Plate Resistance

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K3ZS:
I usually run 500V at 100mA, I think that gives me about 30W out on my AF67.   I made a fixture so that I can use a series light bulb (no CFL's here) to knock down the power.  That way you can tune up to full power on the AF67 and make the linear happy.   I have heard that it isn't good to lightly load it at reduced power when using the AM modulator, maybe someone would know if this is true.   I had to experiment with bulbs, have one for 80M and 40M, depending on the linear.   Mine is a GOnset GSB-101 and has an untuned input but uses a series coil to the cathodes.  For the light bulb I just used a regular electrical octagon box and lightbulb fixture.    I put on two coax receptacles and a switch to short out the bulb when not using the linear.   Its cheap and adds to the glow of the tubes while the bulb flashes as you talk.

flintstone mop:
hey Bob,
There's something neater than light bulbs!!
Someone here mentioned using an NPN power transistor and a 1meg pot and control the screen of the 6146. I can turn power up and down to the 12-15 watts that I need when driving a leanyear without worrying about loading.
fred

Steve - WB3HUZ:
This should probably do ya Fred.

http://www.amwindow.org/tech/htm/drivepot.htm

K3ZS:
Fred and Steve, the pot and transistor looks like a good idea.     With incandescents being outlawed, it looks like a good solution.   There is an empty space across from the modulation level that some use for drive pots.    One question I have, do you have to lower your audio gain on the modulator when decreasing the screen voltage to prevent overmodulation?    I am using the zener diode method of hard limiting, mainly to protect the mod transformer.    I was wondering if this would still be effective when lowering the screen voltage.    I would think if the plate voltage stays the same it would still be OK.    I started my career in electronics after the tube era, still rusty on the fine points of tube based electronics.

flintstone mop:
YUP the screen is the best way to control that particular tube. And definitely yes, reducing the modulation. The modulator will be in heaven and the plate voltages all stay the same.
Thanks Steve, I have used that ciruit many times for DX100's, to control drive instead of a wirewound pot wearing out.
Fred

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