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G4EBT > VALVES 16.12.05 14:42l 103 Lines 4646 Bytes #999 (0) @ WW
BID : 6E1630G4EBT
Read: DL1LCA GUEST OE7FMI
Subj: The wonderful acorn valves!
Path: DB0FHN<DB0FOR<DB0SIF<DB0ROF<DB0ERF<DB0FBB<DB0IUZ<DB0GOS<DB0EEO<DB0RES<
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Sent: 051216/1234Z @:GB7FCR.#16.GBR.EU #:17730 [Blackpool] FBB-7.03a $:6E1630G4
From: G4EBT@GB7FCR.#16.GBR.EU
To : VALVES@WW
A bit of nostalgia, taken from an IRE paper written in 1935, which shows
how far technology has progressed in 70 years, especially when considering
for example mobile phones, and the speed at which the latest CPUs operate.
Back in the 1930's when valve technology was still evolving, there was
a desire to extend the operating frequency of valves above their upper
limit, which back then was 100 MHz. (or to be more in keeping with the
terminology of the 1930s, 100 "Mcs").
The development of two remarkable "acorn" valves - the 954 and 955,
overcame the limitations of valve design to enable valves to operate
at frequencies as high as 40 Cms, pushing manufacturing techniques to
their very limits back then.
The small size and novel structural design allowed compact and convenient
receiving equipment to be built. They were tough, small in size and low in
weight, and were used by the millions for military uses. Even today, the
world is still awash with these wonderful valves, and they have often been
put to good use by constructors.
Early work on the extension of the HF limit of receiving equipment using
conventional RF amplifier circuits using standard valves proved that the
amplifier section of the RX became less and less effective at higher
frequencies, and at 100 MHz became virtually useless.
Similarly, the detector section and oscillator (when one was employed) of
the RX became less and less efficient, and though they operated above the
limits of the amplifier section, they too ultimately became inoperative.
Experiments with circuitry brought about slight improvements, but it
became evident that it was the valves themselves that dictated the upper
limit at which designs would operate.
It became apparent that it was the large size of the valves and their
electrodes that led to these limitations, and the development of the
"acorn" valve was to overcome those limitations.
However, for mechanical reasons, it wasn't possible to reduce all the
dimensions of a valve to the same degree. For example, a reductions by a
factor of four in all the linear dimensions of a typical valve would
result in grid rods of only 6.24 mils diam, grid wire of only 0.83 mil
diam, a cathode sleeve of only 12.5 mils, and a cathode coating of only
0.75 mil thickness.
To manufacture components to such minute dimensions within the limitations
of manufacturing techniques of the day, and all the attendant engineering
problems made it necessary to arrive at a practical design which would be
robust, could be reliably manufactured, would be robust enough to give
reliable operations, yet meet the performance criteria.
The acorn valve was the outcome, and was a triumph of engineering
ingenuity and versatility. It was absolutely the smallest that could meet
all the criteria. It was made as both a triode and pentode.
It operated with a heater voltage of 6.3v, either AC or battery, will
work down to 65 V HT, with maximum plate voltage of 180v for the triode,
and 250v for the pentode. It does indeed look like an acorn, or a largish
grape. its internal construction was designed to minimise inter-electrode
capacitance - quite an achievement, given the proximity of the electrodes
to one another, and the need for robust valve pins.
A 16 page paper written by Bernard Salzberg and DG Burnside of RCA
Manufacturing Co can be downloaded in PDF format, with diagrams of
the internal construction and pictures of the valves from:
http://www.clarisonus.com/Archives/TubeTheory/Acorn.pdf
It seems that very few on packet are interested in construction, but
for anyone who might be nostalgic for the days when magazines such as
"Practical Wireless" was both practical, and about "wireless" rather
than featuring little more than of reviews of the latest hobby radio
"walkie talkies" and re-cycled articles from the archives, and when
that wonderful little magazine "Radio Constructor" was still in
existence, go to:
http://vintageradio.me.uk/radconnav/acorn.htm
That link will go stright to acorn valve designs, but it's a wonderful
website which has loads more other stuff from days gone by - "roll your
own" Denco coils etc, compenent data, practical designs using valves and
transistors, and lots of excellent links.
Who say the internet is "killing radio"?
It's like saying books are killing poetry.
Hope the info is of interest to someone out there.
73 - David, G4EBT @ GB7FCR
Eddystone User Group Member
G-QRP Club Member No: 1339
QTH: Cottingham, East Yorkshire.
Message timed: 12:26 on 2005-Dec-16
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