R. Measures wrote:
>
> On Nov 7, 2005, at 3:17 PM, David Kirkby wrote:
>
>> R. Measures wrote:
>>
>>> Push-pull or parallel 4cx1000As?
>>
>>
>> as in the weasel, - push pull with a U-shaped anode line.
>
>
> My suspicion is that 2-tubes in push-pull is more problematic than 2 in
> parallel.
Well the two 4CX250B amp worked very well. IT was known to be one of the
best 2m designs. The size of the crossover flapper caps needed to be
reduced (in fact it was stable without them). I gather this is common on
modern sockets.
>
>>
>>> ---- What is the resistance of the grid swamper?
>>
>>
>> I can't recall if there was one at all. I had no desire to waste power.
>
>
> Grid termination is important because it provides low SWR to the exciter
> and it influences stability-- especially in cases such as this where
> there is zero grid-current.
Input VSWR was OK, no problems there.
RF feedback was measured by injecting RF in the input and measuring it
coming out the output socket with no DC applied, but DC paths in tact.
We managed to get that to in excess of 50 dB isolation. Of course it
will have no doubt changed at full power as tube capacitances changed,
but stability was never knowingly a problem - it never took off.
Perhaps one day I'll dig it out of my garage and put some photos on the
web.
--
David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
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