Page 83
RF CIRCUITS
INPUT CIRCUIT
An input impedance-matching pi network circuit for each
band
is
connected by Band-switch wafer 1. After passing
through the matching circuit, the RF driving power
is
coupled to the tube filaments by C32. Capacitor C21
equalizes any RF voltage difference between the filament
leads.
TUBES
The amplifier tubes are connected in parallel in
a
class
B
grounded grid- circuit. RF driving power
is
applied to the
filaments in the normal cathodedriven configuration. As
mentioned in "Power Supply" section, RFC 3 holds the
filaments above RF ground.
Pins 2, 3, and 4 of each tube are connected together
internally. Each of the three grid pins
is
bypassed to ground.
This combination of RF chokes and capacitors provides
a
predetermined level of negative feedback at the tube grids to
further reduce intermodulation distortion.
PC1 and PC2 are parasitic chokes in each tube plate lead to
suppress any VHF parasitic'oscillations.
The positive side of the power supply
is
connected in
parallel to the tubes through RFC 1.
Cooling
air
is circulated around the tubes by the fan.
OUTPUT CIRCUIT
The tuned output circuit of the Amplifier
is
a
pi network
composed of plate tuning capacitor C55, loading capacitors
C56 and C57, and coils L6 and L7.
Band-switch wafer 2 progressively shorts out the unused
portions of coils L6 and L7. The coil turns in use are tuned
to resonance by Tune capacitor C55. Load capacitor C57
is
tuned to complete the impedance match between the tubes
and the load connected to the RF OUTPUT. On the 80
meter band, fixed capacitor
C56
is
switched in parallel with
C57 to provide the additional capacitance required on this
band.
If a DC voltage is unintentionally applied to the plate output
circuit,
RFC 6 will provide a DC path to ground, thus
short-circuiting the high voltage supply and opening the
circuit breakers.
ALC CIRCUIT
Approximately 60 VDC ALC threshold voltage
is
available When the RF driving voltage
at
the junction of R21-R22
at
the junction of resistors R4 and R5, which form
a
voltage
exceeds the ALC threshold voltage, Dl8 will rectify the
'
divider across the bias supply winding of transformer T2. C5
negative half-cycles. C51 and C53 act as filters and RF
is
an RF bypass, and
R
11
is
an isolation resistor. bypasses: R23
is
an isolation resistor.
C47 couples some RF driving voltage to voltage divider
The negative voltage appearing at the ALC connector may
R21-R22. C48 and C49 are frequency compensating
be
coupled back to the exciter to reduce its gain and help
capacitors for R21 and R22, respectively.
reduce "flat-topping" of voice peaks due to overdrive.
To Next Page
Loading...
