Message 20733 in TECH.WW
From: G8MNY
Date: Mon, 13 Apr 26 07:06:00 Z
Newsgroups: TECH.WW
Subject: Valves (Tubes)
Message-ID: <57132_GB7CIP>
Path: N2MH4|GB7BED|GB7YEW|GB7CIP
R:260413/0718Z 30161@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260413/0718Z 9724@GB7BED.#21.GBR.EU LinBPQ6.0.25
R:260413/0711Z 16843@GB7YEW.#79.GBR.EURO LinBPQ6.0.24
R:260413/0706Z @:GB7CIP.#32.GBR.EURO #:57132 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
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, and of course in lots of old kit.
Here is some valve information for anyone who wants to brush up on the basics.
HT VOLTAGES
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
HT.
So care must be taken if you don't want a nasty HT nip or even DEATH!
HEATERS VOLTAGES
Only special cold cathode and 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 and
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
stages.
CONNECTIONS
Like lamps Valves have limited lifetimes 2000 hours for some of the bigger ones
and >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, UX4, etc.
_
/ \
Soldered on Central Grid
valve base pins Locating Pin
Many valves have top cap connections usually for Anodes on Tx and 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 and it is useless!
With the all glass construction and 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
structures between them. \_____/ * = Heaters
Punched Mica sheets then plan view
support the supports.
VALVE NUMBERS
There are several systems! Some reflect the heater voltage and 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
and include direct equivalents. But like transistors, many applications may not
need a direct equivalent, but electrode connections and heater voltages are
most important to get right!
CATHODE
The Cathode can be directly heated
by current through the Cathode wire, Indirectly Heated
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, and can be attracted to
the Anode electrode if it is positive (or very near) with respect to the
Cathode.
ANODES
These attract the electrons and have to
or may even be externally finned to
do this.
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.
DIODES
These use the 2 electrodes in an Anode
\ /~\ /
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).
Anode
HH Cathode
RECTIFIERS 350VAC 350VAC
These are just powerful Anode1 Anode2
Bi-phase designs are \ /~~~~~\ /
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.
These have a Control Grid 47K Large inverted
There is quite high capacitance between the Anode to the adjacent Grid and 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.
Common Grid mode (drive on the Cathode) )RFC
For large Tx valves the triode may even be run with no bias and 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.
TETRODES
a control Grid (G1) and a screen 10K )||( LS
Tetrodes are commonly used in === 220R
BEAM TETRODE A
PENTODES A
screening may be used.
Variable gain (mu) can be obtained Stage
if the pitch of the G1 wire in Gain
this feature to control gain. 1 2 3 4 5 -Volts
Anode IF
BEAM MIXERS
A good performance mixer can be A1 A2
the balanced local oscillator.
MULTIPLE VALVES At Ap
Sometimes in a diagram only part H H C
of the valve may be shown in a
part of the circuit.
A A'
INDICATOR TUBES
Often called magic eye or tuning
indicators, these give a varying A1 A2
at HT is phosphor coated, or used
as an accelerator anode to phosphor
on the glass to give the display.
BEAM TUBES
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 and can produce very useful gains (e.g. 40dB) and powers (kW) at some
astonishing frequencies (10s of GHz).
GAS FILLED
Gases are normally not wanted in valves as it ionises and poisons the Cathode.
But cold Cathode tubes using neon, or other gasses do make good shunt voltage
regulators (like zeners), and even triggered timebases using a Thyratron. The
DOT or sometimes hatching, represents the gas inside the envelope.
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, and nowadays Mercury
vapour ones make fluorescent lamps and the heart 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
Starter C
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
← Return to Bulletin List