First of all - thanks to all that took the time to assist me in trying to
find out the cause of the extreme excessive grid current in this original
Ameritron AL-80 (not A or B model) - Single 3-500Z grounded grid.
The hard way, I finally figured out that this AL-80 came with the wrong
meter and face in it. It is the one pictured on the instruction manual and
in all the advertisements back in 1984, or so, and would not have been
suspect except that I had ruled everything else out. The schematic calls
for a 0-1ma meter with an internal resistance of 450ohms and shunt of
1.5ohms. That calculates to 0-300ma scale for which there is not one. In
addition, to further aggravate the problem, the meter in the amp has an
internal resistance of only 400ohms.
The above translates to an actual grid current of 125ma reading as about
325ma on the 750ma current scale. I now wonder if some of you thought I was
crazy when I was telling you the grid current readings I was getting. No
wonder you questioned my tuning procedures - No wonder I questioned them
myself. I started out with a Globe Scout back in 1971 and believe I must
have learned something by now after all.
There is an email in the archives that describes problems with the original
AL-80's. At the end of the email it says that you can tell if you have one
of the originals if it has a s/n of before 350 or has a meter face with only
2 scales. But, in the list of problems there is no mention of any problem
with the metering. Mine has a s/n of 313. The email indicates that most of
those originals were recalled and rebuilt. So, this may be the first
documentation of the meter problem on the Internet (or anywhere else
publicly for that matter - I called Ameritron early on and was given no
clue).
The meter itself is accurate, so the remedies are obvious.
As you may remember, there were other problems, too. I had to modify 3
sections in the tuned input circuit and 3 sections in the tank circuit.
Plus I added 25 ohms of negative feedback resistance. The little amp works
perfectly now - right up to the specs of the AL-80B plus a little grid
current due to lower plate voltage on mine and minus about 100 watts out for
the same reason.
73 all from snowy NW Okla,
Lu K5YP
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