▼ Menu

Recent Valve Developments

K D Rogers, Popular Wireless, May 27, 1933.
Extras ▼


The new ways in which valves can be used, as well as the development of special 'tubes' for the new schemes, are outlined for readers in this review.
Pentode valves show up to great advantage in Quiescent Push-Pull amplifiers, an example of which is Illustrated here.

It is not so much of completely new valves that I want to write here, though there are some new 'tubes' that have recently been placed on the market, it is the new uses to which many of the well-tried and popular valves can be put that concerns this page.

The development that most tickled the general public was probably quiescent push-pull, with its attractive double benefit - saving of HT, and increase of output power.

This brought a much-maligned valve into further prominence. I refer to the pentode, which has been the subject of more discussion and argument than any valve since broadcasting began.

Quite Large Currents

But whether for or against, we had to admit that the pentode was a somewhat greedy valve, though its powers of amplification and its power output could not be denied. The smaller pentodes, like the PM22A, Pen220, PT2, and so on, are less hungry for HT, and they give a surprising output for a small grid-voltage input. Large valves, such as the Mullard PM22, Mazda Pen220A, Cossor 220PT, took quite large anode currents, and so were not looked on with favour by dry battery users.

Quiescent push-pull altered things in this respect, and so we experienced a revision of interest in the large, steep-slope pentodes. It was almost like having new valves to have a new system of using old ones (even if that system was first thought of in 1915 though never generally used or advocated).

Steep Slope Valves

Following close on the heels of quiescent push-pull came the grid-current form of push-pull, generally referred to as Class B amplification.

This can be carried out with suitable transformers with ordinary triode output valves, but it is better obtained with special double push-pull valves, in which the two grids and anodes are enclosed in one bottle. The various valve concerns are busily engaged in developing these valves, two models of which (Cossor and Mullard) give in a three-valve amplifier a total quiescent anode current of about 7 mA or less at 120 Volts HT, with a peak maximum output of about 2 W.

Naturally, the anode current goes well up at this figure, for one cannot get output watts without putting any power into the valve. The big point is that the average anode current consumption is pretty economical.

Double diode mains valves for full-wave rectification or for automatic volume controlling have been developed, while the use of a separate valve for automatic volume control of large mains sets has long been used.

The double diodes incorporate a triode section so that the valve will amplify LF as well as rectify, while I believe DD Tetrodes and Pentodes are in the offing.

HF pentodes, giving greater stage gain than the popular SG valve, are on the point of release to the anticipating public.

Both mains and battery valves are being improved from time to time, and as an example I may draw attention to the Micromesh 2 Volt output valve which has recently appeared the PB1.

This is a valve with a slope of 4 mA/V, having an amplification factor of 16 and an AC resistance of 4 kΩ. It is an excellent output valve, being economical With HT (it requires 11 mA at 150 V, with 4-5 Volts bias, and 7 mA at 125 V, with the same bias.

The fact that the bias cannot be altered though the anode potential drops 25 Volts, gives a good idea of the steepness of the slope of the valve's curve. At 100 Volts the bias has dropped 1.5 Volts, being then 3 Volts, and the anode current is 6 mA.

Though not a valve, I must mention the Ediswan DLS1 thermal delay switch for mains sets. It. looks like a valve, and fits into a standard valve holder, and is certainly unbeaten as a satisfactory means of controlling the switching on and of of the HT of a mains unit.

The Very Latest

Being in a vacuum the contacts of the Ediswan thermal delay switch cannot arc, so that the switching is wonderfully silent.

Finally we cannot let this brief retrospect conclude without reference to the valves that have created more excitement than any for a long time.

I refer to the Marconi and Osram Catkin range of AC valves. These have just been introduced to the public. I can say that I have had some on test for some time now, and can definitely express my delight not only at their structural rigidity but their general mechanical and electrical efficiency.

So far only four Catkin types are available, but I understand that others will follow in due course, and I for one am eagerly awaiting their debut.

Special Variable-μ

Another recent arrival to our laboratory (it will be released to the public on June 1st, I believe) was the Mullard short grid-base variable-μ SG battery valve. It was used, with the new Mullard Class B valve (PM2B).

For the benefit of the others let me explain that the valve is designed to give the full volume control that is normally obtainable from a variable-μ valve of the best type, With a variation of grid bias of only 6 Volts.

This means that a bias battery of 6 Volts is all that is required for the full controlling of the valve, a very different thing from the 16 Volts that is usually required for battery variable-μ valves when close to the local station.

As a matter of fact, except when close to the station the average variable-mu valve can be used with only 9 Volts bias, but in the same circumstances the Mullard PM12M needs but 4.5 Volts. This makes it ideal for use with Class B, sets where the 4.5 Volts battery is quite sufficient for the bias of the driver valve.

Use browser back button to return.