Specifications

from ever happening. The letter "A" points to the RF input coupling capacitors, which are a
pair of 0.02µF @ 1kV disk caps. A single 0.01µF @ 2.5kV silvered-mica cap would be a
good substitute. The SM cap being a more temperature stable type, and able to carry more
RF current without heating up.
The letter "D" points to the filament line. I feel that a choke is needed, even with tubes that
are "indirectly-heated" cathodes.
Here is my reasoning as to why; RF voltage potential differential between the cathode and
the heater could cause an arc-over under certain circumstances. To prevent this type of
damage, the heater is RF coupled to the cathode by a single 0.01µF @ 1kV disk cap. This
placed both at the same RF potential However RF coupled to the heater must then be
blocked from propagating back down the filament line. So, a choke must be installed on the
line.
The choke I used, is a toroid with bifilar # 14ga. Teflon® insulated wire. I used a 1" i.d. gray
mix type. I wrapped about 6 turns on the core, and the inductance is 130µH. The small size
of the core mounts easily near the base of the sockets in a corner of the plenum box.
The letter "E" points to the grid grounding tabs.
The letter "B" points to the "Tune" detector for the cathode. This circuit did not appear at all
in the main schematic, so I had to plot it all out, and find the values of the parts in it. There is
a similar circuit in the plate circuit located near the plate decoupling caps. These two
circuits form what is known as a "phase-detector" and or "gain-comparator", of which a
sample signal is sent to the control board, and displayed on the plate current bar-graph as a
"tuning aid". This feature aids the operator in tuning the amp for best gain & linearity.
This feature works well, except for 10 meters where it was a bit erratic, likely due to a non-
linear response of the detectors. For the most part, the tuning aid works well, and should
help the "less experienced" user tune up well & quickly.
Control board
This is the main control center of the amp. All the LED displays are driven from this board,
all time-delays and relay sequencing is done here too. Operational cathode bias of 9Vdc is
derived from this board also. The control board biases the PIN diode T/R system as well.
The only mod I did to this board, of which I'm willing to discuss is that of the dial lighting
behind the "Tune" & "Load" knobs. I replaced the original incandescent bulbs, which are
short-lived, and hard to get at, with super-bright jumbo LEDs. I replaced R-3 on the control
board, which is a 51 ohm 1/4 watt resistor, with a 5k ohm 1/4 watt resistor. The original light
bulb sockets can be used with the new LED, but care must be observed in the polarity of the
LED when installing it in the socket. Then, bend the leads of the LED 90 degrees toward the
front panel. Using LEDs will allow the user the choice of several colors. I chose red. The use
of LED should mean that they will effectively last a life-time.
Some strange errors were encountered on the control board. The board I have did not
match the schematic in that the part numbers were scrambled, but the circuitry was the
same. Scrambled part designators can really make circuit tracing difficult. Perhaps the board
I have is a later release than the schematic, or vise-versa. Another possibility is that it is a
variation of the "export" version, differing from the "domestic" schematic.