> Hello,
>
> I hope that some of the more experienced can give me some advice. I
> am working on conversion of an older homebrew 4cx1500b to a gu84b.
>
> The best description of the problem is that the higher the frequency,
> the less the output. 160, 80, and 40M due pretty well. On 20 and above
> loss of power output is
> very high. Another interesting occurrence: With about 20 watts drive
> on 20M,
> output is about 30 watts. 2 - 4 seconds, the tube begins to amplify and
> power out
> goes to 400 or so (during this, input swr changes a lot - 1.3 to 2.5 to
> 1).
>
> I have a mov tied to the grid input pin and screen input pin at the
> socket. Is there too much capactiance from these devices?
::You bet. What's the value of the MOV (P/N, etc)? Many have
hundreds of pF capacitance, which reduces with applied voltage. Your
symptoms seem to indicate this -- the MOV's "charging" up to less C with
applied signal, and it takes a while.
-WB2WIK/6
> The tube is
> grid fed via a .16uh inductor tied
> to a 50 ohm 100 watt resistor to ground.
>
> Plate voltage under load 2000
> grid voltage under load -60
> screen voltage under load 325 +
> cathode tied to ground
> tank circuit tuned to 2600 ohms
> drive 30 watts
>
> also, I have too much tune capacitance on 20M..... I cant set vacuum
> variable in the tank circuit low enough...
>
> ideas please.
>
>
> Lynn, NO9Z
>
>
>
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