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PW Research Dept. Popular Wireless April 15, 1933.
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More power; less average HT current consumption! That is what Class B amplification offers the battery-set owner. And here is a compact, easy-to-build adaptor that will enable any listener who has a one-LF-stage battery set to try Class B without any but the most trifling alterations to his set connections.

The method of applying the adaptor is simplicity itself. In Class B two LF valves are required; the first is known as the driver, and the second is the special double valve. The driver is usually a small-power valve and it delivers power to the B circuit.

Obviously, then, if we tack a B valve on to the output power valve of an ordinary set we get the elements of Class B amplification. The only snag lies in cases where the set already has two LF stages, and the addition of the B valve would constitute a third. In such instances one of the two stages would have to be cut out.

Simple Connections

Note the special 'Class B' valve holder with its numbered sockets, which are marked in the sketch below.

But the majority of battery-set users employ one LF stage, preceded maybe by a detector and one, or two, HF, or by the detector alone. This part of the set does not concern us. What we are interested in is the fact that by simple connections we can use the power valve in the output stage of the set as the driver in our Class B system.

And this can be done very simply, as will be seen. In the main there are three forms of output circuits generally used in battery sets. The first is the direct plate-to-speaker feed from the power-valve; the second is a shunt feed through a filter-choke scheme or a 1:1 ratio transformer (incorporated in the set); and the third is either of the above methods used with a pentode valve.

These will be considered in order. Our purpose, of course, is to design a small unit, containing the necessary parts for the output section of a Class B amplifier, in such a way that with the minimum of connection it can be tacked on to the output of the power valve in the set to form a complete B stage.

Now, given that the set's power valve is suitable as the driver valve, all we need is the means of coupling that valve to the Class B valve, and a means of feeding the output of the latter into our existing loudspeaker.

Let us work backwards. We have a loudspeaker which is either a high-resistance type, or a moving coil with incorporated transformer.

Attaching to Set

Thus we need the B output matching transformer on our adaptor (the usual QPP type is OK here). In front of that we naturally have to put our double B valve, and in front of that we need our driver transformer to couple the B valve to the set's output valve. And that is all.

Now for the means of connection. The B valve needs a filament supply, via two flex leads from the B valve holder. It also needs HT.

This can be obtained from the set's supply, so we connect the HT terminal of the output transformer on the adaptor to the HT terminal of the driver transformer, and run a flex lead from there for attachment to the set.

There remains only a connection to the P terminal of the driver transformer, and another flex lead provides that. So we have an adapter with four flex leads from it, and with two terminals (on the output transformer) which will be used for the loudspeaker connections. As the B valve needs no bias we do not have to worry about that.

Now let us take case 1 of the three instances mentioned previously, that of a set with direct feed from output valve anode to loudspeaker.

Easy to Use

This set will have two loudspeaker terminals, one (LS-) going to the valve's anode and the other (LS+) to HT+. Such a set is easy to use with the adaptor, for all we have to do is to take our speaker off the set's terminals and connect it to the two terminals marked 1.7:1 on the adaptor transformer, next we take the two flex leads from P and HT on the drive transformer and connect them to LS and LS+ respectively on the set.

Finally, we take the two filament flex leads on the adapter and connect them to the two filament terminals of the output valve holder, making sure that the lead that comes from No. 4 terminal on the B valve holder goes to the negative filament terminal of the power valve holder (of course, we are assuming a 2 Volt LT supply).

Modifying Set's Output

Extreme simplicity is one of the outstanding features of this, the first Class B Adaptor for the home-constructor. It was in PW that details for constructing. the worldbs first short-wave adaptors appeared, so this latest device follows a precedent that proved of world-wide popularity.

No bias adjustments need to be made if the set is working from 120 Volts HT, but if it is operating from more or less, the HT voltage on the power valve (and thence the B valve) must be made 120, and the bias of the power valve adjusted to suit. All B valves are designed for operation on 120 Volts HT.

In the ease of No. 2 type of set (choke output or 1:1 transformer) we make the same adaptor connections except for those that go to the output of the power valve (from P and HT on the drive transformer). When the set has a filter or transformer output the P flex should be taken direct to the anode terminal of the output valve holder, this terminal being disconnected from anything else. The HT flex then goes to the 120 Volt HT terminal of the set that feeds the filter choke or output transformer.

The third case, where a pentode is used, is rather difficult. In the first instance the output filter or transformer device must be removed, and the adaptor connected as outlined in case 2, and then the pentode must be replaced by a power triode (something of the order of the LP2, P215, and so on, being suitable). Bias readjustment, of course, is necessary.

Increased Power

There is a further possibility, and that is that in the event of a pentode being used the output choke or transformer may have been dispensed with, the pentode being taken direct to a pentode tap on the loud-speaker transformer, or even to a specially wound pentode speaker.

In this case the triode must again be used, and necessary speaker impedance adjustments undertaken to make it suitable for triode, instead of pentode input.

The adaptor will take but a short time to build, and a few moments only to connect, but it will give a genuine Class B output, with all the advantages of increased power and decreased HT current consumption that accrues from that form of amplification.

Components List featuring the Cossor 240B Class B valve

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