What's 6ZP1?

6Z-P1 is a famous Japanese power pentode tube, designed for audio output stage of cheaper radio receiver.
The outline is ST-12 glass type tube with small shell six pin, called a UZ base in Japan. The electrical characteristic is similar to type 38 output tube, except for a heater current rating 0.35A (a heater power 2.205W) and a high mutual conductance.
This is the cheapest tube to be able to obtain 1 W audio output power with a higher sensitivity than American type 38, under a lower electrical power consumption than types 41 and 42.

6Z-P1 was developed during 1945-1946 as a 6.3V version of 12Z-P1 which was developed by Tokyo Electric Co. (Toshiba) in 1939, as an output tube of Japan standard transformer-less radio receiver. It was used in 1950s mainly for the cheapest radio receiver named the Popular Type*. Furthermore, it was often used in an audio amplifier as a pinch hitter of American type 42 and in a small Ham radio transmitter as a final tube.
It was said that a certain Ham guy had succeeded in the experiment to obtain a 10W output from 6Z-P1 amplifier using water cooler. 6Z-P1 had been manufactured until 1980's as a spare part of radio receivers.

* A deluxe receiver that used the American pentode 42 was called the "standard" type radio receiver. Furthermore, the highest one used 2A3.

6W-C5 is a Japanese penta-grid frequency converter tube.
Outline is a ST-12 glass type tube with small-shell and seven-pin, which is called Ut base in Japan. Electrical character is the same as US 6SA7/6SA7GT, except for heater power of 6.3V and 0.35A (2.205W).
Several years after the second world war, status of electricity supply was not so good. The oscillation-stop in the superhet receiver using old 6A7 converter was often observed, when the line voltage drop. Therefore, a new converter 6W-C5 was developed by Toshiba in 1947, as a special version of American 6SA7 with a strong heater to avoid such oscillation-stop.

Another important feature is that 6W-C5 has an outline of the old ST type. It means that each tube-company could use old mass-production machines for manufacturing a new 6W-C5.

The common feature of 6Z-P1 and 6W-C5 is the heater power 0.35 A, which has a strange value, compared with American tube standard 0.3 A.
The reason WHY??
This extra heater power 16 % was useful to keep the emission of tube against the AC line voltage drop, observed under the poor power supply system in several years after the war.

6Z-DH3A is a single diode single triode tube for detector and audio amplifier of radio receiver.
The outline is ST-12 glass type tube with Japanese UZ base. This tube was developed as an economical version of American type 75, which removed one of twin diodes. Therefore, the electrical characteristic is the same as 75 and 6SQ7.
First, a top grid version 6Z-DH3 and a 12V single ended version 12Z-DH3A were developed by Toshiba in 1947, then 6Z-DH3A as the 6.3V version was developed by NEC in 1948.