>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 do 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?
Yes and there is no reason to have it there.
>The tube is
>grid fed via a .16uh inductor tied
>to a 50 ohm 100 watt resistor to ground.
>
A better way to feed DC bias to a grid is connect the DC to the cold end
of the grid R, and double-bypass (perhaps 1000pF in // with 0.02uF) it to
ground. The resistor should be close to the grid - unless you are
building a VHF-oscillator.
>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.
>
Reduce 20m tank L.
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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