[Amps] W6PO 2m amp

TexasRF at aol.com TexasRF at aol.com
Wed May 9 19:14:04 EDT 2007

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!
Gerald K5GW
 In a message dated 5/9/2007 5:32:30 P.M. Central Daylight Time,  
hduff at 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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