Interesting points.
1) I measured the air gap of the capacitor with a feeler gauge and it is about 0.012". If
air breaks down at 30 kV/inch, then it is good for only 360V. 1500W at a 1:1 SWR is about
273V into 50 ohms. So I think you may have a point here! I am going to try to actually
measure the breakdown voltage to get more information.
2) Are you sure your choke wasn't damaged by heat rather than RF? I intend to put a piece
of supercharger hose over the anode to conduct the hot air out of the enclosure. It will
also serve as insulation against radiated heat. The nearest point of the choke to the
anode is 1-5/8". I can move the choke, but I wonder if anyone else cares to comment about
this.
3) The bandswitch is much better than the usual ones from the standpoint of
current-carrying capability. There is also plenty of clearance between the contacts and
any metal parts. The only issue is the distance between the contacts, which is certainly
as great as those in the switches in Alpha, etc. amplifiers -- never mind the ones in
AL-1500's, etc.! So I am not worried about it.
On 5/29/2013 4:03 PM, Bill Turner wrote:
ORIGINAL MESSAGE: (may be snipped)
On Tue, 28 May 2013 17:15:33 -0700, you wrote:
Thanks. I'm not the best metalworker, and I'm no kind of an engineer, so my
work doesn't
compare with that of some of the guys who regularly post here. But hopefully it
will be of
interest.
REPLY:
Hi Vic: I have three comments on the 4CX1000A amplifier which might be of
use to you.
1. The load cap appears to be a standard broadcast band variable. If that is
so, it's voltage rating is right on the edge. As long as your SWR is 1:1 it
will probably work, but much above that and you can expect some arcing at
that power lever. Once an air variable arcs, small blisters form at the
point of the arc and it becomes even more prone to arc. If you ever
accidentally transmit into the wrong antenna, bye-bye load cap.
2. The plate choke is much too close to the tube's anode, IMO. I made this
mistake once and eventually the choke burned. Even though there is only a
few pF between the anode and the windings, at the extreme voltages present,
a lot of current can flow especially on the higher frequencies. You might
get lucky and not have a problem, but watch for burned insulation on the
turns nearest the anode. Not only that, but you are inadvertently setting up
a second parallel resonant circuit with that same capacitance and a portion
of the plate choke. Again, you might get lucky, but beware.
2. The band switch looks awfully small for a legal limit amp. A REAL legal
limit bandswitch should be about four inches across and cost several hundred
dollars. One should not skimp here. Ask Carl how many undersized band
switches he has replaced over the years. :-)
Otherwise, looks good. Having fun?
73, Bill W6WRT
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Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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