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VK3ABK > VALVES 20.12.05 13:12l 77 Lines 3785 Bytes #999 (0) @ WW
BID : 15733_VK3BBS
Read: DL1LCA GUEST OE7FMI
Subj: More Wonderful Acorns.
Path: DB0FHN<DB0THA<DB0ERF<DB0FBB<DB0BI<DB0NOS<DB0EA<DB0RES<OK0PPL<OK0NAG<
9A0BBS<VK6BBS<VK3AVE<VK3AYM<VK3DSE<VK3BBS
Sent: 051220/0147Z @:VK3BBS.#MEL.VIC.AUS.OC #:15733 XFBB7.04h Bid:15733_VK3BBS
From: VK3ABK@VK3BBS.#MEL.VIC.AUS.OC
To : VALVES@WW
Hello all vacuum heads.
I suppose the ancient art of vacuum technology is all but dead, but those
who grew up with high voltages and heater supplies will remember how a
valve, or tube was the prime component in most of our Amateur radios and
test equipment.
Bulletins have zeroed in on the 'acorn' tube (sorry, but all my experience
has been with 'toobs' and most of the disposals of electronic war surplus
that I saw contained American components). The 'acorn' variety was only one
type, but they certainly played a big part in vhf (uhf?) airborne radios.
On a shelf, within reach of the keyboard, I have several 'acorns' and a
short search today located a vhf tunable converter (ex USAF) containing
four 954 RCA acorn tubes in sockets 40mm OD! Some bulletins told of ex
wartime equipment on vhf, which would at the time be called UHF, and in many
cases 'unstabilized' (not crystal controlled) tunable receivers. The device
that I have is type R-1/ARR-1, a down-converter, tunable from 34 to 58 MHz.
This converter would have been the 'front end' of an aircraft receiver, most
likely on HF, doing the same job that we did in our VHF/UHF receiving in
the 1950s and 60s. My first 2 metre tranceiver, in 1949, used a single 955
acorn triode as a modulated oscillator/super regenerative tranceiver.
A one tube transmitter and receiver used with a dipole antenna!
Here are the 'acorn' types that I have, and some that I don't.
These all had radial pins that fit into an anular socket.
954...A pentode detector/amplifier. Anode and grid at top and bottom.
955...A medium mu triode. Anode, grid, cathode and heater in radials.
956...A remote cutoff pentode amplifier/mixer. Anode and grid as for 954.
958...A medium mu triode. Centre tapped heater, no separate cathode.
959...A sharp cutoff pentode. As for 954/956 but centre tapped heater.
These tubes were used for only a few years until better 7 pin and 9 pin
varieties were available. These included 6BC4, 6BN4, 417A, 6AB4, and 6AF4,
all triodes that had lower noise figures, due mainly to lower electron
partition noise than pentodes. Later, triodes in 'cascode' gave the same
low noise with more gain. Somewhere in this tube development came the much
smaller 'Nuvistor' but these were developed for TV front ends, and had just
ordinary noise figure. High power TV transmitters made up for the difference.
All of these tubes were, in turn, just as important and better than the acorn
types, and had the advantage of a smaller socket. Radial pins of the acorns
permitted short leads (low inductance) and increased spacing (low capacitance)
which with close spacing of the electrodes (reduced electron transit time)
and so, higher operating frequencies. Nine pin 6BC4s had 4 grid connections
and 2 anode. The 6AM4 had 'five' grid pin connections! All done to reduce
inductance and increase stability and operating frequency.
My other mementos of a misspent youth, are several planar grid triodes that
extend the radial pin idea with an anular ring grid connection. One type is
small, less than 10mm in any dimension, with a button-disc anode, a ring grid,
and two 2mm dia buttons for heater connection. This must be the 'final fling'
of the vacuum triode before transistors came on the scene.
In the 'good old days' I could handle these tubes, and see them. But now I
use surface mounted MMICS and GasFETS that require extreme care and much
optical magnification. The glow of vacuum technology may be the thought that
keeps these old tubes on my shelf where I can see them amongst my transistors.
Maybe, as there is a return to tubes in the audio racket, acorns and frame-
grid triodes will, one day, find a new place in our world.
I hope so. :-)
73. Dick. VK3ABK.
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