A rare dual-grid (European)

Certain vacuum tubes dating from the 1920s and 1930s have become quite rare. To bring radios from that era back to life, one can replace defective, unobtainable tubes with electrically equivalent ones. This process sometimes requires electrical and/or mechanical modifications. Repairers used to perform such work during periods of technological transition; consequently, many 1930s radios are still fitted with "Rimlock" rectifiers— that is, a rectifier tube using the "Rimlock" technology introduced in the 1950s. Today, it is restoration enthusiasts who face these challenges—challenges that are, incidentally, fascinating.
(*) Ersatz: substitute product.

Replacing the A441N double-grid tube

This legendary A 441 tube is one of the first double-grid tubes. It allowed mixing the reception frequency and the local oscillator frequency to obtain the intermediate frequency in the first superheterodyne circuits. Its characteristics are summarized below. It has become rare and therefore expensive. Moreover, whether one owns one or two, there is hesitation in installing them during the tests of vintage radios being restored. It is tempting to replace it with an "ersatz" as they said on the other side of the Rhine. Some believe that this substitute must be based on an electronic tube (for example, a pentode); the less purist will opt for the solution based on semiconductors proposed below.

Transistorized Ersatz for the A441N double-grid tube

(M. Mur)

The schematic below by M. Mur allows the construction of a biFet transistor ersatz

fig.1 BiFet transistor Ersatz

The BF961 datasheet is here.

Replacing an A441N with a Russian 2Ж27Л(2SH27L) tube

(Based on an article by Wolfgang Holtmann published on the Radiomuseum forum)

This tube is heated at 2.2V/57mA. It is necessary to drop 1.8V with 31 Ω, distributed across the 2 filament terminals by 2 resistors of 15 Ω (¼w). Restorers who practice this type of substitution remove the tube's envelope (delicate operation). Today, the most difficult thing is to obtain the Europe 5 pin socket. The 100K resistor, which degrades the tube's slope, and the 150p capacitor allow approximating the characteristics of the antique double-grid tube. This tube seems to be easier to find in Germany.

Two datasheets, unfortunately only in Russian, can be consulted here.

datasheet #1.

datasheet #2.

fig.1 2SH27L

fig.2 2SH27L

fig.3 2SH27L

fig.4 2SH27L

For information, the prices of European tubes in 1936 with the prices corrected in constant euros
(Click on the figure to enlarge)