The magazine of the Melbourne PC User Group More on Archeology of the V.1501 Roger Riordan |
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I graduated in Electrical Engineering from Melbourne University 50 years ago,
and in our course (after we had learnt all about physics, chemistry, maths,
surveying and strength of materials, to say nothing of alternators, transformers
and other heavy stuff) we had one lecture on those new-fangled semiconductors,
and spent the rest of the time talking about valves. |
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I have called Vic Jan's uncle, because his hand-blown envelope and cruder
internal wiring clearly belong to the generation before Jan. However the packing
that Frank Osowski describes does sound like WW2. Vic may have been bought for a
transmitter as a spare, and never used, but the Broad Arrow marking suggests he
was bought from disposals, and that the two valves are of similar age. Perhaps
Vic was manufactured in Britain in a factory using prewar machinery, while Jan
was made by RCA in the States with the latest machinery.
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Vic and Jan both have directly heated cathodes. In other words, the filament is
also the cathode, and the wire spiral inside the plate, which Frank thought was
the cathode, is the grid. My first move was to consult the modern archeologist's
favourite tool. I Googled '1501 triode' and immediately got a hit
http://www1.sphere.ne.jp/teihaku/sinkukan/sinkukan-e.html. Unfortunately this
led to a picture of a forced air-cooled V.1501 in a museum, and even if Frank
had known that Vic was a triode, he would not have realised that this was the
same thing, as it looks completely different. I tried many combinations, but
could not get another hit, or find a data sheet. However I did find a data sheet for Jan in my office. Her envelope is 115mm in diameter and 160 mm high, so she is only half Vic's height. Her filament has a cold resistance of about 0.15 ohms, and draws 10 amps at 10 volts. As Frank guesses, the filament resistance rises rapidly as it heats up, and Jan is very power hungry. When the filament is lit up (as it is in Figure 1) Jan would almost pass for a decorative lamp. The maximum rated plate voltage is 3000V DC, and the maximum rated average current is 0.4A. At full power output she will deliver 635W of RF, and she could have powered a prewar commercial radio station. At full load the plate will glow a pretty orange-red colour. Vic is much taller than Jan, but there is a lot of waste space inside his envelope, and his plate is smaller, so his ratings are probably substantially lower. I have only put Jan to work once, when OTC ordered a 2000V, 100mA regulated power supply from us, and I needed a variable load to test it. She glowed very nicely at full power! |
As Frank surmises, in those days valves were assembled largely by hand, and Jan
also has a hand-written number on her base, as can be seen at the bottom of
Figure 2. Frank also comments on a strange structure at the top of the valve,
and suggests it could be some form of overload device. A rather similar
structure can be seen at the top of Figure 2, and a darkened mirror-like area,
which is directly opposite it, can be seen near the bottom of Figure 1. This
structure is in fact the 'getter'. This consists of a metal loop, partly coated
with a relatively volatile metal (possibly calcium or strontium), mounted close
to the envelope of the tube.
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Ideally there should be no gas at all inside the envelope of the finished valve,
but there will always be some gas left after the air is pumped out, and the
purpose of the getter is to deal with it. After the innards are sealed into the glass envelope, the exhaust tube is connected to a vacuum pump, and the whole assembly is placed in an oven. The valve is then "soaked" at a high temperature for some hours to drive out as much as possible of the gas adsorbed on the surfaces of the internal components. Then the exhaust tube is sealed off. The small glass nipple, visible in the middle of Figure 2, is the remnant of the exhaust tube. An induction coil is then placed near the getter to heat it. This vaporises the volatile metal, and the metal vapour streams to the inside wall of the envelope where it condenses. On the way it sweeps up most of the residual gas molecules and traps them in the metal film. This condensed metal is the black film at the bottom of Figure 2. It would have been unusual for Vic to have been packed in an original box if he had been removed from service, but perhaps he had developed a non-lethal fault, like low emission, and had been kept as an emergency spare. Thank you, Frank, for an opportunity for a trip down memory lane. It is staggering to realise how far electronics has progressed in the last 50 years. Then mantel radio makers boasted their radios had three, four, or (Wow!) even five valves. Now we can buy PCs with a clock frequency of 3GHz, and nearly one billion transistors in the microprocessor. |
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But I think the biggest pleasure in moving from valves to transistors was the
realisation that one was now free from the ever present worry that the slightest
miscalculation could be rewarded by a potentially lethal shock!
Roger Riordan AM, BEE (Melb), MIEEE.
Reprinted from the March 2005 issue of PC Update, the magazine of Melbourne PC User Group, Australia
