Copyright © 2019 KG7TR. Technical information on this site may be shared in the interest of promoting the hobby of amateur radio. I do ask that you give proper credit to KG7TR for my equipment designs.
“The Real McCoy” is a vacuum tube, single sideband (SSB) transmitter that operates lower sideband (LSB) only, at any frequency from 3.5 to 4.0 MHz, with nominal 100 watts peak envelope power (PEP) output. I built this radio primarily for use on the west coast Vintage SSB Roundtable that meets Tuesday nights on 3.895 MHz. Its design and construction meet all of the vintage criteria for use on that net.
In 1952 Lew McCoy, W1ICP (Silent Key in 2000), founded the company that bears his name. The McCoy product line spanned several decades, and consisted of quartz crystals, quartz crystal filters, crystal ovens, and other frequency controlling devices. Lew McCoy was also one of the pioneers of amateur radio. He authored many articles in QST and CQ magazines, and was known for explaining technical subjects in practical terms for his readers. I had the pleasure of meeting Lew at the Ft. Tuthill hamfest in Flagstaff in the early 1990s. So like it says on the box, and as a tribute to Lew and his many contributions to the hobby, I decided to name this radio “The Real McCoy”.
This button will take you to an old QST ad for McCoy filters.
Top view of chassis is shown at the left. The PA uses a pair of 6883s (12 volt version of 6146). Perforated aluminum shields have been removed from the top of the PA and low-level/VFO compartments for the photo. The VFO uses the familiar variable capacitor and coil from ARC-5 transmitters. McCoy filter is seen at the left center. Ten tubes are used, and they all face inward on the original U-shaped chassis from the scope. This provides optimum cooling air flow down the center. The fan at the rear is mounted over the hole that originally housed the CRT socket.
Bottom view of chassis is shown at the right. The two original large holes at the rear were covered with perforated sheet to mount the PA tube sockets. This also allows cooling air to flow up over the tubes. The high voltage rectifiers and filters are housed in the shielded compartment at the upper left. This area contained the CRT high voltage supply in the original scope. As with most of my other radios, toroids were used for the low level inductors.
New rear panel fits beneath the original plastic frame of the scope. A 12 VDC fan pulls air in from the original ventilation holes at the front of the cabinet and exhausts it from the rear. The radio runs very cool. A large compression trimmer above the coax connectors provides coarse loading for the PA pi-network.
The photos at the left show the left and right sides of the chassis respectively. The toroid power transformer is a modern unit from Antek. It is very efficent, runs totally cool, and fits nicely in the space available. On the left in front of the high voltage compartment is the small 365 pf PA loading capacitor. A larger unit would not fit, so the mica padder on the rear panel was added to provide the additional capacitance needed for 80 meters. The filter choke mounted in the middle of the PA pi network components, as well as the low voltage transformer on the front panel, are examples of how parts had to be located wherever there was space. The VFO tube (mounted in the white ceramic socket) and associated components are located at the very front of the right side of the chassis, where they pick up minimal heat from other tubes.
The radio was packaged in the cabinet and chassis from a parted-out Tektronix 465B oscilloscope. New front and rear panels were fabricated. The main tuning knob appears to be from a BC-221 Frequency Meter. The frequency is displayed on a six digit counter/display module from MPJA. It uses blue LEDS. The meter appears to be from an old CB radio, and shows Power Amplifier (PA) plate current or ALC action on new custom scales. All water-slide decals were made using the techniques described in my homebrewing tips document.
The block diagram is shown below. A solid state mixer circuit using a vintage MC1496 IC had to be incorporated late in the build to feed the counter/display module, after it was discovered that its firmware could not perform the required IF minus VFO offset calculation. The mixer circuit is mounted at the top front of of the chassis, right behind the counter/display module. Regulated 12 VDC powers the solid state circuits and fan, as well as the filament of the VFO tube for good stability.
The Real McCoy
Construction of the radio was prompted by acquisition of a McCoy 9.0 MHz crystal filter and matching carrier crystals from NU6X. This was a new-in the-box (NIB) kit, as seen above. The filter inside was the premium “Golden Guardian” model, manufactured sometime in the 1960s by McCoy Electronics in Mt. Holly Springs, PA. Its specified 6 dB bandwidth of 2.7 KHz is printed on the filter itself. What looks like a QST ad for McCoy filters was inside the box. It was scanned and can be seen at the button at the bottom of this page.
This button will take you to the latest schematics for the Real McCoy.
At this button you will find a comprehensive technical description of the Real McCoy, including photos you can expand for more detail.