Check Out this Listing On
eBay
Imagine my surprise as I was lurking through eBay when I spotted this item. For those who will not start a homebrew project unless there are circuit boards that excuse has been removed! Snap this item up as when these are gone... they are gone. It is a proven design, and you have chance to claim, "I homebrewed a rig". (Well almost).
So back to the homebrew 20M SSB Hybrid Wireless set. Long ago I realized homebrewing a rig had a lot to do with real estate. Whether it is the space allocation on the front panel or the area needed for the circuitry... it is all about the real estate availability.
When I built the 20M Shirt Pocket SSB Transceiver the front panel (2X2 inches) was the minimum size needed to fit the I/O. Control knobs, jacks and switches all needed space and after an exhaustive evaluation, the 2X2 inches was the minimum. Fat Fingers also had a "finger" in that pie!
But also important is the physical arrangement of the tubes (or devices) that minimizes the wiring while at the same time considers unwanted feedback paths. The last thing you do is drill holes and the 1st thing is some paper layouts of the devices. In the new 20M rig, there is a bit of cheating based on tribal knowledge and having built the 40M version.
The layout above was developed over a 60-year period from the 1st metal plate that was used for the 40M version to the latest 20M Hybrid Wireless.
There is some "Tribal Knowledge" to soak in and I will try to point that out to you.
1. The right and left sides of the photo are essentially inputs and outputs. On the left side we have the Microphone Input (12AU7) and Audio Output from 1/2 of the 12AX7 where the other 1/2 is also the Product Detector.
2. On the right side the output from the 12BE6 Transmit Mixer goes to a Band Pass Filter and on to the Driver stage which I am keeping as Solid State. The signal coming from the Antenna Relay (not shown) passes on to a 2nd Band Pass Filter which is ahead of the 12BA6 Receiver RF Amplifier stage. One Band Pass Filter could be used and relay switched between these two parts of the circuit, but I used two to minimize any possibility feedback paths.
[Experience with the early Bitx20 showed how using a common relay could create a feedback path. Others saw this same problem.]
3. Our Digital Electronics (not shown) provide a LO and BFO signal. The LO goes to the 6BE6 Receiver Mixer and the 12BE6 Transmit Mixer. Thus, an LO port to the right of the Heathkit Filter and midway between the 6BE6 and the 12BE6 would provide an optimal location. BTW we found that in the 40M Hybrid Wireless that a ferrite core homebrew 2.5 MHy Plate Choke could also be used as a matching transformer to the 50 Ohm Band Pass Filter.
4. The BFO signal is supplied to the 6JH8 Balanced Modulator on Transmit and the 12AX7 Product Detector on Receive. Thus, a BFO Port to the left of the Heathkit filter and mid-way between the 12AX7 and 6JH8 is a good location. Some distance from and a bit of shielding is needed for this port, so the Filter is not picking up the BFO signal.
5. In this design and layout only one tube is used on both transmit and receive. That tube is the 12BZ6, the 1st IF amp stage. A small value Silver Mica cap samples the signal on the plate of the 12BZ6 and that feeds the 12BE6 which is the transmit mixer. Thus, the need to have the 12BZ6 and the 12BE6 in close proximity.
A nightmare in the 40M wireless was to not think about space under the chassis for terminal strips to hold parts. Allow some extra space around tube sockets and mounted parts that provide anchor points for 4, 5 and 6 lug terminal strips.
With this 1st sketch of a layout the next step is to place actual part sizes on a new sheet of paper. I have some metal washers that are the same size as 7 pin and 9 pin tube sockets. So, it is a simple matter to draw around the washers to represent the tube locations. The ARC-5 IF cans are quite larges and I used a cardboard cutout to represent their footprint.
The Heathkit filter I have is an older and larger footprint than the one shown in yesterday's blog. I need to absolutely verify that it works and if OK will use it in this rig. The larger sizes simply add an air of authenticity and aura of an older wireless set built with some parts from 2025.
The Digital LO with some in-depth thought can be a module mounted to the front panel and that avoids having to provide horizontal real estate.
Front panel controls would include the Audio, Mic Gain, RF Gain and Carrier Balance. Switches would include USB/LSB select, MOX, as well as Mic Input and Headphone output. Since the Driver will be broad band solid state the only 12GE5 Final Amp tuning control will be a Tune Cap using a fixed Loading Cap like in the Hallicrafters SR-160. The Heathkit Meter could be used for TFMS (Tune For Maximum Smoke).
My seat of the pants says a 12X12 inch chassis using an aluminum base plate of that size. Thank you, Lauren Bezos, as your OM sells 1/16-inch plate in that size. BTW get the 3003 or 5052 type aluminum which is easily bent, shaped and drilled. The 6061 is much more difficult to work without heat treating it first.
The back apron should include space for an 11Pin "octal type" power plug, BNC antenna connectors, RCA jacks, bias pot adjustment, ground lug and fuse for the DC circuits.
Them that know can make things go.
73's
Pete N6QW