In message <000801c061d5$c99fdf40$d84cef9b@telkomsa.net>, Wynand van Wyk
<zs6arf@telkomsa.net> writes
> Greetings,
> ?
> I have started work on a new amplifier (80 - 10 meters) utilising
> the popular grounded grid design but with a slight difference than
> the norm as I use four valves in parallel, of the AMPEREX 4-250
> type.?The main reason is I have ample supply of 4-250's (as
> compared to the usual 4-400 or 4-500 one reads about in designs).??
> I still?would like to break the 1?KW barrier on output.
> ?
> I have a similar working one I build in 1999 but although it is
> working it is not very efficient and prone to destroying chokes and
> by-pass capacitors.? Power output is also way below what I expected
> .. and not breaking the 1KW barrier.
> ?
> I run the valves with +/- 3,400 Volts on the plate and the
> filaments have two transformers connected in parallel giving the 5
> Volts at +/- 15 Amps.
> ?
> I need advice on how to improve performance and limit component
> failures.
> ?
> I know I have to look at the capacitance of the four valves in
> parallel as this will impact on efficiency especially at 10 meters.
> ? I AM JUST NOT SURE WHAT I AM SUPPOSED TO BE LOOKING AT.
> ?
> My plate choke will also have to be properly designed, as
> previously I took it out of a YAESU 2100Z and just transferred it
> to the homebuild version.? It works but surely is partly the cause
> of the lower than expected power output.
> ?
> I also now want to add a tuned input circuit as previously I just
> put a ATU beween the rig and amp input.
> ?
> Any advice, especially on the use of the four valves in parallel be
> it practical or an indication that I should?use two valves will be
> appreciated.
> ?
> ?
> Wynand van Wyk
> ?
> ZS6ARF
> ?
Have a look at http://www.vcnet.com/measures
This will tell you all you need to know about plate chokes and more. I'm
currently re-building my 4-250 amp and have hit the very same problem,
so at this moment I've redesigned the plate choke and added a secondary
choke in series at right angles.
What Richard says about chokes is spot on - try it yourself with a dip
meter!
For 10 metres 4 x 4-250's maybe too much capacitance as even 2 is just
manageable.
You will probably need a blower as opposed to muffin fans with 4 anyway.
Is it grounded grid or grid driven?
Paul
GW4WWE
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