Pete,
That does seem strange. Grid current should be close to zero for
this tube. I ran it with just a tiny bit of positive grid current
(about 0.1 milliamp) when working SSB. For CW or any mode not
requiring class AB1 for linearity you can run up to about 2 mA
positive grid current despite what the tube data sheet says. I have
run them that way for years on CW with no problems. Efficiency is
noticeably better with the higher grid current but you should be
seeing 55 to 60% as long as you are driving it to the point that
grid current just barely starts going positive. Note that some
negative grid current is typical at low drive levels, and I have
noted poorer efficiency when operated in that region. However your
efficiency is lower than it should be regardless.
The only changes I made from published design were in the input
matching circuit. Many builders have experienced problems arriving
at a suitable input match with the original circuit.
Can you check the approximate spacing between plates of the plate
tune capacitor and the output coupling capacitor? I not longer have
my amplifier but seem to recall both were in the 5 to 8 mm range. It
*may* be possible to arrive at some incorrect tuning that appears OK
but isn't.
Paul
Pete Lindsay wrote:
> 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@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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|