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?
Carl
KM1H
----- Original Message -----
From: "Steve Flood" <kk7uv@bresnan.net>
To: <rfamplifiers@yahoogroups.com>; <amps@contesting.com>
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
> KK7UV
>
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