[AMRadio] Pi-Net vs Link Couple


W5OMR/Geoff w5omr at satx.rr.com
Thu Sep 22 17:59:31 EDT 2005


Byron Lichtenwalner wrote:

>Geoff
>Your last discussion on the L/C ratio of the tank intrigues me.  Was the
>circuit not resonate with the extra (now removed) turns?
>
>On the link tuning, what series cap. value are you using?  Does it load
>heavier when at max. cap or minimum?
>
>Byron, W3WKR
>  
>

It was resonant, but I wanted to draw a bit more current in the final.  
Not necessarily at the 4kV of DC that a 250TH can take, (4kV @ 250mA 
would be quite suffiecient, as long as I had enough drive to compensate 
for exceeding the plate dissapation by only running 45mA of grid drive 
per tube - for a pair, I'd need more like 150mA at that kind of current) 
but what I wanted, was around 1500v @ 350mA.

I even started using a 40m coil (since it only had 2 turns less than the 
modified 80m coil) and added some capacitance from an added split-stator 
cap to resonate the final on 75m, but I find that the last 100 to 150mA 
of plate current does not result in an appreciable increase in power 
output.  Yeah, I could get 400mA @ 1500v, but the output was only around 
200w.  Changing back to the previous coil, I can make around 350w of 
carrier input, but the plate current is only around 240~250mA.  1500v @ 
300mA = 450w DC input and I'd like to keep the current up, while I 
reduce the voltage.  It would also be around 5000 ohms of Z, vs
a reduced current at higer voltage (say 1500 @ 100mA) is 15000 ohms.  
HUGE swing in impedance, which makes for a huge difference in audio.

If I could get a starting figure on what size capacitor to use, I'd add 
one.  I'd like to use a variable capacitor, but am unsure of the value 
range.  400 to 1000pf?  Less? More?

I'm asking... I don't know.

--
73 = Best Regards,
-Geoff/W5OMR






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