Like Phil sez,. pay particular attention to the tank cct. This is a good reason
to use a vacuum variable for the tune c. The rf feed capacitor from the tube to
the tank should be robust and be able to handle 30 amps of rf current at 10m
since it has to handle the tank circulating current from the tank cct. since
the tube plate c is in parallel with the tune c. and in your case you have a
lot of plate c. with all those tube in parallel. Pay attention to the lead
lengths to the tank coil from the tubes and to the band switch. Every 1/4 inch
make a difference.
gl with the project es 73 Steve
ps The home brew 8877 amp here is within 3% output from 160m to 10m
"Phil (VA3UX)" wrote:
> At 01:47 PM 12/9/2000 +0200, you wrote:
> > Greetings, I have started work on a new amplifier (80 - 10 meters)
> >utilising the popular grounded grid KW barrier on output. Power
> >output is also way below what I expected .. and not breaking the 1KW
> >barrier. I run the valves with +/- 3,400 Volts on the plate and the
> >filaments have two transformers connected in parallel giving the 5 Volts
> >at +/- 15 Amps. I need advice on how to improve performance and limit
> >component failures.
>
> I wouldn't be surprised if some of the low efficiency problem is due to the
> design of the output tank network. Get a copy of Ian White's Pi network
> calc spreadsheet from his website and go through it from start to finish.
> You might be surprised to find that the impedance the tank has to match
> into (on the tube side) is quite different from the apparent calculated
> load impedance of the tubes. This can make an *enormous* difference.
>
> As an example, about 10 years ago I helped modify a dual 3-500Z amp here to
> improve the power output on 20 - 10 meters. We did two things : 1) took
> plates out of the tune capacitor to lower its minimum capacitance, and 2)
> slightly repositioned the inductance taps for 20, 15 and 10 meters. We
> changed nothing else. Result : power output on 10 meters increased from 850
> watts (before modification) to 1400 watts after modification. This is a
> clear indication that the tank was not presenting the proper load
> impedance. Noteworthy improvements were also seen on 15 and 20 meters.
>
> Having a tuned input will be another major piece of the puzzle in your
> case. Again, use Ian's spreadsheet for the network values. I also have a
> very easy-to-use spreadsheet for Pi and Pi-L calulations that can help you
> with this but it doesn't do some of the tube calcs that Ian's does. It's
> free for the asking.
>
> Phil
>
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