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Mystery tube

Jacob Roschy Ernst Erb Martin Renz Bernhard Nagel 
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Forum » Valves / tubes - Semiconductors » Very rare valves or unknown (not in RMorg) » Mystery tube
Harri Lumme
FIN  Articles: 7
Pict.: 5
28.Apr.21 22:52

Count of Thanks: 1
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I am evaluating a home-made receiver from about late 40's. It has a strange two-tube TRF design and I am going to publish a model page about it unless I find a similar existing page.

The receiver may have been sold as a kit(?), but the tubes are much older than the receiver. The front end tube is a 38 but the second tube is very mysterious. It seems to be a diode / pentode combination and my preliminary analysis suggests that the diode part is used as half-wave rectifier for the B+. The pentode part is the audio amplifier.

The tube is manufactured by Purotron and there is also number 181 many times embossed on the tube base, which has the 7-pin-B pinout.

The diode and the pentode have separate cathodes and it seems that the heaters are wired in series. The pentode suppressor grid is internally connected to cathode. The diode plate has a large surface area, which suggests that it may be designed to act as a rectifier(?).

The heaters of both tubes are wired in series, so probably both have 0.3 A heaters.

I enclose two images of the tube. One showing the top of the tube and a second one showing the lower electrode connections and the base. I hope they help in the identification.

Thanks for the help.



Jacob Roschy
Jacob Roschy
D  Articles: 1697
Schem.: 17
Pict.: 68
29.Apr.21 07:29
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Hello Harri,

it seems to be the 12A7.

Best Regards,


Harri Lumme
FIN  Articles: 7
Pict.: 5
29.Apr.21 08:09
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Thanks a lot.

It was so easy...