[Amps] Grounded Screen 4CX1500B

[R.Measures]

On Nov 7, 2005, at 10:48 AM, David Kirkby wrote:

> 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.

Push-pull or parallel 4cx1000As?
>
> 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.


---- What is the resistance of the grid swamper?

cheerz, David
>
> 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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>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org