b. INTERMEDIATE STAGES. - Following
the master oscillator a type 6AK6 is employed in an untuned, Class A amplifier
stage. This stage completely isolates the master oscillator from the remaining
tuned stages. The 6AK6 drives a series of three frequency multiplier tubes,
the first of which is a type 6AG7. The operating frequencies at the plate
of the multiplier tubes for the different bands is given in the following
table.
1ST MULT.6AG7 | 2ND MULT.7C5 | 3RD MULT.7C5 | |
80m | 3.5 mc | 3.5 mc | 3.5 mc |
40m | 3.5 mc | 3.5 mc | 7 mc |
20m | 3.5 mc | 7 mc | 14 mc |
15m | 5.75 mc | 10.5 mc | 21 mc |
11m | 6.8 mc | 13.6 mc | 27 mc |
10m | 7 mc | 14 mc | 28 mc |
c. POWER AMPLIFIER STAGE. - A type
4D32 tetrode power amplifier tube is used in the PA stage. This tube always
operates as a straight amplifier. The plate circuit is tuned by a combination
pi-network and "L" network which is band-switched along with the multiplier
stages. The combination network reduces the output impedance to around
50 ohms on all bands by means of inductance and capacitance switching.
The output network will actually operate satisfactorily with antenna impedances
in the range of 26 to 200 ohms. It is also affective in reducing harmonic
output of the transmitter. During phone transmission, the screen grid and
plate of the 4D32 are both modulated. Plate and screen voltage is obtained
from the high voltage supply while filament power is obtained from the
low voltage plate supply transformer. The tube is biased with 75 volts
of fixed-bias plus some grid leak bias.
4.2.2. AUDIO CIRCUITS. - The first and second audio amplifier consists
of a type 6SL7 tube operated as a cascade amplifier. A volume control,
R205, is located in the grid circuit of the second amplifier stage. The
driver stage employs a type 6SN7 tube with the two triode sections operated
in parallel to drive the modulator stage. The modulator stage utilizes
a pair of type 807 tubes connected in a push-pull circuit and operating
class AB2. The output of the modulator is coupled to the final amplifier
by transformer, T202, to modulate the plate and screen of that stage. During
CW operation, the secondary of the modulation transformer is shorted out
by S302A. Bias for the modulator tubes is adjustable by R305, is obtained
from the low voltage supply and is regulated by the voltage regulator tube,
V304, type VR-75. The secondary of the modulation transformer has a 500-ohm
tap provided for supplying 60 watts of audio power to an external load.
4.2.3. HlGH VOLTAGE SUPPLY. - The high voltage transformer is energized
when the contacts of relay, K301, are closed. The high voltage supply employs
two type 5R4GY rectifier tubes connected in parallel in a full wave circuit.
The output is filtered by a single section choke input filter. This supply
furnishes voltage for the plate and screen of the final amplifier and plate
voltage for the modulator tubes. The amount of output voltage from this
supply may be either 600 volts or 700 volts, depending on the position
of the tap switch, S305, in the primary winding of the high voltage transformer,
T302. For the same power input, the efficiency of the final amplifier improves
with the higher operating voltage. The tube manufacturer recommends no
more than 600 plate volts for phone operation, but this is for CCS rating.
A pair of 15-ohm resistors are connected in series with HV plate transformer
primary for "tune-up". These are shorted out when operating.
4.2.4. LOW VOLTAGE 8UPPLY. - Transformer, T301 furnishes power for
both the voltage plate supply and the filament of all tubes in the transmitter.
T-301 is energized by closing the LV switch, S304, Three separate windings
on transformer, T301, furnishes filament power to the tubes, The low volt-
age plate supply employs a type 5Z4 rectifier tube in a full wave circuit
with a two section choke input filter. This supply has a total output voltage
of approximately 315 volts, 240 volts is supplied to the audio amplifier,
oscillator, buffer and multiplier stages. Bias voltage for the modulator
and final amplifier stage is furnished by this supply, It also supplies
voltage for the operation of relay, K301.