[Amps] Help with ARRL handbook design 4CX1000A 2m PA

[Pete Lindsay]

Paul,

Thanks for the reply. Yes I think you are correct, it is not a problem
with the design but my specific amp, but how to find it?! I can drive
the plate current to 800mA with about 10-15w of drive, which agrees
with the expected figures in the design. Even at at 800mA the output
power is only about 700W. I guess your amp worked first time and
didn't need any tweaks or changes from the original design as
published? What control grid current did you run at, the published
design does not mention a figure for that, should it be close to zero?

Pete

2008/7/22 Paul Kelley N1BUG <paul.kelley.n1bug at gmail.com>:
> I think there must be some problem unique to your specific
> amplifier. I built and used that amp for several years with good
> results, efficiency close to 60%. I have read your subsequent
> responses to other replies. How hard are you driving it? What is the
> plate current with drive and how much output are you seeing?
>
> Paul N1BUG
>
>
> Pete Lindsay wrote:
>> Hi Guys,
>>
>> I have a 2m PA built around the design in the 1986 ARRL handbook
>> chapter 31 using a 4CX1000A. It was built by a friend to a very high
>> spec and looks like an exact copy of the original design. The problem
>> is it has never really worked that well. The problem lies in the
>> output efficiency which, at best, is only around 30-35%. I am using
>> 3KV on the anode and have tried several tubes. The screen grid and
>> control grid currents act as you would expect when
>> increasing/decreasing the loading, but the output is just not there
>> (measured on several different power meters). I would be very
>> interested if anyone out there has successfully built one of these
>> amps and managed to get a reasonable output efficiency. Other amps I
>> have built with 8877/GS35 etc all work fine, but this one has me beat.
>> Is there a critical part to the design which needs close attention to
>> get it to work? Any help appreciated!
>>
>> Thanks
>>
>>
>> Pete, G4CLA.
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