Tube radio in FRISK case


completed in Jan. 2012


Size: 2.8" x 1.3" x 0.42"(except protrusion of knob) 



*Tube radio built in FRISK case.
*It can drive only earphone but is 3 tube super het. with I.F amplifier.


Back side

Control part


使用球    : CK6418 x2 6977/DM160 x1

回路構成:周波数変換 中間周波増幅  グリッド検波
        CK6418    CK6418    6977/DM160

Block diagram:     Mixer-oscillator   IF-AMP    Grid-Detector
                         CK6418       CK6418       6977/DM160

Tube line up : CK6418x2 6977/DM160 x1


・A電池: ボタン電池LR44x1 1.5V
・B電池: コインリチウム電池 CR1616x6個 18V

Batteries requirement:

*A-batteries : LR44-AG13 Alkaline Button Cell x 1 1.5V
*B-batteries : Lithium Coin Cell Battery CR1616x6 18V






性能、他 :



Motive for making this radio and policies :

I heard that making various things into FRISK case is popular, and so, I think my project should realize tube radio in FRISK case, and begun the work.
But dimension of inside of the FRISK case is only 67mm x 28mm x 8.5mm (2.6" x 1.1" x 0.33"), and begin with devising circuit and structure to make the radio into FRISK case any way.

About schematic:

Pentode mixer-oscillator and I.F amplifier using two CK6418, and after that is grid detector with 6977(DM160).Circuit diagram of pentode mixer-oscillator is almost the same with other radios posted on this web site, and that of I.F amplifier is very ordinary.
Originally, CK6418 was developed for hearing aid, is smaller than ordinary sub-mini tube and its filament drain is only 10mA, very little, and so I use this type of tube.
Actually, 6977(DM160) is not a "tube" but a kind of indicator, it's likely that indicator for computer in early days. But its structure of electrodes and characteristics are just those of "triode", and is very small. So I use 6977(DM160) for grid detector. Because 6977(DM160) is really a indicator, its plate (having structure like grid) glow greenly by plate current.
Choke coil is favorable for plate load, but having no room for choke coil, so, resistor (volume control) is used.
At first, I planned regenerative detector directly after mixer-oscillator using two-CK6418s only (without I.F. amplifier). But its performance was insufficient, and more, there were still enough room. So I tried 3-tube super-het having I.F amplifier.

Inside part of dotted line in schematic diagram including tubes, I.F transformer are built on circuit board of 0.5mm thick and wired, as PWB-unit. All parts outside of dotted line in schematic diagram including batteries holders, variable capacitor, power switch, volume control, ferrite stick antenna, oscillator coil are fit to lower FRISK case directly and wired each other.
Power switch is under the volume control, so, for wiring switch, there are some small holes at bottom of the case and also holes for trimmers of variable capacitor.
B-batteries can not go into the case as they are, because. total height of B-batteries is 9.6mm. So, bottom case has hole of same diameter with CR1616 B-battery. and the hole is sealed with tin plate of 0.2mm thick. also work as minus terminal of batteries holder.
Variable capacitor is used without its back cover, even so its height is about 10mm, and can't go into the case. So, part of top of the case is cut out and top of variable capacitor is exposed from top of the case, and hidden by tuning dial.
I.F transformers and oscillator coil are converted from 7mm by 7mm square I.F transformer for transistor, using core only. Its original terminals are removed, and height are lowered to 7.5mm.
PWB-unit and lower case are assembled and wired separately, and thereafter, united each other.

Mechanical Structure:

All stations within about 30 miles can be received with built-in stick antenna in my home (ferroconcrete structure). And at night some distant station can be received. Sensitivity is pretty well.
But earphone is drove directly by output of detector and load of detector is resistor, so, volume of sound is not so loud. Capacity of batteries are small, and especially, A battery last only 1 hour or so.
Even though far from practical use, I think my original goal, that is; to confirm the possibility of tubes to make radio in the small FRISK case, are achieved.

Performance and others :



Schematic diagram

*Schematic is only a reference and can be use freely But I can't be responsible for any result

真空管 tubes

6977のプレートが光ります plate of 6977 glow

PWB unit

PWBユニットを外した状態 without PWB unit



Top; without tuning knob and sticker

Bottom; without sticker

部品配置 Parts arrangement