On Sun, 30 Mar 97 08:18:20 -0800 Rich Measures <measures@vc.net> writes:
>How are the tubes being blown, Carl?
Darned if I know, I get the amp after the damage and the tubes go into
the trash.
>What is the breakdown V of the AL-82 bandswitch?
Ameritron rates it at 6KV 20A but it is just a standard
Centralab/Electroswitch JV series (new E4 series) as used by Alpha, Amp
Supply, MLA-2500, etc. The old Centralab and new Electroswitch catalog
rates it a 3KV AC and 17A max.
> Is the AL-82 bandswitch similar to the AL-80 bandswitch?
No, the AL-80 uses the 2500 series switch as was in the SB-220, SB-1000,
Clipperton -L.
Have you measured the resistance of the VHF suppressor resistors (Rs)
in an AL-82 whose bandswitch arced? In the AL-80?
No I toss them and install my own version which I developed ages ago to
tame the SB-220 and SB-200 on 6M.
> In the AL-80, Rs, damage is often not visable--even though an ohm
>meter test indicates a large change in resistance. IMO, when HF
(10m/12m
>mainly) damages Rs, external signs of overheating are apparent.
>However, intermittent VHF parasitic oscillations are so brief that
external
>damage is seldom visable. I have seen suppressor resistors that
appeared to
>be shiny-brandnew , whose resistance suddenly quadrupled some time
around
>the occurrence of a 'big-bang'. ..
Any 2W carbon comp resistor will change value at high RF levels (over
500W per resistor but with a parasitic coil of course) and is frequency
dependent upon how much. That is why you see them fried on 10M, etc.
Parasitics and high harmonic content just magnify the problem. A pair of
150 Ohm 2W carbons in parallel will last indefinitely, even on 6M at the
600-700W per tube level. The coil is a 2.5" long by .25" wide strip of
thin Mu-Metal bent in a U shape on the outside of the resistors. The
resistors barely get warm to the touch even under sustained key down
testing. The metal passes HF/6M RF but is a dead stopper to the second
harmonic of 6M which is damn close to the natural reasonance point of
3-500Z's. I had a double whammy to lick (no comments, I have no
secretary).
Since I did this work initially back around 1972-73 on the SB-200 I was a
bit amused to read your articles about 14 years later! With the
increasing prices of carbon comps I may have to try some of those metal
oxides. What is a good source of 3 to 5W versions?
> It seems me to that a HV-RFC fire or arc on 160m, 80m, or 40m is quite
>unlikly to be caused by 160m, 80m, or 40m RF.
I agree, but it is unlikely to be VHF either, it is more likely to be
quite close to the operating frequency and a design flaw associated with
the Pi-L.
Look at it this way for a moment, we have three 3-500Z amps with "some
sort" of a problem; the SB-220, TL-922 and AL-82. We have others that,
to my knowledge, have never had problems; L-4 and L-7 ( discount the poor
PS); Ten-Tec Centurion; various Henry's; PT-2500A (after they redesigned
the Pi-L !!). Even the LK-500 series is plenty reliable; I've beat the
hell out of one for 10 years and it still runs fine.
Why is the 3-500Z so squirrely when the 3CX800, 8877, 8874 and the like
are not. VHF gain is not the answer since those latter tubes will run
full bore in the 250-450 MHz range. Why are some 3-500 designs bullet
proof? Can anyone see a common thread?
73....Carl KM1H
>Rich
>
>
>R. L. Measures,ag6k,805-386-3734
>
>
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