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Kit 811KP includes two Zener bias diodes. These diodes are conservatively rated when installed
in the airstream of the fan. The 811KP Zener diodes connect in series with the transformer
white center tap lead. The banded or striped diode end connects toward the transformer Fig.
32, since they are Zener diodes. Each Zener diode is rated at 5-watts and adds 3.9 volts of bias.
The Zener’s are safe to 1.3 amperes continuous duty when in open air.
I suggest you use one Zener diode in addition to the six rectifiers used by Ameritron for a total
of about 7.3 volts, or two Zener diodes in series if your amplifier does not have the bias rectifier
string.
There will not be any noticeable increase in IMD from the additional bias, and the tubes will run
a lot cooler. Please look at this dissipation graph. Anything over 65W is over the ICAS tube
rating.
The diodes can be soldered in by cutting the white wire near the fan. Strip both ends, slide large
heat shrink over the white wire well away from the diode(s). Make hook leads in the diodes.
Crimp and solder them to make connections. Use either one or two diodes as required.
Figure 28 preparing and connecting Zener’s
This kit also includes a small 100K 2–3-watt metal oxide resistor. There are multiple points this
resistor can be connected; without a great deal of operational change. This resistor can go
from any tube pin to chassis or from any point on any filament wire to ground. You can even
attach it to the Zener mod as shown, since this is a convenient place. This resistor is necessary
to keep GDT’s from “ticking” or “clicking” on receive on some bands with some tubes. The 100K
resistor’s position on the Zener is meaningless so long as the other lead can reach a ground
point. The resistor can also go underneath the amplifier tube chassis, or on the input board.
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