1j42a (1j42a)

This is a dual control grid rod pentode similar to the 1j30b and  1j37b. In Russian it's marked 1Ж42А

See also Russian Subminiature Tubes for general Information.

The 1j37b is down to 12V on Anode and G2 for operation and about 59mA filament. The 1j42a can be used with 6V HT and is only 15mA filament

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I found this Electrometer circuit for the 1j42a (1Ж42А) . I presume it would work also for the  1j37b (1Ж37Б).

The power  needed  by  the  amplifiers  from  the  sources  (battery  or rectifier with  stabilizer) does  not  exceed  5  watts.
The  greater  part  of  the  power  is consumed  in the dropping  resistances  of  the  filament  circuits.  The  required power  decreases  a  great  deal when  the  filaments  are  supplied  from  a  separate source  (in the  case  of  a miniature  broad-band  amplifier [1 x 1j42a + 2 x 1j24b]  down  to 0.3  watt  and in the  case  of  a  semiconductor  amplifier  down  to 0.5  watt).
Thus,  the  tube amplifier,  having  somewhat  beter parameters,  may  not  be  inferior to the  semiconductor  amplifier  from  the  standpoint  of  economy [5W in example].

Electrometer using 1j42a and 1j24b (1Ж42А and 1Ж24Б)

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