[AMPS] 3-500Z on 50MHz

[km1h@juno.com]

On Thu, 19 Nov 1998 08:24:46 +0000 Mike Willis <m.j.willis@rl.ac.uk>
writes:
>
>I am building an amplifier for 6m with a 3-500Z. I have 2.6kV EHT, 
>plenty
>of drive and am looking for something like 500W of output. I can get 
>300W.
>
>I have had two problems. 


Three problems maybe Mike. I tried replying to your AL-80 on 6M query a
few weeks ago but the mail kept bouncing.

>
>Firstly a nasty parasitic at 190MHz that did not go away with the 
>usual
>shunt resistor on the anode link. It required an HF style parasitic 
>choke
>made from 2 turns of 1/4 inch copper sheet close wound on 3/8 dia, 
>shunted
>with 2 47ohm 2W metal film resistors. This works, but I worry about 
>losses
>at 50MHz.


If it doesnt burn up the losses are minimal.


>
>Second problem is the output circuit. It tunes OK but either the grid
>current is too high, or the efficiency too low. To me this implies 
>the
>impeedance transformation is not correct. 


That amp will easily do 650W+ on 6M with 60-70W drive.

What are you using for an input circuit.....you did not mention one.



The circuit is a pi-L with 
>(up
>to) 15pF Tune, 4T dia 2" of 1/4" pipe, 200pF Load and then 5T 1/2" 
>close
>spaced. 


I would scrap the PI-L for the time being and get it going with a
straight Pi. If necessary use a good external filter for
harmonics....input and output.


>I think I might need 5 turns on the first tank coil, but then the 
>tune
>capacitance will then be very low. The tune capacitor is a silver 
>plated
>wide spaced butterfly of 30pF per section with a very low minimum C.
>
>Good advice appreciated.


The tank coil can be very simple and still be efficient.  I use the
original SB-200 10-15M coil stock for 6M at 750W + and it is only #8
wire. 
Similarly I use original SB-220 parts at 1200W.
And LK-800A parts at 3KW.

With a single 3-500Z the tank coil can be 5T  #8  ( US gauge) of 1" ID.
Space  turns over about 1.5".
Or as in the SB-220 4T 3/16" tubing, 1.5" ID spaced the same. 

A GDO is most helpful. The Tune cap should range from 7 pf min to 15-20pf
max 
( easy to construct from higher value caps). With everything connected,
the tube in the socket set the Tune cap to mid range, the Load to about
150pf and then adjust coil spacing for resonance with the GDO. This will
get you very close.

Ig should be 125-150 ma and Ip around 400 ma when tuned
properly....assuming the tube had rated output on 10-15M in the stock
AL-80. 

73  Carl  KM1H


It >
>Mike
>
>
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