Geoff
Don't have the tube characteristics here, but from your description your
changing the operating point of the amp by changing L/C ration. Or the
"transformer effect" (turns ratio of link to plate coil) gives a better
match to the antenna)
If you have a 1000 pf that will take the voltage, start there. If you
loading to an untuned link, I bet you can load that thing like your using a
bucket of bricks.
Byron, W3WKR
----- Original Message -----
From: "W5OMR/Geoff" <[EMAIL PROTECTED]>
To: "Discussion of AM Radio" <amradio@mailman.qth.net>
Sent: Thursday, September 22, 2005 5:59 PM
Subject: Re: [AMRadio] Pi-Net vs Link Couple
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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