The R.4 was a hard vacuum valve developed from the R type to
meet a specific need for a heterodyne receiver. See also:R2
The lettering on the envelope reads: Valve wireless
receiving R.4 Pat. No. 4411. A10528. The other side reads: Osram
GEC Made in England. R.4. The Naval version is the NR4.
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zaterdag 03 december 2016