Carbon Microphone Amplifiers – Eric Vast

This article is a general account of my encounters with the above, and for a very detailed technical analysis I refer the reader to the Brown version described in Volume I number 4. of Vintage Wireless. I attended in my youth Hendon County School, where the Upper forms ran a Scientific Society. We were a co-educational school but very few of the girls ever joined our ranks tint ii we put on a show of “ Wireless “ for all and parents.

At first we had little to show, just a rather nasty receiver constructed by myself and two other lads. However external lecturers were permitted with the Head’s approval. Accordingly we invoked the aid of the local Guru, one “Buzz Perrin who ran a shop on Hendon Broadwav. This was a young man who sold batteries, model steam engines, electric motors, and the occasional radio. He was not adverse to lecturing on the subject and offered a demonstration.

On the day there was a very good turnout of parents etc as at that time there was little on the air. You could get Eckersley on 2MT from Writtle once a week, some amateurs, BBC tests, Time signals from the Eiffel Tower, and not a lot else. There were even a few people present who had never heard a broadcast.

After our home product had performed to polite applause, ‘Buzz” put on a good show with his professional receiver, however the sound level was rather low. After a while he produced a small box which was connected up and a louder signal resulted. Occasionally the signal wilted, but a knock on the box brought it back. It sounded as if the box was giving out around two watts or so. When the audience left myself and a few others were shown as much as could be seen of what went on in the box. It was one of Browns Microphone Amplifiers. The output of the Receiver was fed to a driver which agitated carbon granules and made an enhanced signal. The Brown amplifier did not have much gain, only about 10db in the version on show. That was my introduction to a Microphone Amplifier.

Some years later when the Picture Transmission Equipment was being installed at the Daily Express, one of the American engineers involved was a most interesting character. He was a senior member of AT&T of about retiring age, who had started with them as a lad in I 880. He knew the equipment well and could operate it. Most of his time lie had spent with Long lines. I think that the trip to the UK was probably a going a way present. He told me that around the turn of the century, Microphone Amplifiers were in common use on the American telephone networks. The version commonly used had a gain of some thirty Db arid a low level output to suit the handset of that time.

They were so reliable that a serious attempt was made to go from one side of America to the other using ninety units in series. He said that you could understand the speech at the end of its 2500 mile trip just about but that its daily reliability was only about thirty percent. The service was after a short period of commercial use withdrawn. It is curious what large amounts of distortion speech will stand before it becomes completely useless.

There was a mainly Carrier circuit in the Middle Fast in 1944 from Tripoli in the East to Tripoli in the West (part of the time, also) whose diabolic roughness I once looked into at Cairo. A 1 Kw sine-wave when displayed on a scope, came out as a series of comb-like vertical structures, yet you could still understand it. Probably this trans-USA effort was the Microphone Amplifiers greatest day.

In the period before the 39/45 war, I was working for that part of the Western Electric organisation which dealt mainly with Cinema and Studio apparatus. However American Western Electric sold in Europe a quite outstanding Deaf Aid equipment which more than held its own against a growing number of models using valves. The amplifier it used was a small and very efficient Carbon Microphone Amplifier. Because they wanted these devices repaired and serviced, our base in north-west London was chosen.

We had an Acoustic Laboratory and trained people. I suspect that the work put in on these amplifiers, years before, was paying off. The amplifiers were fed from a rather large Carbon Microphone that held the gain control. The Microphone Amplifier itself was quite small, about one inch in diameter and three inches long. All the inside and the operating parts were very heavily gold plated. The carbon granules were spherical graded pieces of a special grade of’ Anthracite from a defined seam in a particular coalmine in Pennsylvania. Carbon granules have to be very much all the same size to prevent packing. They gave us a couple of Kilos of these to get us going. We not only repaired ailing units but tested people and set up the equalization.

The amplifiers had lots of gain and coped with the aged deaf with their five octaves of six Db per octave. Unfortunately the units were very, very expensive, and trade was not all that good. A lot of the returned amplifiers suffered from heavy staining on the gold plating. This was doubtless produced by minute leaks iii the sealing aided by temperature variations producing a cycle of low pressures inside. Years later I think I solved this one. I was working at Shepperton Studios at the time.

We were troubled by tiny spots appearing in the captions These were a single frame of 35mm film in a glass sandwich. With the aid of a very high powered microscope I sorted out fibres from clothing and floor coverings, earth particles and the like all held in a brown goo. I had a friend, a government chemist who was rather good at micro-analysis, he diagnosed my Lady Nicotine. Probably the plating stains were from smoking. Anyway the war intervened. When I returned after it this industry had packed up, but I did manage to get an old deaf aid for my mother. Somewhere along the line it disappeared. A pity it would make a nice exhibit.

NB – Early Brown models appear to have a hard carbon button pressed against an iridium point as a “Microphone”. Diagrams exist which show a push-pull arrangement of granules.

Eric Vast – November 1996.

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