G8MNY > TECH 02.12.23 11:40z 347 Lines 16537 Bytes #24 (0) @ WW
BID : 61545_GB7CIP
Subj: Valves (Tubes)
Sent: 231202/1049Z @:GB7CIP.#32.GBR.EURO #:61545 [Caterham Surrey GBR]
To : TECH@WW
By G8MNY (Updated Sep 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Although Valves are considered old hat nowadays, they are still used in big Tx,
RF heating systems, microwave ovens, & of course in lots of old kit.
Here is some valve information for anyone who wants to brush up on the basics.
Most valves use quite high voltages, the High Tension e.g. 90, 120, 200, 250,
350, 1k5V 2kV, 3kV etc. Only a few special Rx valves use voltage as low as +12V
So care must be taken if you don't want a nasty HT nip or even DEATH!
Only special cold cathode & gas filled valves need no heaters. The heater power
can be quite large compared to the total power. For battery radio valves heater
voltages of 1.4V or 3V are normal. The most common voltage is 6.3V & for mains
heater chains where all the heater in a set are in series, either 0.3 or 0.1Amp
are the 2 common standards with a dropper resistor (line restistance cord), AC
capacitor, or diode to make up the mains supply voltage. Negative temperature
thermistors were commonly put in mains chains, to take the warming up shock
from the cold low resistance heaters.
On diagrams the heater chain/wiring is normally not drawn with the rest of the
Like lamps Valves have limited lifetimes 2000 hours for some of the bigger ones
& >20,000 hours for CRTs etc. Turning them off & on is oftem more limiting than
leaving on! Most come with a pin connection base standard for easy replacement.
These are give names like... B7G, B9A, MO, IO, P, P5, B9, B7, B5/4, UX7, UX5,
│ │ Top Cap
│ │ ___
│ │ Getter──.˙Ř˙. / \
│ │ Marks │ │Glass │ ┌───┴─────┴───┐Finned
│ │/ │ │Envelope /~\ │ 4CX250B │External
│ │ │ │ │ │ │ │Anode
\ / │ │ ──=======── └──┬───────┬──┘
├───────┤ │ │ \_/ G2 ┌┴───────┴┐~\Ceramic
│ │ └┬┬┬┬┬┘ │ Screen└─┬┬┬┬Ď┬┬─┘ Insulator
└ĎĎĎĎĎĎĎ┘ Through glass Acorn Ring ││││║││
║║║║║║║ wire pins Type └─Ż
Soldered on Central Grid
valve base pins Locating Pin
Many valves have top cap connections usually for Anodes on Tx & power valves
like TV sweep tubes, but may also be a sensitive control Grid on Rx valves!
Getter marks are shiny silver/black areas where Magnesium has been vaporised
onto the glass to catch the last remnants of gasses in the evacuated tube. A
white getter mark indicates air has got into the valve & it is useless!
With the all glass construction & a good magnifying glass, it is often possible
to determine not only what the internal constructions are (diode, triode,
pentode etc.) but often also their pin out connections. But I have seen
tetrodes assemblies wired up as a "rectifier diode internally" though (must
have had a surplus?).
INTERNAL CONSTRUCTION _____
Most valves are cylindrical / \ Anode
design with the Anode & /.˙'''˙.\ ˙ = Grid Wire
Cathode electrodes as ( Ý (*) Ý ) Ý = Grid Support
coaxial tubes with wire grid \'˙...˙'/ ( ) = Cathode tube
structures between them. \_____/ * = Heaters
Punched Mica sheets then plan view
support the supports.
There are several systems! Some reflect the heater voltage & valve type, while
others have no meaning at all.
EBC33 has 6.3V heaters (E), having Diode detector (B) & a Triode (C) with
electrical & connection valve type 33.
CV123 is a UK military type, often with loads of commercial equivalents.
12AU7 is a 12V heaters (2x 6.3V) double triode.
A good valve data book will give you all the information for the type number &
include direct equivalents. But like transistors, many applications may not
need a direct equivalent, but electrode connections & heater voltages are most
important to get right!
The Cathode can be directly heated
by current through the Cathode wire, Indirectly Heated
or indirectly heated with insulted │ │ Glass Cathode
wire inside the Cathode tube. \/~\/ envelope │ /~~\ │
Electrons boil off the hot Cathode │~│ \│_/\_/
electrode, normally helped buy an Directly │ ││
emitting coating enabling lower Heated Wire Heaters
temperatures to be used (dim red glow). Cathode
Without the coating the Cathode has
to be very HOT, more like a lamp!
Directly heated Cathodes on AC supply often use a centre tapped AC supply, or a
"hum dinger" pot across heater line to minimise hum due to the heater voltage.
The boiled off electrons form a cloud in the vacuum, & can be attracted to the
Anode electrode if it is positive (or very near) with respect to the Cathode.
These attract the electrons & have to
absorb kinetic energy from the fast │
moving electrons & radiate it as heat. .-│-.
On power valves Anode structures are / ─┴─ \
designed to radiate the heat away, │ Anode │
or may even be externally finned to
If the electrons are very fast then they can dislodge electrons in the Anode
causing secondary emission from the Anode to other electrodes. To stop this
some power valves use carbon/carbonised Anodes which better soak up electrons
to stop most of this unwanted emission.
These use the 2 electrodes in an Anode
evacuate tube to conduct current .-│-.
one way. / ─┴─ \
\ /~\ /
Note that electrons flow the '│-│'
opposite way to current e.g. -ve to Directly
+ve, as electrons were discovered Heated
after the properties of current Cathode
had been determined.
Unlike solid state devices, there is no voltage drop across valve signal diodes
as the nearby Cathode electron cloud actually gives up electrons freely to the
Anode (space charge current).
In Rx valves, they are often / ─┴─ \
part of a multi valve single tube G1 ─┼ - - - │
for IF detectors & AGC detectors D1 ─┼─┤/~~\├┼─ D2
plus the 1st AF amp stage. \ /\│ /
RECTIFIERS 350VAC 350VAC
These are just powerful Anode1 Anode2
versions of the diode .˙│-─-│˙.
used in power supplies. / ─┴─ ─┴─ \
Bi-phase designs are \ /~~~~~\ /
│4V 2A├───>+350V DC
Voltage drop may be up to 30V @100mA. Heater
Efficiency diodes rectifiers used in old TV stages, unusually have the Cathode
as the top cap with very good Cathode to heater insulation for several kV.
TRIODES +150V HT───┬─
These have a Control Grid 47K Large inverted
between the Cathode & Anode ├─────┤├─── Output
that reduces the flow of .-│-. 0.01uF
electrons. Gain "gm" or "mu" 10n / ─┴─ \ 250V
is in Anode mA per Grid Volt. ─┤├─┬─┼ - - - │
The Grid is usually negative Input │ │ /~~~\ │
with respect to the cathode, 1M \ /\ │/
& an AC bypassed cathode grid│ ~││~├───┐+
resistor is a common way to leak│ HH │ ===
obtain this bias. │ 1K │ 25uF
There is quite high capacitance between the Anode to the adjacent Grid & this
causes high frequency negative feedback (Miller effect) in these high impedance
circuits. The electron flow is proportional to the Grid voltage, but also the
attracting Anode voltage, so there is considerable negative feedback in the
triode, which keeps the Anode impedance low in common Cathode mode.
┌─ +1500V HT
Common Grid mode (drive on the Cathode) )RFC
is used in some PA & RF amp designs ├─────┤├─── Output to
with its reduced power gain, it has .-│-. 1n 3kV pi Tank
the advantage of no Miller effect as / ─┴─ \
well as applying current NFB, making │ - - - ┼──┐
the stage stable, quite linear & with │ /~\ │ _│_
low impedance drive. \_│ │_/ ///
RF Drive ┬─┤├──┤ │ RF Floating
└───┤├)─┤ Heaters & Cathode DC
For large Tx valves the triode may even be run with no bias & very large
amounts of Grid current, when the Grid is pushed positive by the drive signal,
but the Grid must be designed for this, or it will be destroyed! In data books
you see mode of operating "class A B or C" with suffix "1 or 2", 1 indicates NO
grid current 2 is with Grid current.
These have 2 Grid electrodes, HT───┬────┬_ ___
a control Grid (G1) & a screen 10kK )||( LS
Grid (G2). The screen Grid │ _)||(___
provides screening for G1 │ .-│-. 25:1
from the Anode reducing the │ / ─┴─ \
Miller effect & also └┼ - - - │
maintaining a + attracting Input─┼ - - - │
electrode other than the Anode │ /~~~\ │
reducing the Triodes NFB \│_/\_/
effect & increasing gain. 100uF┌─────┤ ││
Tetrodes are commonly used in === 220R
AF output stages. ──┴─────┴─
BEAM TETRODE A
These have extra earthed .─│─.
electrode structure (attached / ─┴─ \
to cathode) shielding much of │ ] [ │
the Anode structures electrical G2 ─┼─ - - \│
effects from the Grids. G1 ─┼─ - - ││
The Grids may also be exactly │ /~~~\/│
in line to improve efficiency, \_/\_│/
by keeping G2 currents low. ││ │C
These have an earthed suppressor .-│-.
Grid (G3) between the screen / ─┴─ \
Grid (G2). This also does Grid G3 ─┼─ - - |│
Anode shielding, but also reduces │ - - ─┼─ G2
secondary emission effects, G1 ─┼─ - - |│
improving gain & linearity. │ /~~~\└┼─ Screening
For RF signal amplifiers an outer \│_/\_/
painted on or inner electrical C│ ││
screening may be used.
Variable gain (mu) can be obtained Stage
if the pitch of the G1 wire in Gain
manufacture is made variable, then 100%┤˙..──--.._ Normal Valve Cut
increasing the -ve bias will slowly 75%┤ ''˙˙.. \ Off & Distortion
reduce the small signal gain of the 50%┤ Variable''\˙..
valve rather than just cut off the 25%┤ Mu Gain \ ''˙˙.._
whole valve. Rx AGC & Tx ALC use 0%┼────┬────┬────┬────┬──~ Bias
this feature to control gain. 1 2 3 4 5 -Volts
With more electrodes / ─┴─ \
some valves can be a Suppressor G5 ─┼─ - - │
mixer plus oscillator │ ─ - ─┼─ G4 Osc Anode
using the same │ - - ─┼─ G3 Osc Grid
electron flow. Screen Grid G2 ─┼─ - - │
RF signals G1 ─┼─ - - │
│ /~~~\ │
A good performance mixer can be A1 A2
made using a balanced out of .-│-─-│-.
phase Anodes A1 & 2, & a pair / ─┴─ ─┴─ \
of beam bending deflection P1 ─┼─┤ ├─┼─ P2
plates P1 & P2 fed with the │ - - - ──┼─ G2
balanced local oscillator. G1 ─┼── - - - │
The amplified radio signal on │ /~~~~~\ │
G1 is then fed in turn to A1 or \│__/~\___/
A2 depending on of the phase of C│ │~│
the balanced local oscillator.
MULTIPLE VALVES At Ap
As vales are quite big it is .-│-────-│-.
quite common to include more / ─┴─ ─┴─ \
than one in a glass envelope if │ ┌ - - - │
there will be no interference G1t ─┼─ ─ ─ │ - - ─ ┼─ G2p
(e.g. not multiple RF stages). │ │ - - ─ ┼─ G1p
Here a detector Diode, audio D1 ─┼─┤ /~~~~~~~\ │
Triode & audio output Pentode \ /~~\ │ /
all share the same Cathode. ~~─│─--─│~│~
Sometimes in a diagram only part H H C
of the valve may be shown in a
part of the circuit.
Some push pull VHF/UHF Tx valves .--│-────-│--.
use twin beam tetrode assembles / ─┴─ ─┴─ \
with one Cathode & G2, but two │ ] [ ] [┐ │
G1s & Anodes. Internal Anode G2 ─┼─ - - -──- - -│ │
to Grid neutralising capacitors G1 ─┼─ - - - - - -│─┼─ G1'
between A'- G1 & A - G1' are │ /~~~~~~~~~~\│ │
possible with push pull valves \___ /~\ ___│/
in one envelope. ~~│~│~~ │C
Often called magic eye or tuning
indicators, these give a varying A1 A2
area of green fluorescent display .│-───-│-.
depending on the signal. They are / ├──┐ │ \
commonly twin triodes, where the │ ─┴─ │ ─┴─ │
anode of the first is the beam G1─┼ ─ ─ └─ ─ ─ │
altering grid of the second triode │ /~~~~~~~\ │
& connected to HT with a high value \│___/~\___/
resistor. The second triodes' anode C│ │~│
at HT is phosphor coated, or used
as an accelerator anode to phosphor
on the glass to give the display.
Many types of special electron beam tubes exist other than the Cathode Ray Tube
they include Travelling Wave Tubes, Klystrons etc. Most amplify or self
oscillate & can produce very useful gains (e.g. 40dB) & powers (kW) at some
astonishing frequencies (10s of GHz).
Gases are normally not wanted in valves as it ionises & poisons the Cathode.
But cold Cathode tubes using neon, or other gasses do make good shunt voltage
regulators (like zeners), & even triggered timebases using a Thyratron. The DOT
or sometimes hatching, represents the gas inside the envelope.
.-│-. .-│-. Trigger / ─┴─ \ .-│-──-│-.
/ ─┴─ \ / ─┴─ \ Gate ─┼ - -°- │ / ─┴─ ─┴─ \
│ ¨ │ │ ¨ │ Grid │ /~~~\ │ │ ¨ │
\ ─┬─ / \┌ o / \│_/\_/ \_/~~~~~~\_/
'-│-' Start/-│-' │ ││ │~~~~~~│
NEON REGULATOR HEATED HEATED HIGH CURRENT
80V 2mA 150V 80mA THYRATRON GAS RECTIFIER
NOTES: OA2 = 150v much typ 1007 cold kathode (octal)
------ OB2 = 90v See book: (US militair app's)
(From ON4CBL) 150C1K (octal) EEV-GenE. ...
More complex neon ones make up NIXI number display tubes, & nowadays Mercury
vapour ones make fluorescent lamps & the hart of Plasma TV display panels.
With liquid Mercury as a Cathode, large ones use (before the silicon diode)
to be used as large poly phase rectifiers, with 3, 6, or even 12 Anodes, many
Amps at several kVs can be rectified this way.
A1 A2 A3 A4 A5 A6
/ ─┴─ ─┴─ ─┴─ ─┴─ ─┴─ ─┴─ \
\_ ┌ o _/
See my bulletins on "Early AVO Valve Tester", "Old Valve Radios", "Microwave
Ovens" & for PAs "4CX250B PA Regulated Supplies" & "4CX250B PA Timing &
Control", & also "Reforming Caps" or "Oscilloscopes" for CRT information.
Why Don't U send an interesting bul?
73 De John G8MNY @ GB7CIP
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