RF Deck Construction



Below are photos of my small machine shop and test bench, both of which were essential to this project. The metalwork turned out to be more difficult and time-consuming than I expected. (I've learned my lesson and in the future will use commercial enclosures!) The amplifier project fell into five phases of roughly equal time: (1) circuitry and mechanical design, (2) metalwork, (3) printed circuit board layout, (4) assembly and construction, and (5) testing and debugging. The test bench photo, below right, shows the amplifier and power supply during final testing.


Machined square bars interlock at corners. A single screw secures the joint.
Completed frame. Tolerances were maintained to within 0.015 inch to keep frame true..
Frame & side panels require 60 tapped holes!
Chassis punches make tube socket cutouts easy
Completed chassis metalwork
Checking to see if front panel fits
GU74B tube sockets
Finally, time to mount components

Printed Circuit Boards

Control Grid Reg. PCB (drawn with Omniglyph. Get it HERE)
Screen Grid Voltage Regulator PCB
Front Panel PCB holds most front panel components
QSK PCB (see June 2015 QST for writeup)
RF Sense PCB (attaches to input RF Connector)
High Voltage Monitor and fuses


Construction Details


Input attenuator provides a passive 50 ohm load for the exciter and is rated at 100W. No frequency compensation is needed for 160m and 80m.
Detail of plate choke construction. The form is made of fiberglass tubing with solid drilled and tapped end caps.
Completed plate choke has 230 uH inductance. Phosphor bronze hardware is used in strong RF fields to avoid heating.
Detail of parasitic suppressors, plate choke, and blocking plate capacitors. Teflon sleeves route the hot exhaust air to vented openings in the top enclosure cover.
Detail of the RF sensor PCB mounted on the input RF BNC connector, and the two back-to-back Jennings RJ1A vacuum T/R relays. Feedthrough capacitors bypass all the DC leads into the tube compartment.
This inductor, shown here disassembled, started life as the tank coil in a Gates AM broadcast transmitter
Here is the completed tank coil, all spiffed up and cut to the target inductance of 16.5 uH. Not shown is a tap for the 80m inductance. The high voltage cable between the RF deck and the HV power supply uses RG6 coax with reverse polarity "SHV" BNC connectors. This is a safe, rugged way to route high voltage. You may not recognize this interior view of a Bird 1000W wattmeter slug. This slug was modified to give a 3000W full scale reading on a 50 uA panel meter.
This is the "L2" coil for the tank circuit, fabricated from 3 T-200-2 toroid cores, taped with fiberglass and wound with Teflon-insulated 10 AWG wire. The 80m tap has not yet been installed. Detail showing the plate tune and load capacitor mounting. Shaft couplers route the rotors through the sub-panel to Jackson Bros. planetary reducers that provide vernier tuning.The two doorknob loading caps overheated during testing and were changed in the final design. These solid state timing modules are an easy, inexpensive way to ensure an accurate warmup time for ceramic-metal transmitting tubes. These are widely available on eBay for about $10. The GU74B requires 180 second warmup.
Look closely and guess who dropped and broke the $150 vacuum relay on the right. Ouch! There are five vacuum relays in the amplifier RF deck, plus another three in the power supply. Here's a tidy way to eliminate the frayed ends of coax cable shields. That, plus Teflon-insulated hookup wire, stainless steel hardware, and the liberal use of Ty-Wraps, make for clean wiring. If it doesn't look good, it isn't good! There's a lot of paperwork involved in any large homebrew project. Down the road, you'll thank yourself if you make the effort to keep well-organized detailed notes and records on your work.
Breadboard mockup of the screen voltage regulator. During testing, I found a mysterious motorboating problem that took days to diagnose and fix. Breadboard mockup of the control grid regulator circuit. The switching properties of numerous vacuum relays were evaluated before settling on the Jennings RJ1A for the QSK T/R relays. A digital oscilloscope made comparisons easy.


Completed Amplifier With Covers Removed

Amplifier left side with panels removed.
Amplifier right side with panels and tube cooling sleeves removed



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