Bob Shannon's Page
4 Tube Radio
The first photo is an overview of the radio in its case, with one of its plug-in coil sets sitting inside the cover. Those coil forms are two inches in diameter, so this is a rather large radio.
The second photo shoes the panel layout. The tubes, from right to left, are 12AF6 RF amp, 12AF6 regen detector, (fine tuning dial), 12DZ6 AF amp 1, 12DZ6 AF amp 2. The RF gain control is below the RF amp and detector, main tuning vernier center, and the regeneration control below the audio amp section. The tubes are mounted 'Radiola style' using telescoping shield tube sockets, so only about 1/3 of the tube is above the panel.
The panel itself is .125 aluminum plate, very stiff, so the receiver is very stable.
The third photo shows the underside of that plate. The RF amp grid tuning coil is in the lower right and the detector transformer at the lower left. Main tuning, and fine tuning caps are along the vertical center line. You can just see how each tube is built on a smaller subchassis mounted to the front panel on standoffs. This spaces the tubes just right for that Radiola-style mounting, so you can see them glow.
I've built 3 sets of plug-in coils. Two cover from below the AM band up to the 160 meter ham band. The third coil covers a bit over 90 to 60 meters short wave. Performance copying SSB hams is excellent.
But with only two tuned circuits, and capacitive coupling (not to
mention stray reactance) this receiver starts to get too wide in its
selectivity at the higher frequencies. While the set is perfectly
usable, its clearly not a good performer above 4 mhz.
5 Tube Single Band SW Radio
My goal was to build a 5-tube set using very weak link coupling to build a dedicated short wave receiver. I wanted good selectivity up on 49 meters for DX short wave listening.
I decided on two tuned RF amps, with RF transformers between the antenna jack and first RF amp and between the first and second RF amp. Both RF amps are 1T4's, with screen grid control over RF gain or attenuation. The second RF amp feeds the detector, again by weak link coupling. The detector is also a 1T4, and its plate drives an interstage transformer feeding a two 3S4 tube audio amplifier.
The photo shows the polished aluminum front panel of my five tube
short wave set!
The controls are from right to left, RF gain / Attenuation, Regeneration, A+/B+ toggle switch, the Main tuning vernier dial, Dial light toggle switch, Fine tuning, and AF gain. The box-like gizmo over the top of the vernier dial is a home made dial illuminator thingy.
It works like this, there is a glass slide with an etched (vertical) line in it. This glass plate is mounted to an aluminum angle-stock bracket on spacers off the front panel so its aligned with the index mark on the vernier dial. A NE-2 neon bulb is arranged with a carefully placed bit of tinfoil behind the angle-stock so the bulb shines down incidently onto the index mark and dial.
The tinfoil takes the light from the other side of the bulb and edge-lights the glass slide so the etched line glows brightly against the indirectly lit dial and index mark. This eliminates any parallax between the dial markings and the stationary index mark caused by the thickness of the dial, and it provides excellent illumination.
I set the series resistor for the neon bulb carefully so the dial light also serves another important function. As the battery voltage drops, the dial light will eventually fail to turn on, even with the radio off.
This is a clear sign that the radios performance has started to fall off (sharply) due to low B+. Before it reaches this point, the dial light will start normally after switching the set on, but a few minutes later it will not turn on again after switching the dial light off momentarily (because the starting voltage of the NE-2 is much higher than the sustaining voltage).
The speaker grill is simply old perforated aluminum sheet with a satin finish, taken from the bottom of some salvaged chassis. The knobs are from Radio Shack, of all places.
The fourth photo shows the rear of the chassis. The first and second RF amps are on the right, near the A+ battery. Eight 9-volt battery make up the B supply. Another 9-volt cell against the front panel, just below the speaker is the C bias supply for the audio amps.
The antenna connection is a BNC connector at the corner of the chassis on the right hand side of the image. An antenna matching trimmer cap is just below that.
The detector is at the rear of the chassis, on the right of the image. The two 3S4 audio amplifiers are along the front panel.
The second photo shows the bottom of the chassis under the first and second RF amps, and you can see the RF transformers. The 3-gang variable cap is partly seen in the lower left of the image, with two glass piston trimmers to match the stages.
The next photo shows the other half of the tuning capacitor. The detector stage is at the top left, above the audio amplifier. Two audio interstage transformers are mounted to the sheet metal divider that keeps the stages isolated. feed through capacitors distribute all the supply voltages to the sections with wiring that is kept above the chassis.
The 3-stage variable cap is something of a beauty, found at a Hamfest, it came out of some old military command set receiver, and uses ceramic shafts and zero-backlash gears to turn slotted plates pre-tweaked to match the three sections. I turned a shaft coupler from an extra lenght of 1/4 inch potentiometer shaft using a hand drill, vice and a file, and mated this cap to a new production vernier dial from Ocean State Electronics.
The very last shot shows how all the stages are arranged.
I'm very happy with this receiver first for its performance, secondly for its appearance.
I've listened to China, Viet Nam, England, Germany, Japan, South America, India and Cuba, dozens of european countries, some island nation in the south pacific who's name I cannot even pronounce with this little set and a long wire hanging outside in the trees.
With the right tweaking of the coupling, the selectivity and sensitivity are quite good, and the receiver is very stable and easy to operate, controls are smooth and effective. I've added padder resistors to spread out the 'right zone' on the RF gain and Regeneration controls, so those are extra easy.
A set of batteries ($) runs the set for well over 10 hours, the exact life is not exactly known, but its at least reasonable, if not fairly decent.
Schematics will be along shortly. If you have questions, please e-mail me.