Would you like to have more "talk power"? Would you like
to not have to worry about over
modulation? Would you like to protect your modulation transformer? If you
answered yes to any of these questions, then read on.
The circuit described below is so versatile, it will give your
rig more "talk power", keep you from over modulating, and provide
protection for the modulation transformer - all at the same time! And best
of all, the circuit is simple, easy to build and install.
Figure 1 is a schematic of the circuit.
It consists of a number of zener diodes in series with a silicon diode.
This string of diodes is then placed across the secondary of the modulation
transformer as shown in Figure 1. That's all there is to it. But, how does
The key to the circuit is the zener diodes. Zener diodes are
a special type of silicon diodes that are designed to operate in the reverse
conduction area (reverse biased). In this region, the voltage drop across
the diode will remain relatively constant no matter what the current through
Zeners are often used in voltage regulation circuits in a manner
similar to the more familiar gas regulator tubes (OA2, OA3, etc.) Zener
diodes are specified by voltage and power dissipation. To design a zener
regulator for, say, a 5 volt supply, you would use a 5.1 volt zener. As
long as the input voltage to the circuit was greater than 5.1 volts, the
zener would keep the output at 5.1 volts.
The power rating would depend on the amount of current the zener
would be required carry in its regulation duties. Zeners are commonly available
in one and five watt varieties and look like small silicon diodes. Higher
power handing types are also available and are generally of the stud mount
Now, back to our circuit. When the audio voltage on the secondary
of the modulation transformer has the polarity as shown if Figure 3, the
silicon diode is reverse biased and the zeners are effectively disconnected
from the secondary. This condition exists during the positive cycle of the
When the polarity is reversed, during the negative cycle of
modulation, the silicon diode is forward biased and the zeners are reversed
biased. The zeners now regulate or limit the negative voltage on the secondary
of the modulation transformer to slightly less the the B+ value of the final.
This regulating action means the B+ to the final is never cut-off, thus,
eliminating over modulation on the negative cycle of the modulation.
To the top
To design a zener limiter
for you rig, determine the B+ voltage on the final amplifier. If your rig
has an appreciable drop in the B+ under high levels of modulation, determine
the lowest value to which B+ voltage drops. The total zener voltage of the
zener diode string (the sum of zener voltage of each diode in the string),
should equal about 50 volts less than the B+ voltage previously determined.
The total power dissipation of the string (the sum of dissipation of each
diode in the string), should equal about half the power capability of the
Now, the trick is to get the zener string to meet the two conditions
above. Since zener diode are commonly available in voltages from a few volts
to about about 200 volts, the number used will depend mostly on the power
dissipation requirement. For rigs up to about 100 watts output(B+ of 1000
volts or less), 5 watt zeners are the most economical. This type of limiter
is not recommended for higher power rigs. Instead, use the Three Diode Negative
Although the zener regulation action will clip the negative
audio peak near 100% negative peak modulation, the resulting distortion
is minimal. Remember, at the instant the clipping occurs, the voltage on
the final tube(s) is only about 5% the normal B+ voltage. This also means
the instantaneous final plate current is also only about 5% of normal (or
what you read on the transmitters meter which reads the average value).
Knowing power is the product of voltage and current (P = EI), then the input
power to the final at the instant the negative peak clipping occurs is only
0.25% normal. Thus, the distortion products created by the clipping will
be no worse the 0.25% of the input power or unnoticeable. For example, if
you are running 100 watts average input, the power input at the instant
the negative peak clipping occurs is just 0.25 watts!
With the circuit in place, the negative peak modulation is absolutely
limited to slightly less than 100%. If your rig has sufficient modulator
power, you can crank up the audio gain some, achieve higher levels of positive
peak modulation, and never produce splatter. Remember, splatter is produced
by modulation in excess of 100% on the negative peaks, not the positive
peaks. Modulating over 100% on positive peaks will make you audio sound
louder at the receive end or give you more "talk power".
The circuit also provides protection for the modulation transformer.
A big killer of modulation transformers is large transients created by modulation
in excess of 100% on negative peaks. Since the zener circuit prevents negative
peak modulation from exceeding 100%, no damaging transients are produced.
Long live your modulation transformer!
Back to the General Technical Page