INTERNAL ADJUSTMENTS
There are only two internal adjustments. These are pre-set at the factory and should need no adjustment by the operator. Because both adjustments must be made with the bottom cover off and the amplifier power on, they are dangerous. Voltage in excess of 800 VDC is present at several points both top and bottom whenever the Power Switch is on with the power plug inserted in a live receptacle regardless of the position of any other switches. The Standby Switch does not cut off the high voltage; it just prevents operation of the keying circuit. If no qualified technician is available with high-voltage experience, we recommend you send the amplifier back to the factory if internal adjustment or repair is necessary. The bias adjustment will need to be made whenever one or both power amplifier tubes are changed or have been in use for a long period of time and have changed with aging but are still usable. The meter adjustment pot should never have to be reset unless there has been a failure of a component in the metering circuit necessitating replacement. For readjustment instructions refer to Servicing Section.

Tube Replacement
It will be necessary to remove the top cover; because it will require resetting of the bias pot, the bottom will also have to be removed. Take out the three hex head black sheet-metal screws on each side and the four similar screws on the bottom. Both top and bottom covers may now be removed. It is not necessary to remove the rubber feet to get the covers off.

WARNING: Be sure power cord is disconnected from the wall receptacle; and if the amplifier has been used recently. discharge the high-voltage power supply capacitors to ground with a metallic screwdriver before trying to remove the 6LF6 tubes.

The replacement tubes must be the short version similar to those included as original equipment. Otherwise they probably will short out to the top cover when it is replaced. 6KD6 tubes can be used in the amplifier if the plate connectors are changed to the smaller size used with the 6KD6 tubes. Before inserting the new tubes, inspect the base pins to be sure they are straight and evenly spaced. When inserting into the tube sockets, match the gap in the ring of pins on the tube base with the gap in the ring of holes in the socket. Make sure all tube pins are lined up in the center of each hole in the tube socket and the tube is pressed evenly into the socket. Moderate pressure will be required, and gently rocking the tube will assist in getting it started. Never use excessive force. You may bend a tube pin to the extent of cracking the glass. rendering the tube useless by destroying the vacuum or even breaking the PC Board which holds the tube sockets and many of the components. Damage of this kind is not covered by our Warranty.

Setting Bias

WARNING: Dangerous high voltage in excess of 800 VDC is present at several points in this amplifier. Only qualified persons should turn this amplifier on or attempt to make adjustments with the covers off.

The resting current in the cathode circuit is measured by an indirect method. The voltage drop across a 2 ohm 10 watt resistor is measured and the current calculated from the formula Current Equals Voltage/Resistance or I = E/R. Since we know that the resistance is 2 ohms, we have to know only the voltage drop to set the proper idling current. The proper resting current without drive to the amplifier is 75 milliamps. From another form of the above formula, E=IxR, or E= .075x2, E= .15 volts. This then is the voltage we are adjusting the bias pot to plus .15 volts is quite small so a voltmeter capable of reading voltages below 1 volt is needed. Refer to the schematic and to diagrams for points of measurement. The .15 volts is measured from the junction of C4, R2 and L5 on the power amplifier PC Board (90T) to chassis ground.

The bias adjustment for the correct voltage is made by the small PC type potentiometer located at one end of the power supply PC Board (90P5). This is identified As R14 on the circuit diagram. Allow 5 minutes after first turning on the amplifier to be sure the cathodes have reached full operating temperature.

Meter Adjust Pot
This is the only other internal adjustment in the amplifier. It limits the amount of meter deflection during transmitting. It should never need adjusting after it is set at the factory unless there has been a component replacement in the meter circuit. Even then, unless the potentiometer itself was replaced, it is doubtful that readjustment would be required. The potentiometer is located on the bottom of the amplifier, and at one end of the power amplifier PC Board (90T). It is R15 on the circuit diagram. While transmitting in Hi Power, adjust R15 for the desired meter deflection.