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Re: [Amps] [rfamplifiers] Unexpected loading capacitance

To: "Steve Flood" <>, <>, <>
Subject: Re: [Amps] [rfamplifiers] Unexpected loading capacitance
From: "Carl" <>
Date: Tue, 2 Jun 2009 22:09:08 -0300
List-post: <">>
I agree with Gerard about needing more drive.

Also what are you using for a plate choke?

Coupling cap?

I assume the safety choke is at least 1.5mh

Most amps come up a little short on 160. What about 40?


----- Original Message ----- 
From: "Steve Flood" <>
To: <>; <>
Sent: Tuesday, June 02, 2009 9:55 PM
Subject: Re: [Amps] [rfamplifiers] Unexpected loading capacitance

> Looking further into my lower-than-expected power output on my 3x813 
> amp....
> 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?
> Steve
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