything
wrong before I think about buying another tube.
Any guidance will be appreciated.
(Pix of the amp can be seen at http://www.va3to.com )
Thanks, Hugh VA3TO
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