> I've read the archives and know that this is a common problem. but is it a
> tube quality issue, or something about the amp. perhaps a parasitic or
> filament current inrush problem? Either way I've lost confidence in both
> Amperex 3-500Zs and the AL-80B. I want to dump the unit and get something
> more reliable.
Rod,
What were the code dates of the failed tubes?? That is a four digit number
like 9838 which means 38th week of 1998.
There is no parasitic problem and inrush problem. First, it is technically
impossible for a parasitic to cause that damage.
Second, the amplifier has a very effective inrush limiting system.
Even if you put a new Amperex 3-500Z into an old amp that has lasted for
years, a very large percentage have grid-filament failures. Richardsons has
replaced bad tubes, but it doesn't help the reputation or make anyone happy!
I don't work for Ameritron now, but I did do some consulting for them when
this tube problem started. Historically the AL80's were a very reliable
amplifier. Ameritron outsold every other manufacturer of any amplifier with
both the 811 and 80B amplifiers. The problems with 3-500Z's started when
Eimac decided to quit making 3-500Z tubes. Ameritron went to Amperex and
after a good first batch of tubes Richardson's inventory NOS (new old stock)
dried up.
When Amperex (owned and controlled by the US monopoly called Richardsons
Electronics) restarted the tube line, it had been years since they made a
3-500Z. If you look at code dates, you can see how old "new" Amperex
3-500Z's and other tubes are. Their sales people told me Amperex was in a
new facility someplace, due to low production demand and closing of the old
Amperex plant. It not only took a while to get tubes, the filament-grid
problem occurred. Being a "protected" foreign manufacturing facility, or a
source Richardsons does not want others to learn about, Richardsons does not
allow engineers here to communicate directly with engineers at the
manufacturing facility. Even e-mails were edited and processed by and
through Richardsons.
Mid and early 1990 code dates are good, everything 98 and later has been a
problem of varying degrees.
I did life-testing on new tubes and found that tubes with the lowest G-K
breakdown voltage failed most rapidly. I had a test fixture that cycled the
filaments on and off in about eight tubes at a time, and applied negative
grid voltage through a SCR latch system. No HV was applied, and about 20% of
the brand new tubes failed within a few off-on cycles or when tapped with a
screwdriver handle. Nearly all the tube failed in a few days, and the
problem is the filament helice bends over and touches the grid.
All of my life-test data went to Richardsons, and I never was allowed any
direct communications with Amperex. I'm not sure if Ameritron is currently
using that test fixture or not, or if the problem has been or will be fixed
soon.
I don't know what you will find that is a tube amp that is going to be
reliable, or when a better source of 3-500Z's will be found. Short of
getting a good tube, there is nothing you can do except keep changing tubes
until you get one that lasts. If you don't want to do that, perhaps you
should get rid of the amplifier and try to find something with a more
reliable tube. This is like the 8877 problem of the 80's, and may take a
while to resolve. This may very well kill sales of what was once the most
popular amplifier sold.
73 Tom
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