Mike,
According to the description of the 220 MHz W6PO amp in W6SAI's Radio
Handbook, here are the operating conditions with 3000V on the plate:
Single tone plate current 667 mA
Idling plate current 54 mA
Cathode zener voltage 12 V
Single tone grid current 48 mA
Power input 2000 W
Power output 1230 W
Apparent efficiency 61 %
Drive power 48 W
Power gain 14 dB
With only 20-25W drive, one might expect numbers roughly half of these
levels. Presuming you are using a 100 mA grid current meter, I would
think you'd easily see 25 mA of grid current and only about 333 mA of
plate current. Tube age and "goodness" could shift these numbers
somewhat. Still, I think you're seeing too little grid current and
slightly high plate current. This seems to indicate too heavy loading of
the amp.
Dave Olean's advice to move the loading plate away from the tube is the
right thing to do. Apply drive power carefully, dip the plate tuning
and note DC power input and RF output. Move the loading plate a little
closer to the tube and repeat the process, possibly with a little more
drive to keep the grid current in the 30-50 mA range. You should see
increasing power output with roughly 60% efficiency (RF output/DC
input). When the efficiency starts to drop off, you've gone too far.
Dave/K8CC
On 12/31/2014 8:19 AM, Dave Olean wrote:
I show very little grid current. Roughly 3000 volts .500 mill of plate
current 1500 in 200 out. 20-25 watts drive Ft736 driver. Mike
Those readings seem ok. I would think that 20 watts would drive it to
500 ma. I would make sure that you could move the plate loading
"plate" far away from the tube. Are you sure that the plate circuit
goes thru resonance? Maybe it never does! You might check that too.
Use a grid dipper or antenna analyzer.
If the loading control has little effect, I would also try looking at
the input with some sort of return loss bridge or antenna analyzer.
You could also put a Bird 43 type instrument in and see what the VSWR
looks like when you apply drive power. I remember I had a problem with
my new amplifier and it was a poor connection on a ground on the input
circuit around the tube socket. The input impedance changes with drive
level, so a VSWR check is only valid when the amp is being driven with
RF, but you should still see some sort of poorer match with almost no
drive.
Dave K1WHS
----- Original Message ----- From: "mike repinski via VHFcontesting"
<vhfcontesting@contesting.com>
To: <vhfcontesting@contesting.com>
Sent: Wednesday, December 31, 2014 4:16 AM
Subject: [VHFcontesting] 222 amp
One other observation. The controls do not seem to operate as sharp
as my 2 meter amplifier does. Kind of sluggish. The loading does not
seem to do much. Mike WB8PFZ
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