[AMRadio] Pi-Net vs Link Couple

Byron Lichtenwalner w3wkr at verizon.net
Thu Sep 22 18:08:37 EDT 2005


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" <w5omr at satx.rr.com>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Thursday, September 22, 2005 5:59 PM
Subject: Re: [AMRadio] Pi-Net vs Link Couple

> Byron Lichtenwalner wrote:
>>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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