[AMPS] Building AMP with four valves in parallel (4-250) - any guidance ?

[Phil (VA3UX)]

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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