Shawn Tayler wrote:
> Hi Guys,
>
> Playing with my 30S-1 has gotten me thinking about a project. I've been
> wanting an amp for 50Mhz for a while and I have 2 4CX1500B's lating around
> as spares. One would make a nice amp on 6M and I think I'd like to do the
> grounded screen arrangement like the 30S-1.
Many moons ago (10~15 years) a friend G8WYI and I built an amp with a
pair of 4CX1000A's on 2m. The unit never did perform properly and is
sitting in my garage in a pretty much useless state.
The screen was at 0 V, the cathode at -325 V, the grid at -325-x (x set
the grid bias voltage according to the Eimac data sheet). HT was around
3 kV, with the negative returned to the cathode and not the chassis.
The basic RF (but not DC) layout was similar to the weasel amp for 2m
using a pair of 4CX250Bs which I'd built before and worked well. The
anode was U-shaped, the grid a pair of quarter wave lines.
There were three problems with this amp.
1) It was all too easy to get grid current flowing. The 4CX1000A is
rated for zero grid dissipation, but it did not take much drive to get
5mA of grid current per tube flowing. We never pushed it any further for
risk of damaging the tubes.
2) The power output was too low. We got only a few hundred Watts at 5 mA
grid current.
3) The linearity was really poor, with obvious distortion on SSB even at
only a few hundred Watts of output power.(Of course, this is related to 1).
We tried various loading techniques, used a spectrum analyser with
tracking generator to adjust the "flapper" capacitors for minimum RF
feedthrough. I think we had > 50dB isolation at 144 MHz, so it was well
in excess of the gain and so should have been stable (which it was).
I thought I had some notes about this disaster on http://www.g8wrb.org/,
but I either removed them at some point (I can't think why) or they are
on the site but I can't find them. I'm 90% sure I wrote a web page on
it. Perhaps when I have a bit of time I'll look over my hard disk(s).
They are possibly on a backup tape, but they are too much hassle to search.
We had someone else look at it, but he had no better ideas. I had
discussions with G4FRX (now GW4FRX) who said the 4CX1000A is a
particulary nasty tube. But the problem was never resolved.
Good luck if you manage it. We tried but failed!!
--
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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