Thanks for the answers to my questions.
What precipitated my question was a specific situation. I have a contest
station in Barbados and I have a number of amplifiers down there. The main
amps, a AL1200 and Centurion have been extensively repaired and work just
fine. The fallback amp, which I leave out for renters and use in
emergencies is an AL80A. I have been very impressed with the AL80A as it
makes surprising power and seems to be able to take a beating. I also have
a TL922 down there which makes much less power, particularly on 10, when
operated in the CW mode.
In a back storeroom, I found a another AL80A that has a sticker that said
"very bad" stuck to it. I opened it up and it looks like somebody tried to
fix it and gave up. It also had an aborted attempt to install the step
start mod. I decided to take it home and try to fix it. My plan was to get
it working as a backup amp and bring the TL922 home and sell it.
In working on the AL80A, I got a manual where I found out about the 400mA IP
limit. I am certain that I was running the other AL80A much higher than
that. While the amp did not malfunction, I was concerned about the quality
of my signal. Hence my question about the limits. If 400 mA is the real
advisable limit for signal quality with one 3-500, then the TL922 is
probably the better choice. If the TL922 can be run in CW mode in the SSB
position (several have told me that they do this all of the time) then the
TL922 is definitely a better choice than the AL80A as a backup and there is
no point carrying amps down and back.
The old AL80A is actually not in bad shape. The whole thing was taken
apart, cleaned, and had some repairs to ancillary circuitry and now works
with an old tube I had laying around. The amp seems just as good the one I
still have in Barbados. The only obvious defect was that the little coax
that feeds the cathode was broken off. On the other hand, the grid shunt
meter resistor had been replaced (with the wrong value) and the grid current
meter was dead (including the needle being bent). I opened the meter and
the internal resistor was burned to the point that I could not see the color
rings and the meter movement was open. I can't imagine was could have
caused that to happen to the meter when the rest of the amp was in pretty
good shape.
This 3-500Z plate current question seems to have hit a raw nerve. Are the
characteristic curves for the tube published anywhere on the internet so I
can decide for myself?
73, Tom W2SC
-----Original Message-----
From: Georgens, Tom [mailto:tgeorgen@lsil.com]
Sent: Tuesday, March 18, 2003 9:54 PM
To: 'amps@contesting.com'
Subject: [Amps] TL922A and AL80A
Hello All
The TL922 manual says "DO NOT under any circumstance OPERATE your exciter in
CW mode when the linear is in the SSB mode. Failure to head this warning may
cause immediate, extensive damage to the amplifier""
"REPEAT: DO NOT OPERATE CW IN THE SSB MODE"
I always wondered about this comment. The amp runs in CW mode at about
1700V and about 2700V on SSB (Under load). On CW, the amp barely puts out
as much power as my single tube AL80A. The statement above is remarkably
emphatic but I wonder how accurate it really is when the SSB tune up
procedure allows key down in SSB mode.
My question is what is the weak link that prevents operating CW at the
higher voltage and is there a mod to get around this?
As long as I am asking questions, the AL80A manual says to tune up to 550 mA
of Ip and then back off to 400. I think the 3-500Z max Ip is 400 mA. Is it
really OK to drive it at 550 mA? If so, is the amplifier still linear at
this level? Is 400 mA the destructive limit or is it the point at which
tube aging will accelerate? Also, in a grounded grid configuration like
the AL80, at what current would linearity be lost? Is there a reference
with the appropriate curves that I can use to answer this question?
Thanks in advance
Tom W2SC
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