A rare triode (European)

Some vacuum tubes dating back to the 1920s and 1930s have become quite rare. To bring radios from that era back to life, one can replace defunct, hard-to-find 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, equipped with a rectifier tube using "Rimlock" technology, which was introduced in the 1950s. Today, it is restorers who face these—admittedly fascinating—challenges.
(*) Ersatz: replacement product.

Replacing an A409 triode

Triode tubes for battery-operated radios date back to the 1930s. Tubes such as the A409, A410, A415, etc., are hard to find and/or often unusable. While waiting to find replacements, the radios remain silent. One can therefore try to temporarily replace them with more modern tubes. Examining the tube's datasheet suggests that it can be replaced by a number of other tubes.

Vintage Radio Notes

Substitution example

(Source: JC Jardine)

The "Miniature - 7-pin - battery" series from the 1950s may be suitable (DK92, DF91, DL95... = 1AC6, 1T4, 3Q4...).
These tubes, which were used in portable receivers, are still available at reasonable prices.
The comparison table below regarding anode current and voltage indicates that the DL95 (or 3Q4) tube is suitable.

	Heater Voltage (V)	Heater Current (mA)	Anode Voltage (V)	Anode Current (mA)	Bias Voltage (V)	Transconductance (mA/V)
A409	4	65	80	3.5	-9	0.9
3Q4	2.8	50	90	7.7	-4.5	2

The 3Q4 pentode will be wired as a triode. To do this, the plate and screen grid will be connected together.
The remaining tasks are to convert the base connections and adapt the heater filament voltage. Paradoxically, this is the only technical difficulty.

To lower the heater voltage from 4 V to 2.8 V (a drop of 1.2 V) at a current of 50 mA, a resistor is placed in series with the filament:
R = 1.2 / 0.050 = 24 Ω ...is therefore necessary.
This resistor will be distributed across each filament terminal. We will therefore place a 12 Ω (1/4 watt) resistor on each filament terminal.
The DL92/3S4 tube (same pinout, same heater) is also suitable.