Front view of matching power supply/speaker unit (PSSU). I originally wanted to use a single 6 x 9 speaker but couldn't find a decent one. So I ended up using two 4 inch square speakers because I had them. Not conventional perhaps, but it works. Switch on right turns on AC power. Switch on left controls filaments. In the down position only those filaments used for receive are turned on. In the up position the extra filaments needed for transmitting are powered up. That way I can just listen if I want without wasting power and tube life.
This radio is without a doubt my most complex project to date. It is a triband SSB transceiver covering 80, 40 and 20 meters, with 100 watts PEP output. It uses all octal tubes, 19 of them. All the tubes used were in production prior to or during WW II, with the exception of the 6146 finals (which were introduced in 1952). You can see the tube lineup in the block diagram below. Most circuits are cloned from the Octalmania radios - why reinvent the wheel?
This radio has been completed and has been used on the air with fantastic success. It was featured in the June, 2016 issue of Electric Radio. Schematics, tech writeup and parts list are available at buttons below. Along with the transceiver I built a matching dual 813 linear, which provides a 100% homebrew SSB station using WW II and older tubes.
Top view of PSSU. Transformers, choke and even speakers were all hamfest finds. The plate transformer on the left is a weird one made by Wahlgren, and is specified as 111 VAC rms input, 50-700 cps. At any rate it generates just the right B+ voltages and doesn't even get warm in use. To its right is a 6.3 VAC at 10 amps transformer for the filaments. Above that is a 6 henry choke for the +250 VDC supply. Choke input arrangement provides excellent regulation.
Below is the block diagram for the transceiver showing the tube lineup and general configuration. Unlike most commercial tube type transceivers, this one uses separate tuned circuits for the receiver front end and transmitter driver and PA.
Typical band switch section. All circuits use toroid coils tuned by ceramic trimmers. I use toroids because they are available, cheap, and have known characteristics. Useable slug tuned coil forms in decent quantities and prices are unobtanium these days. The toroids are mounted on 3/16 inch fiberglass pegs that are pushed into grommets. The diameter of the peg is built up with masking tape until the coil has a tight fit, and then the coil is secured with epoxy. Semicircular holes in bottom of shields provide clearance for wiring bundles.
At the buttons to the right you can download latest schematics, parts lists and a detailed technical description. The tech description has pictures that can be expanded for a closer look at the hardware, as well as a section at the end containing some general info on homebrewing techniques used to build these radios.
Octal Tribander Transceiver
Copyright © 2020 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.
Octal Tribander with matching PSSU and 813 Linear shown side by side. This integrated homebrew station has been used on the 20 and 80 meter Vintage SSB Nets, and was featured in the June, 2016 issue of Electric Radio magazine.
Inside of PA cage. A rotary inductor is used in the PA because I had a new one, and it avoids the complication of extra linkage to rotate a switch in the PA cage off the main band switch shaft. Plus, with a variable inductor you can find the optimum inductance without fooling around with coil taps. The front panel dial is a 15 turns counter normally used for precision pots. It works fine in this application.
Rear view with cabinet installed. Internal relay switches metering zero pots between S meter and ALC circuits.
Bottom view of PSSU. The "economy" configuration is used so only one plate transformer is needed. Unit puts out nominal +800 VDC at 250 ma, +250 VDC at 150 ma, -125 VDC at 10 ma, and +20 VDC at 150 ma (for relays and digital display). Angle stock runs sideways on chassis and picks up two mounting studs each from plate and filament transformers to beef up things.
Rear and top views. A mix of glass and metal tubes is used. Metal tubes have an integral shield that comes out on pin 1. Main advantage to glass tubes is that they glow in the dark. Like Octalmania, this radio uses the 2.1 KHz INRAD crystal filter (#2309).
Bottom view. A lot of thought went into laying out the circuits. Being a transceiver, several signals are common to transmit and receive circuits and end up crisscrossing around the center of the chassis. The sheet metal work was onerous and probably took longer than the wiring. The bandswitched circuits run down the chassis centerline from front to rear. Seven ceramic switch wafers provide 16 poles of switching.
Rear view of PSSU. A 4 amp breaker protects all circuits. A label on rear of this unit and transceiver identifies power requirements and pinouts.
Octal Tribander front panel. It works great. I used a digital frequency readout because making an old fashioned mechanical readout with 1 KHz accuracy is an impossible task. But the actual VFO is genuine ARC-5 analog stuff using a common as dirt 6SK7 in a Colpitts circuit. Front panel labels and custom meter face were designed using Microsoft Visio. Original meter scale plate was painted white and the new custom scale was glued to it with spray adhesive. Panel labels were printed on laser water slide decals from papilio.com. Panel was first painted with light gray primer, decals were applied, and then it was sprayed with clear matte finish. See Homebrewing Tips for details.