4
voltage transients induced by lightning, inductive switching, or electrostatic discharge.
This limits
transient to about 350 volts absolute peak. Any substitute GDT
’
s should be vetted for performance.
GDT’s do not clamp.
GDT
’
s ionize at a certain voltage, becoming a near-short once ionized
2.)
Instructs you to directly ground grids in older version that had grid equalizing resistors. This is a critical
update. The grids should always be directly grounded. THIS IS A MUST DO in any grounded grid amplifier
that floats grids from chassis.
3.)
Increases or adds bias. Bias improves tube life and efficiency without noticeably hurting IMD with 3.9 volt
5 watt Zener bias diodes. If your amplifier has a string of bias diodes, you add only ONE of these diodes in
series with the white center tap. If your amplifier does not have a string of bias diodes on the input board,
two diodes are used. The goal is to be around 10-20mA quiescent current per tube. This would be 30-
60mA zero signal keyed idle current in the AL811, and 40-80mA idle in the AL811H
4.)
Add a higher surge rated negative rail clamp diode. This diode protects the grid and plate meters and
shunts. The original diode is a small 1N4001 series diode. The kit diode is much stronger and fails far less
often. This reduces the aggravation of protection diode failures
5.)
Add a 100k resistor from filament to ground, or to a place I provided in later AL811’s that Ameritron for
some reason never used. This prevents a weak “popping” noise in your receiver from intermittent gas
tube ionization with some GDT’s and tubes
6.)
Supplies an additional GDT if you want to further clamp the RF input line to the input circuit
7.)
You
should
remove MOV’s if they are defective or if they are readily accessible. MOV’s do not add
anything worthwhile in protection and MOV’s are failure prone.
They are too far downstream and
beyond the choke to do any good for your exciter. If MOV
’
s fail, they can cause problems
The AL811 three-
tube model normally does not require dropping the back panel unless it has defective MOV’s to
service. The 811H back panel almost always has to be dropped. This is not a big deal if your particular unit was
wired correctly from the factory
. It is a little more work if the transformer wires have been cut too short.
Always make sure the amplifier is unplugged and the capacitors are discharged before starting
this modification procedure. Never attempt to make any repair on the amp with it powered up
and the cover disconnect overridden. There are LETHAL VOLTAGES present inside the amplifier.
In all cases be careful and work slowly.
811PK
All common grounded grid amplifiers should have grids directly grounded with the shortest possible leads.
811H amplifiers with the terminal strips and resistors, as shown below in figures 2 and 3,
must
be upgraded
for safety.! Use of grid resistors, long grid leads, or grid chokes increase chances internal tube arcs might to
continue to the filament and out to the radio. A hard tube short will almost always damage or open the grid
resistors, causing operational issues.
