It is with some sadness but some relief that we are no longer pretending that we can continue with Valve of the month, simply because the list is not endless. We could continue with non-audio valves, but that is not really are reason for being. We will bring you more information about particular valves as they come to us. However, you can glean lots of fascinating facts about all sorts of valves at the excellent r-type.org website here.

Ladies and gentlemen, for your valve entertainment we bring to you an unremarkable audio valve from the 1930s. The Cossor 402P is an audio triode of, by modern standards, a miserable device!
From the start of the AC/DC  chassis era, this is part of a whole raft of series heater devices. This one requires 40 volts at 200mA to light it up. Yes folks, that's 8 whole watts to energise the heater.
Then, with your 200 volts of HT, you can supply a further 3 watts to ultimately obtain your 0.75 watts of audio, which Cossor described as 'adequate for domestic purposes'.
That equates to a little less than 7% efficiency.
Sadly, it took a World War to consign this miserable device to history.

Just when you think you might be running out of audio valves for the Valve of the Month spot, along comes another interesting one. We found this example inside a McMichael compact radiogram (model 545) manufactured in the 1950s. The fifties were an interesting decade of the valve era with several manufacturers trying to influence design. This KT81 was Marconi's equivalent to the American 7C5. It is a B8B or loctal based audio output valve with a pressed glass foot. This is electrically identical to the 6V6 which can give 4.5W single ended or up to 14W as a pair in push-pull configuration. These loctals along with the miniature rimlock devices first appeared in 1947 but became obsolescent by 1951 when manufacturers decided that the B9A form was a better option, probably due to the increased production of tv receivers with higher valve counts. Incidentally, the 6V6 device was available in 9 pin miniature form as the 6BW6. The Chinese 6P1 has a very similar performance but with a different pin-out.

This month, lovely viewer, we have the EF86. An audio pentode.
Mullard introduced this miniature low noise, low hum device to supersede the EF37A and EF40 (octal and loctal valves).
When new they are really excellent valves with an equivalent noise voltage of 2uV and hum voltage of 1.5uV when correctly wired to a balanced 6.3 volt heater supply.
Their lifespan, however, is reduced by electron poisoning of the screen grid, leading to secondary emission and increased noise.
Careful design reduced microphony to a very low level especially if the valve was mounted on a sound insulating valve base, enabling its use in tape recorders with internal loudspeakers and motors sharing the same chassis.
Sensibly equivalent to CV2901 6267 6BK8 6F22 & Z729

This month's curious offering is the PEN383 from Mazda. Our example comes from a Murphy D70RG radio. This radio is one of the first of a generation of the awful  'live chassis AC/DC sets. That is to say, receivers operating without an isolating mains transformer. This was particularly nasty if you hooked up to the 'gram' input, rendering your record player live! The Pen383 is a beam tetrode audio output valve on a Mazda Octal base that was designed for  200mA series heater connection.. The Pen383 could produce just under three Watts of audio (single ended) and was the 'series wired' near equivalent of the PEN45. It has a slightly larger than normal envelope for this sort of valve, measuring 54mm in diameter. Also of note is the internal graphite coating which prevents us from getting a good look at the insides. Thanks to Mazda wiring the heater to pins 1 and 8 instead of the more usual pins 2 and 7, it has no direct equivalents. In summary, a bit of an odd ball.

It's a sorry day here at Rush Radio. We are dropping back to 'Valve of the Month' because it was becoming to erratic trying to maintain 'Valve of the Week'!
Anyway, here to welcome in the new regime, please give it up for the remarkably advanced for it's time, the Mazda QP230!
Introduced to the world in 1936, designed for battery operation, the QP230 is a directly heated double pentode for audio output use. It was designed to operate from a 2 volt accumulator filament supply and a 120 Volt HT battery. The two pentodes being designed to operate as a quiescent push pull output stage in class B1 . It's a veritable thermionic integrated circuit!
Let's look at the spec.
Standard British 7 pin base. Filament requires 0.3 amps at 2.0 volts.
Output power for cathode biased with 120V supply is a useful 0.85 watts for 5% THD, but can operate down to 100 volts of HT and still give 0.57 Watts of useful audio with a drive of 19 Volts pk-pk across the grids.
Optimum load impedance between 16 and 18kohms.

We thought we would go in the opposite direction this week. One of the lowest power audio output valves is the DL96. Designed as part of the "25mA" series of directly heated valves for battery portable radios, which were popular in the 1950s, before transistors became commercially available. Here is the description from the National Valve Museum:

"The DL96 is an audio output valve from Mullard's low consumption dry battery valves from 1954. The audio output is low at 0.2 Watts but considering the low filament power it is quite remarkable.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 46 mm tall.Type DL96 was first introduced in 1953."

Unlike the other valves in the set (which can be operated as series 25mA or parallel 1.4V filaments) the DL96 has a centre tapped filament which could be configured for series 25mA at 2.8 volts or parallel 50mA at 1.4V to get the full 0.2 Watts of audio. There was an extra mode where it was possible to light up just one half of the filament but derive only 0.1 Watts of audio.
This example comes from a Pye battery portable radio, which we will feature on this page soon.