On Dec 31, 2014, at 10:37 AM, David Pruett <k8cc@comcast.net> wrote:
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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