[AMPS] 3-500Z on 50MHz

[Rich Measures]

>
>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. 
>
>Firstly a nasty parasitic at 190MHz 

What is the resonant frequency of the grounded grid?  //  One means of 
reducing feedback at c. 190MHz is to connect one 10-ohm 2w mof resistor 
in series with a 20 or so pF cap. from each fil. pin to gnd.  The lead 
lengths should be minimal.  The idea is that the intrinsic L cancels the 
cap. reactance somewhere around the parasitic frequency, thereby swamping 
the input element at the parasite f..  Unfortunately, this envolves a bit 
of estimation based on how much inductance is between the fil. (the sig. 
input element) and the socket pins.  

>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.
>
Any signs of severe heating on the mof supp. resistors?  What is the 
current resistance?

>Second problem is the output circuit. It tunes OK but either the grid
>current is too high, 

Do you tune for max. Pout with c. 65w of drive?

>or the efficiency too low. To me this implies the
>impeedance transformation is not correct. The circuit is a pi-L with (up
>to) 15pF Tune, 4T dia 2" of 1/4" pipe, 

At 50MHz, due to skin effect, larger copper tubing in the tank might help 
reduce tank loss.  .  Identifying tank loss sources can be done by going 
hammer down for a minute, wait 5 sec., unplug the amplifier, connect a 
100w 500 - 1000 ohm resistor across the hv filter cap., stand on a 
plastic matt, and touch suspect areas in the tank with one finger before 
they have a chance to cool.  If you have an IR-capable camcorder or a IR 
thermometer, that's even better .  

>200pF Load and then 5T 1/2" close
>spaced. 
>
>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.
>
-  later, Mike


Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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