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Re: [Amps] [rfamplifiers] Unexpected loading capacitance

Subject: Re: [Amps] [rfamplifiers] Unexpected loading capacitance
Date: Tue, 2 Jun 2009 21:48:22 EDT
List-post: <">>
Sounds like you might need more drive power. That would increase the plate  
current swing which will lower the plate load resistance. This would 
require  less plate loading C for maximum power output.
When you checked the output network with the 259B was the tube side of the  
network loaded with a resistor? If so, what value resistance?
Gerald K5GW
In a message dated 6/2/2009 7:56:50 P.M. Central Daylight Time, writes:

Looking  further into my lower-than-expected power output on my 3x813  

1.  Checked all tubes in my 813 supply one at a  time.  Only 1 out of 12 
had slightly lower plate current than the  rest.  I selected 3 'good' ones.
2.  Checked calibration on my Ig  and Ip meters.  Good there.
3.  Power supply sag on key down is  3100 down to 2850 on the high setting, 
and 2500 down to 2200 on the low  setting.
4.  Checked my input network again for any mismatches.   No problems there.
5.  Checked continuity of all  three plate  leads to the plate choke.  Good.

The result is still the  same:

- I have to use much higher loading capacitance than what I found  tuning 
the tank with the MFJ259B on the ouput.
- My power output is much  lower than I expected.  I should be making over 
1 kw, and I can only get  850 to 950w.

I'm using the G3SEK adjustable zener bias circuit for  about 3 to 27v of 
bias.  I brought the 10k adjustment pot out to the  front panel so I can have 
another knob to twiddle.

So, here's my next  questions.

1.  Do I figure input power based on the sagged or  unsagged plate voltage? 
 On 160m, on the "high" power supply setting  (3100v), I get 2850v and 
575mA at full 100w drive and just under 150mA  Ig.  That makes about 58% 
efficiency at 950w output.  I am thinking  to raise efficiency I need more 
bias?  I 
was hoping to run Class B at 65  to 70%.  Is that reasonable?

2.  Still can't resolve the  issue with way-higher-than-expected loading 
capacitance.  With the MFJ on  the ouput and the 2.5 mH output safety choke 
disconnected, I could get the  tank to tune to 1:1 SWR easily on all bands.  
Of course I reattached the  safety choke, and try it out with real RF, and 
the load cap settings require  much higher capacitance.

What to try next?


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