Jim Kearman wrote:
> The last time I built an amp I just stuck in an available R-175 choke.
> This time I'd like to roll my own.
> The amp will use a pair of 4CX800As
> to develop 1500 W with Ep about 2500 under load.
> It will be used primarily on 160-40, although
> the available bandswitch will permit 20 if I want to add it.
> So I'm not concerned about self-resonances
> above 14.5 MHz.
>
> I have a 1-inch (25.4 mm) ceramic core that will allow
> a winding length of 4.5 inches (114 mm).
> If I close-wind #24 wire I can get an inductance
> of about 255 uH.
> What interests me is that George Daughters' 4CX1600B amp
> in my ARRL Handbook uses a much smaller inductance,
> albeit with a higher Ep.
>
> This core is the right length for the physical layout
> so I'd like to use it. It would give me about 2900 ohms
> of impedance at 1.8 MHz.
> If I'm understanding the theory, that means an additional
> 30 pF of plate-tuning capacitance to compensate.
> My plate blocking cap will be 3300 pF.
Have you considered using a pair of ferrite cores
made to suppress RFI through ribbon cables,
used as an inductor core?
(see part number 28R1861-000 at Digikey)
I think you could get the required inductance with something like 12
turns of high voltage Teflon or silicon insulated wire wound through
them in a couple cubic inches of well shielded inductance. The inter
winding capacitance would be pretty low, also.
Its main advantage would be the contained field, so you didn't have to
worry about what it was coupling to.
> Another related question: Bypassing the cold end of the choke.
> RF Parts sells .001-uf, 6-kV disk ceramics at a reasonable price,
> especially compared to the same value in a doorknob.
> Would two or three of these be safe to use at the cold end
> of the plate choke?
Since they do not see a lot of RF voltage swing, like capacitors that
take part in tuned circuits, I can see no reason they should not work.
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