[Elecraft] Tube Linear Amp design for basic K2

[Charles Greene]

Robert,

I worked a guy the other day using 400 watts, but it is very 
unusual.  I usually use my K2 /100 at 25 watts and my SDR-1000 at 35 watts.

I made an error on the calculation of power output of the linear 
amplifier.  If you double the input voltage, that is a multiply of 4 
times the input power, as power = E^2 / R.  That also produces 4 
times the output of the linear amp.  Bottom line, if I use an input 
swamping resistor of 100 ohms, I can drive the amp to 400 watts 
output with 5 watts.  Useful addition for a plain ordinary K2.  I 
haven't tried it, just calculations.

73,  Chas, W1CG

At 02:20 PM 6/10/2006, Robert Allbright wrote:
>Hi Chas
>
>Thank you for your interesting info.
>
>I was surprised that you mention using 400w on PSK31
>
>I've never seen anyone mention using more than about 70w on PSK31 as
>high powers are not supposed to be necessary?
>
>Just my comment as when I experimentally tuned up my Acom 1010 amp
>for 400w output I had a very irate neighbour knocking on door!
>
>That's another story and I'm still alive hi!
>
>Since then I've been using 5 - 20 watts cw and psk31 and having fun
>
>73 Robert G3RCE (from the future Islamic Republic of the UK - God
>help us!)
>
>------------------------------------------------------------------------ 
>---------------------
>
>From: Charles Greene <crgreene at cox.net>
>Subject: Re: [Elecraft] Tube Linear Amp design for basic K2
>To: "Wyn" <vr2ax at attglobal.net>, <elecraft at mailman.qth.net>
>Message-ID: <7.0.0.16.0.20060609145715.019de4a8 at cox.net>
>Content-Type: text/plain; charset="us-ascii"; format=flowed
>
>Wyn,
>
>I have an antique Hallicrafters HT33A with a PL172 tube, and  I
>modified it so it will accept lower drive.  Right now when I drive it
>with about 20 watts from my K2/100 in PSK31, I get about 400 watts
>out.  That's a gain of 20 times, or 13 dB.  The tube input is swamped
>with a 50 ohm resistor to help in its stability.  The PL172 is a
>power pentode with 1000 watts plate dissipation, operated in the
>class AB1 mode (no control grid current) when amplifying SSB.  On CW,
>it operates in the class AB2 mode, with a little higher
>efficiency,   A 4-1000A is the nearest new(er) design tube.  I
>thought about how I could make the old guy put out something exciting
>when driven by my K2.  I could swamp the input with a 200 ohm
>resistor and use a 4:1 balun and the impedance to the K2 would still
>be 50 ohms.  Actually, with the K2 automatic antenna tuner, you could
>tune the 400  ohms to 50 ohms.  Same difference.  The tube itself
>does not take any power in class AB1, but the input voltage needs to
>be so much for a given output.  A 4:1 balun would increase the drive
>voltage by a factor of 4 times which would increase the output of the
>amp to 1600 watts if the drive remained the same, 800 watts with 10
>watts drive, or 400 watts PSK31 with the K2 maxed out at 5 watts,
>which is respectful.  From 20 watts to 1600 watts is a power gain of
>19 dB which in my opinion is a little high.  I would be more
>comfortable with a power gain of 16 dB, but you can try it.
>
>The point is, yes you can find a tube that will take a 10 watt
>transmitter to 800 watts or so, but be careful.  Those tubes are
>expensive.
>
>73,  Chas W1CG
>------------------------------------
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