Here is the reply I rcv'd this morning from Tom Raush, W8JI,
who designed the AL-811H for Ameritron.
Some personal stuff at the opening, is deleted, and he did give
permision for me to pass this info along to those of interest::
"Hi Jim,
...snip...
1.) The common failure in 811 and 572 amps is tubes. The
common mode of failure is high vaccuum or gas arcs. Although
most likely under periods of high peak anode voltage
(operation), flashovers can occur under ANY condition when
HV is present. Sometimes the arc itself breaks down the gas
and getters the tube, sometimes a tube is not recoverable.
2.) The resistor in the grid are to improve balance between
tubes. The capacitors keep the grid at chassis potential for
RF, and this is to SHIELD the cathode from the anode. The
grid is the ONLY isolation for feedthrough capacitance. The
values were chosen to minimize unwanted coupling through the
tube and to equalize the drive to tubes over a normal range
of replacement tubes. I wouldn't let a backyard mechanic
without a network analyzer suggest new values based on
emotional opinion.
3.) You can add as much nichrome as you like and you will
have the same tube problems. The problem is not parasitics.
The original suppressors are perfectly [adequate]..
4.) The meters are already protected. 100% of [such] meter
failures are caused by the negative rail of the
electrolytics rising above ground when the HV dumps to
chassis ground through the tube anode to grid flashover. The
current flows back through the grid shunt to the negative
rail, and that is what blows out the grid shunts and meters.
If anyone took the time to trace the current path they would
see a single negative rail clamp diode protects the metering
system.
5.) The "bang" occurs because things outside the tubes move
and arc when you dump over a thousand volts to ground
through five or ten ohms of power supply ESR. If you double
that resistance by adding a 10 ohm series resistor, you
reduce the surge current to half the value. It will still
damage things. 75 amps of HV current isn't much better than
the 150 amperes. If you really want to do glitch current
suppression you should add at least 25 ohms using a HV surge
rated resistor.
Anyone is welcome to do whatever they want, but the fact is
if the tubes are good there isn't a problem. If one or more
is bad you'll have a problem even with dozens of circuit
mods. If you have problems with an 811 or 572 amp it will
almost always just be a tube or tubes. That's because tubes
are cheaply manufactured and inadequately tested and
conditioned before being sold. 3-500Z's are getting that way
also.
Tubes are costly to build correctly, that's why Eimac dumped
glass tubes years ago, and why the people who bought the
glass tube line weren't sucessful. We have to live with what
is being manufactured and sold.
For example, the Chinese manufacturers I've seen don't even
highpot the tubes for voltage breakdown. In the mid 1990's
they were testing production 572B's at 200 watts output with
1700 volts on the anode!! This is fodder for people with an
agenda because it means there will be significant numbers of
field failures related to flashovers.
If you want to increase tube life in the 811 series, plug in
572B's. They are tested about like they should be testing
811A tubes. They have high reserve dissipation, so you are
not likely to "cook" the anodes by overdissipation. The
stated intention when designing the 811 amp was to build a
very cheap amplifier. The cheapest amplifier possible for a
given output power.
There could have been $100 dollars of additional circuitry
like a 25 ohm 25 watt HV fault resistor, a series 50 ohm 50
watt cathode feedback resistor, and even a bigger tank coil
and more expensive meters. Even with all that the tubes
would by far remain the weakest link in the system. There is
little point in installing rear wings on the back of Hondas
with weak 4 cylinder engines that go 100 mph maximum, but
some people want to do it. Myself, I prefer to get the BIG
problems first. Change the tubes. Chinese 572B's are about
as good as old RCA 811A's were at the same power.
73 Tom"
Very interesting information.
73, Jim W6KPI/KH6
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