March 2002

Whoops! Another goof. Please see below.

The Final Word On My Thirsty PM2DX

Back in September 2001, I found a PM2DX amongst my valves. I then knocked it over. Fortunately, the valve didn't break. Strange thing was, the filament took almost 200mA (at 2V) rather than the data book figure of 100mA. I mentioned this in the column and several readers kindly measured the filament current of their PM2DXs. In total, three out of about seven PM2DXs took close on 200mA. Despite what the data books had told me, I reasoned that there must be (at least) two versions of the PM2DX in existence.

The mystery has now been solved by another reader who sent me a copy of an early Mullard PM2DX data sheet. Click here for a copy (214K, 300dpi GIF image). Do a 'save to disk' because it's intended for printing. On the data sheet you'll see that the filament current is given as 200mA.

It seems that quite early on in the PM2DX's production, Mullard did reduce its filament current from 200mA to 100mA. Okay. So what. Well, it just proves that with valves from the early 1930s and before, you can't always trust data books.

Low Voltage Receivers.

As I promised in the December issue of Practical Wireless, this time I featured a single-valve (a 6K7) regenerative receiver which was entirely powered by a 4.5V battery. Yes, that's 4.5V on the heater and 4.5V for the h.t. supply. A friend of mine built a copy and has told me that it does work. Okay, so it's not particularly good, but what do you expect with just four and a half volts!

Before I forget, here is the data sheet which covers the Wearite Medium Wave coils specified for the set. It's a 180K JPEG, at 300dpi and is intended for printing, so do a 'save to disk'. Look for the 'PA2' and 'PA7' coils.

It is important to realise that these low voltage receivers weren't always just novelty sets. In previous times of trouble, they have provided a useful and portable means of receiving radio broadcasts. In the August 1941 issue of The Wireless World there's a low-voltage portable set - designed by S.W. Amos - which uses two P220 triode valves. The filaments are powered by a 4.5V battery, while a 9V battery provides the h.t. supply.

The same author published a similar design (pictured) in the January 1942 issue, this time using two modern (for then) type N14 1.4V pentodes. Click on the image to get a 300dpi JPEG copy suitable for printing (48K). Although the set uses the typical arrangement of regenerative detector followed by one stage of audio amplification, the tuned circuit is unusual in that it has a very high L/C ratio. The 800uH coil is for the Medium Wave band, and is around four times the usual value. Such a high inductance maximises the dynamic resistance (L/CR - gain, to you and me) of the input tuned circuit making the set as sensitive as possible.

A Modern Low-Voltage Receiver

Low voltage valve receivers are becoming quite popular. They give much of the satisfaction (and nostalgia) associated with valve receivers, but have none of the usual high-voltage safety issues. One prolific constructor who has turned his attention to these sets is Lutz Gemerski DL4OBG. Lutz has designed and built a two-valve receiver which works extremely well. The set uses a twin-triode, regenerative detector/audio amplifier, followed by a frame-grid, remote-cutoff pentode for (headphone) audio output. There's a full article on this set here.

Nearing completion is a superhet design by Lutz. More on that later.

There is an error in Fig.1 on page 42.

The circuit around the heater and cathode of the valve is wrong. Rather than trying to explain, the correct circuit (a rather poor but CORRECT version from the original article back in 1948) appears below.

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