ad5gb wrote:
>I just found a chart that listed the distortion characteristics of varioius
>tubes. I was really disappointed to see how poorly the 4CX1000A ranked amongst
>the competitors. IIRC, I think it was the absolute worst in the bunch.
>
>I got a couple of these things (with sockets) very cheaply a little while ago
>(well maybe longer than that ) and hoped to build something nice for HF out of
>them. I planned to try a grid-driven AB1 configuration at max anode potential
>with the best screen/bias regulation I could come up with but based on that
>chart.... it doesn't look like the most wonderful of things to try to do.
>
>Anyway to make these run more satisfactorially? Cathode drive /w NFB?
>Grounded
>screen (float the cathode below ground) Anyone with any experience and
>distortion figures better than in that Eimac chart I found?
>
>Thanks
>
>
>--
>Randall D. DuCharme (Radio AD5GB)
>
>Powered by FreeBSD!
>The Power to Serve
>
How about trying to use RF negative feedback, where some of the output
is coupled back to the input in anti-phase? Negative feedback will
generally improve the linearity. I've not tried it myself, but would
suggest something like
a) A splitter on the input, so the drive power is the sum of the power
from your transmitter and some fraction of that fed back from the output
of the amplifier.
b) A coupler on the output, that couples off a small fraction of the
output power back to the input.
The difficult part is to get the phase right. Get it very wrong and
thing will turn into an oscillator. But if the phase is correct, this
will improve linearity. I can't be bothered to explain why negative
feedback reduces distortion, but there are plenty of web references to
it - this one is about audio amps, but the same principles apply.
http://members.tripod.com/~gabevee/nfb.html
One way of ensuring you have the phase right (without the use of phase
measuring equipment) might be to first try this with a small fraction of
the signal coupled back. Then adjust the phase of the coupled signal.
When the signal is directly out of phase, the gain of the amplifier will
be lowest. So you adjust the amp for the lowest gain. Perhaps some of
these things from the likes of MJF could be used too.
Of course, the above paragraph indicates the gain of the amp will be
reduced, but that should not be a problem with a tetrode like the
4CX1000A, as the drive power required is not a lot.
At HF, it is easy to delay a signal with a length of coax. Since the
coupled signal will be low power, thin coax will do. You would probably
need to switch in different lengths for different bands. To get a
half-wave shift on 160 m, you would need about 80*.67 metres of coax
(80m since its a half wave, and 0.67 for the velocity factor of the
cable). Quite a bit I admit, but not too bad considering the coax could
be thin. Something like RG174 (about 3mm diameter and cheap) would be OK.
PS, good to see you are running FreeBSD. Not using that myself (running
Solaris on a Sun), but Solaris (for both Intel chips and 64-bit Suns is
a free download).
--
Dr. 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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