Wow, sounds like you had some fun Dick.
My only contribution to your situation is to confirm that the windings on
the plate choke will move when you have a catastrophic problem with the
tube/s.
We had a 4cx250 amp which had one tube shorted internally and when we
switched on the ht, of course the fuse went qrt but not before all the
turns of the plate choke had bunched up to one end of the former.
It did not go open circuit.
We had been given a box full of those tubes and quite a few of them were
duds, so we rewound that choke and replaced the fuse quite a few times
while we were testing them all. I remember it was fun at the time. (A
couple of guys, a box full of tubes, a case of beer. Can't think of a
better way to spend a Saturday afternoon.)
Maybe the bunching up of your plate choke was caused by the sick tube and
because of that, it had some funny resonances which affected the meters on
40m and and said good bye on 75m.
73 de Alek,
VK6APK
At 02:54 AM 05/08/1999 -0400, you wrote:
snip
>2. I lifted the tube deck to clean underneath it and to clean the lower
>fans. Everything looked fine under there, but I did notice something funny
>about the plate choke -- it doesn't look very pretty. The windings are kinda
>bunched up in a couple of places. The insulation appears OK and the turns
>aren't loose -- it's just that some of the turns are over other turns, even
>though there's still some room left on the form.
snip
7. The exception was 40. Meter behavior was *very* odd there. Tuneup was
>normal until I reached max drive. Then, at max power output, the plate
>current meter was showing maybe a couple of hundred mils and the grid
>current meter was showing something like 50 mils -- with both power meters
>showing a full 1500W output! If I fiddled a bit with the tune and load
>controls, I'd get a tad more output power and the plate and grid meters
>would suddenly jump up to normal reading and the transformer would buzz like
>it was under full load. No change in output power -- still max. I really
>couldn't set the amp at that sweet spot -- the meters would drop to those
>weird low reading and the transformer buzz would subside. But power output
>was still max. I figured something very odd must be happening with the
>meters on that band.
>
>8. But then I tuned up on some of the other bands and the meters behaved OK.
>I tried tuning into resonant antennas on several bands, and got full output.
>Finally, I decided to see whether the amp could load up in the 75M phone
>band on my 80M CW dipole. The output network looked as hefty as my old
>SB-220 (which tunes into 3:1 fine) and since there are no PIN diodes to fry,
>and the amp has an ALO circuit to detect high VSWR, I figured this would be
>pretty safe -- WRONG! I tuned up at several hundred watts out from the amp
>and increased drive until the output was about 800W. I thought I saw
>something out of the corner of my eye -- maybe a thin waft of smoke -- but
>thought it was a reflection on my glasses. I bumped the power up to about
>900W -- suddenly the output dropped and I saw smoke coming out of the tube
>compartment. I let off on the key and looked down through the vent holes at
>the tube deck and saw that the plate choke had turned black and copper in
>several places where the wire insulation had vaporized. End of plate choke.
snip
Any help would be *most* appreciated!
>
>73, Dick, WC1M
>
>
>
>
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>
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