[VHFcontesting] 222 amp

David Pruett k8cc at comcast.net
Wed Dec 31 10:37:58 EST 2014


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.


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 at contesting.com>
> To: <vhfcontesting at 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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