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Varmint linear

5

555

Active Member
Mar 17, 2006
424
20
28
Varmint XL-600
just thought i would hook up the old varmint and was hooking up the coax connecters
and came across a little knob that says input
so i got to thinking so i checked my swr on my radio the swr were little high
but the swr from amp going to antenna were 1.1 to 1.2

any input on what this little input knob will do before i go twisting on it

I havent been into the base amp setups to much and lost sight of a few things
but since i have my mobile where i want it

i would hook up my varmint 600 driver and see what i could get going

yes i said varmint 600 driver its for my 5tuber
but just curious as to what the input knob was for
 

Hi 555,
That adjustment is to minimize the SWR between the radio and the amplifier, NOT on the antenna side.

When the amplifier is keyed, the radio isn't feeding the antenna any more, it's feeding the two driver tubes. The input adjustment is to fool the radio into thinking it's driving a 50-ohm antenna with a low SWR, when the Varmint is keyed.

Requires you put a SWR meter between the radio and the amplifier. Keeping the jumpers short, like 3 feet or less tends to help. The internal SWR meter on a base radio may be accurate, or it may not.

With the amplifier keyed and tuned up for max modulated power, the input match is adjusted for minimum reflected power on the radio-side meter. You'll probably notice that turning it ALSO affects the max modulated power coming out of the Varmint when you turn that knob, as well.

In a perfect world, the spot where it shows minimum input-side SWR is is the same knob setting that shows max wattage coming out of the Varmint. In the real world, max drive may be at one spot on the knob, and least SWR may be at another spot. This calls for setting that knob for a "compromise" between the two knob settings. Generally, you should be able to get the input-side SWR under 1.5 without sacrificing more than 5% or 10% of the power coming out. If the amplifier's input SWR is much over 2 to 1, the radio's final transistor may get worked too hard.

You'll also fid that the Varmint's input SWR will change when switching from "Supermod" to "High". The tubes in the Varmint tend to run cooler on the Supermod side.

73
 
hey thanks
i keyed it down tune for max audio and on low it deadkeys
50 to 75 watts and modulates to 275 to 300 watts
on high supermod it deadkeys 150 to 200 and mods to 900 watts
the swr showed around 2.5 to 3
adjusted the input and got it down to 1.8 to 2 swr showing on radio
went out to the shop and looked around found me a 6 foot jumper put it inline and wham its shows the same as on swr going to antenna

thanks for your help
 
well the problem still exists
i forgot to recalibrate the swr on the radio it showed lower swr but after i recalibrated it was still there and i can turn he input left and right but no change so what do i do next
 
I had the same problem with input SWR on my Varmint. There is a small trimmer inside for the input adjustment, but it was worn out.

So, I put a Maco 300 antenna tuner between the radio and the Varmnint and tuned the SWR on the radio flat. (in my case a RCI 2950 DX with the digital self adjusting SWR meter).

Well, it worked. That old Varmint talks all over the nation conditions permitting.
 
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If the minimum SWR is reached with the screw all the way loose (clockwise), you may have the version that was sold wired wrong. Not too many folks checked input SWR in 1978, and a lot of them went out the door with this input circuit just wired wrong. Only fix is to add a small coil, around a half microhenry, in place of a wire jumper.

Don't have any pictures, or diagrams about how to fix that problem. Been doing it from memory for 25-plus years. Pretty sure most published diagrams won't match the way your input-match control is wired.

73
 
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for that varmint 1000
a 1 to 1 1/2 watt deadkey with a 12 to 15 wattswing will give it all she needs any more and you be killing the tubes
 
I have Two XL 600s, one I retubed patched up the best I could. 2watts in swinging 12watts would show 800 on my Diamond SX 100. I found another and drove 5 hours to pick it up in Georgia, long story short I was ripped off. Just simply lied to in a parking lot deal. I have learned so much but reading here. There are a couple out there that know this thing from inside out. To them thanks for the pictures and sharing your knowledge. Neither of my boxes are working at the moment, I have all new tubes and am very apprehensive about putting them in. I want the amp checked from top to bottom, I wish I had the knowledge to do it. 300.00 of tubes I will have to wait to meet or talk to the right person.
73,
Slingshot Daytona Beach Florida
 

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