L4-B PLUS

REPLACING THE 3-500Z WITH A GU74B

L4-B PLUS

REPLACING THE 3-500Z WITH A GU74B IN A DRAKE MODEL L4-B AMPLIFIER

Objectives of the modification:

1. Replace the 3-500Z glass tubes with a GU74B metal-ceramic. 2. Keep the L4 PS power supply as original (no modifications). 3. Replace the RF switching with vacuum relays. 4. Preserve the aesthetics of the amplifier. 5. Maintain an output power cut of 1KW (see unmodified power supply).

Installation: DRAKE L4-B Reference Manual Connect the PTT-ON (VOX RELAY), ALC (XMTG AGC OUT) and RF INPUT cables to the RTX. Connect the RF OUTPUT to the antenna (ATTENTION ROS < 1.5) Connect the multipolar cable coming from the L4 PS Connect the high voltage cable

CAUTION DO NOT ACTIVATE THE AMPLIFIER WITHOUT HIGH VOLTAGE ON THE GU74B PLATE

L4-B PLUS amplifier operation Activate the ON – OFF button in the ON position, the RED light turns on (filament voltage present) Keep the CW/TUNE – SSB button in the CW/TUNE position. Set the BAND and PLATE controls to the required frequency, while LOAD is in position 5. Set the AGC/STANDBY – PUSH control to position 10 and pressed (transmission control not active).

After 60 seconds from activation in ON, the yellow light turns on. After 150 seconds from activation in ON, the instrument lamps turn on (amplifier ready). Set the AGC/STANDBY command – PUSH by pulling the knob the transmission command is unlocked. Activate PTT-ON without RTX power, reading the quiescent current on PLATE AMPERS between 0.08 and 0.1, return to PTT-OFF.

Set RTX for 20 Watt output and activate PTT-ON by performing the tuning procedure. Bring RTX to an excitation power of 70 Watt, go to PTT-ON and refine the tuning, adjust the AGC/STANDBY – PUSH control potentiometer for an increase in excitation while maintaining the Grid Current lower than 30 mAdc (rotate clockwise, towards 10, if it exceeds). In these conditions, 500 Watts are obtained in the CW/Tune position, while in the SSB position, 800 Watts are obtained with a single tone.

In voice with peak reading (Bird 43 set for peak readings) values exceeding 1 KW are obtained in the SSB position.

Circuit for G2 voltage
The 300 Vac voltage is bridge rectified and smoothed by capacitance and a 1.2K 7W resistor connected to a quartet of 0A2 gas-filled regulators which provide 300 Vdc to bias the GU74B screen grid.
A 1/16A fuse is placed in series, before powering G2, to protect the grid.
There is a shunt for measuring the screen grid current, full scale 0.1 A dc.

The functions performed by this part of the circuit are the following:
• Transmission command interface coming from external RCA socket (PTT-TX), active towards GND, current 10 mA dc closed circuit, voltage 12 Vdc open circuit, use of a 4N33. • Transmission command activation delay 2.5 sec, use of a 555.

• Relay RL1 to activate the instrument lamps after 2.5 sec AMPLIFIER READY and enable the transmission command after 2.5 sec PTT ENABLE.
• Relay RL2 to activate RF switching, voltage of 28 Vdc PTT-ON command vacuum relay RLV1 and RLV2 and activate relay RL4 fan speed.
• Relay RL3 to activate the switching of the BIAS voltage to the bias circuit of the GU74B.

Change switching circuit for instrument readings
The switch for selecting the flow rates has been replaced, in particular:
• Plate Voltage full scale 4 KVdc (Low reading scale)
• Grid Current full scale 100 mAdc (Low reading scale: 4)

G2 current reading keep current below 30 mAdc (reduce excitation if exceeded)
• 3000 watts fwd full scale 3 KWatt (High reading scale * 10)
• 300 watts ref full scale 300 Watt (High reading scale)
• 300 watts fwd full scale 300 Watts (High reading scale).