KG7TR.com
Getting started. Major parts were moved around to get the best arrangement and to make sure there were no interferences.
Bottom view. Design is similar to HB-600, with angle stock brace that runs width of chassis to beef up plate transformer mounting. In addition, a right angle shield and aluminum plate are mounted under the toroid transformer to prevent flexing. Two 5 VAC/10 amp transformers are connected in series to provide 10 VAC to the 813 filaments.
At the buttons below you can download the schematic, parts list and detailed technical description for this amplifier. The tech description has pictures that can be zoomed for a closer look at things. For info on cabinet construction and general homebrewing techniques see the Octal Tribander tech description.
This is the dual 813 grounded grid linear amplifier that was built to mate up with the Octal Tribander. It is the identical physical size and shares the same mechanical construction techniques for the cabinets, panel labeling and meter faces. The overall layout and architecture are very similar to the HB-600 linear. As it turns out, vintage 813 tubes can still be found today at reasonable prices. And they are virtually bullet proof. This radio is completed electrically and mechanically, and has been on the air. It was featured in the June, 2016 issue of Electric Radio magazine. Full documentation is available at the buttons below.
2X-813 Linear Amplifier
Out of necessity the amplifier cabinet was built first, before the transceiver or power supply/speaker. Cabinet is made with an aluminum angle stock frame covered with .063 perforated aluminum sheet. It provides a lot of structural strength for the completed unit. Octal Tribander and power supply/speaker have the same type of cabinets.
Top view. Pair of 813s in grounded grid configuration put out about 600 watts PEP with nominal 2,200 volts on the plates. Plate transformer is an 800 VA toroid from Antek. It has dual 800 VAC/500 ma windings connected in parallel and fed into a voltage doubler that is identical to the HB-600. PA coil is an Illumitronic pi dux 2408D4. RF connections are made using braided wire. Coil is mounted on ceramic standoffs.
Detail showing how PA tuning variable is mounted on top of PA loading variable. Both sections of PA tuning variable are used on 80 meters. Only the rear section is used on 40 and 20.
Rear view. SO-239 connectors are for RF input and output. RCA phono jack is for the T/R relay. Two circuit breakers protect control circuits and plate transformer .
HV doubler board. Six 270 uf/500 VDC caps are connected in series. Each is paralleled by a 35k/10watt wirewound equalization/bleeder resistor. Six 1N5408 diodes are connected in series for each branch of the voltage doubler circuit. 25 watt chassis mount resistor is a 0.5 ohm shunt connected in the return lead for the plate meter.
Rear view with cabinet removed. 813s are recessed about 1.5 inches. Plate choke is a National R-175A. Pigtail at rear of chassis connects to fan mounted in enclosure.
The buttons below will take you to downloads of the B&W L-1000-A manual and GE Ham News articles on one of the earliest grounded grid 813 amplifier designs. Also available here is an RCA data sheet for the 813 from 1955.
Front of 2X-813 amplifier. Unit covers 80, 40 and 20 meters. High quality NOS Triplett meters with custom scales monitor plate current, grid current and high voltage. Fan can be switched to full speed when more cooling is desired.