I have built most of my transmitting equipment since getting into ham radio in high school. Several things influenced me to build this amplifier. My interests are in HF and 6M operation and combining those bands into a common/single amplifier as are these bands so combined in modern transceivers seemed like a logical extension of the concept. This is not common in commercial amplifiers. This amplifier represents an hundred years of developments and refinements in high power amplifying equipment and was developed with the ARRL Centennial celebration in mind. It has some features that you just don't find in commercial amplifiers today.
The use of high power on the 6M band while living in a compact suburban setting has been practically unthinkable until recent years with the advent of underground utilities, and now, particularly with the removal of VHF TV broadcast to UHF HDTV.
The basic design begins with the choice of a triode in the usual grounded grid circuit. The several good reasons for this choice are that it is simple, it does not require grid supplies and protection circuitry, linearity is excellent, and input power is not wasted in a dummy load as is required at the input to the grid in grounded cathode operation. In fact, with the grounded grid amplifier the drive power shows up in the output.
Beyond these basics, features are added at the input and output of the amplifier to make it more flexible in operation. There are separate inputs for HF and VHF. Likewise there are separate outputs. Several transceivers can be connected - or one transceiver with two outputs - and separate outputs are provided to the antennas The bandswitch position determines which connections are used when the amplifier is keyed. Two TX key lines into the amplifier are provided for.
The amplifier has some novel switching to accommodate the combination of HF and VHF functions. It has parasitic stabilization as described in my QEX article of September 2003. Geared drives and dials provide means for accurate logging and quick resetting the amplifier when moving between bands. A fast overcurrent shutdown circuit provides protection to the amplifier and power supply. A warm-up timer protects the tube from premature operation.