PEARL Archive of Vacuum Tube Technology

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PATENT SPECD'ICATION
--Application Date: June 5, 1937. No. 15620/I3 7.
Complete Specification Left: March 4. 1938.
496.883
Complete Specificat{on Ar;c;epted: Dec;, S, 1938.
PROVISION AL SPECIFICATION
Improvements in or relatnng te Thermionic Valve Amplifying
Circuits
I, ALAN DOWER BLUMLEIN, of 37, The former, or alternatively, the screening
Ridings, Ealing, London, "V.5, a British grid may be connected to a source of
subject, do hereby declare the nature of potential through a resistance and con- 55
this invention to be as follows:- nected through a decoupling condenser to
Ii This invention relates to thermionic a tapping point on the primary winding
valve amplifying circuits, and is pa.rticu of the output transformer. Other alter
larly concerned with power amplifying l'.ative connections for causing the poten
circuits. tial of the screening grid to vary in pro- 60
It is usual in such eircuits to employ portion to the potential of the anode may
10 as the last valve of the stage a pentode be employed. For example, the screening
valve or a power tetrode. These valves grid may be connected to a tapping point
are provided with screening grids which in a choke connecting the anode of the
are usually maintained at a constant posi valve to the source of anode current. 65
tive potential with respect to the cathode. The effect of causing the screen to vary
15 Valves of this. kind give a greater output in potential proportional to the variations
for a given anode potential than triodes, in anode potential is to cause the valve to
but they suffer from the disadvantage that function somewhat after the manner of a
their characteristic impedance to the triode. It is known that in a triode the 70
anode is very much greater than the ratio anode impedance is less than the ratio of
20 of the 'Steady D.C. voltage to the steady the steady D.C. anode voltage to the
D.C. current. Consequently, in order to steady D.C. anode current. Hence, by
obtain a good power output the imped choosin~ a suitable tapping point ror the
ance of the load must be very much less screening grid the t!haracteristic imped- 70
than the characteristic impedance of the ance of the anode and screen effectively
25 valve. Furthermore, particularly in the combined can be given any dE'sired value
case of either a pentode or a power tetrode between the impedance of a pentode or
valve, which is coupled to its load through tetrode and the impedance of a triode. A
a transformer, it is known that if the load suitable tapping point, in practice, may 80
is removed then for relatively moderate be found to be such that the voltage swing
80 variations in potential of the 'input grid, on the screen is between about a quarter
excessively high output voltages will be and a half the voltage swing on the anode.
produced which may be sufficient to cause For the purpose of describing the in
disruption of the valve or of the output vention more in detail reference will be 85
transformer. made to the accompanying diagrammatic
85 It is the chief object of the present in drawings in which:
vention to provide an improved circuit in Fig. 1 illustrates the application of the
which the characteristic impedance of invention to a circuit embodying a pen-
valves of the kind referred to can be more tode valve, and 90
nearly matched to their optimum loads Fig. 2 illustrates the application of the
40 and in which the danger of an excessive invention to a tetrode valve.
increase of volta~'e on removal of the load As shown in Fig. 1, signals to be am
may be substantlally avoided. plified are applied between the control
According to the invention a thermionic grid 3 and cathode 4 of a pentode valve 95
valve amplifying circuit is provided em- thro'ugh a coupling condenser 5 and leak
45 ploying a pentrode or tetrode valve in resistance 6, the cathode of the valve
which the screening grid of the valve is being biassed by a dropper resistance 7
so connected that the potential thereof shunted by a by-pass condenser 8. The
varies in proportion to the A.C. potential suppressor grid 9 is connetlted to the 100
variations of the anode or the valve. This cathode in the usual manner. The anode
50 result may be atlhievec1 by connecting the 10 is connected through the primary wind-
screening grjd to a tapping point on the ing 11 of an output transformer to the
primary winding of an output trans- positive terminal of a suitable source of
-
C) 496,883
high tension current the secondary wind the primary winding 22, it may be con
ing 12 of the tra;nsformer being con nected to a suitable tapping point on the
nected to a load not shown. The screen choke 20. ]orany other alternative con·
ing grid 13 is connected as shown to a nections are possible. 40
5 tapping point on the primary winding' 11 The circuits described also llave the
so that the screening grid 13 varies in advantage that if tIle loads are remowd
potential in proportion to the potential the output voltage obtained for relatiwly
variations on the anode 10. As stated 8mall potential variations of the control
above, the tapping point may be arranged grids is not as great as with the arrange- 45
10 so that the voltage swing on the screen ments usuall~' employed, since the screen
grid 13 is between about a quarter and a ing grid which is partially effective in
haH the voltage swing on the anode 10. controlling the cathode current will swing'
The arrangement shown in Fig. 2 of in opposite phase to the grid and so pre
the drawings illustrates the invention as vent or reduce an excessiw output voltage. 50
15 applied to a tetrode valve and, in this In some cases the decoupling condensers
case, signals to be amplified are applied 8 and 18 of Figs. 1 and 2 respectivel~- mn~­
betweell. the control grid 14 and cathode 15 not be employed, in which cases negath-e
through an input transformer 16 the feedback will occur tending to reduce the
cathode 15 being biassed by a dropper second harmonic output. The effed of 55
20 resistance 17 shunted by a by-pass con such feedback will, however.. be to in
denser 18. The anode 19 is connected to crease the impedance of the valves so that
the positive terminal of a source of anode in cases where it is desired to reduce the
current through the choke 20 and is con second harmonic output it should be
nected through a; by-pass condenser 21 to arranged that the potential variations of 60
25 the primary winding 22 of an output the screen should be greater compared
transformer the secondary winding 23 of with the cases in which the decoupling
which is connected to a load, not shown. condensers are employed in order to main
In this example of the invention the tain the anode impedance at the required
screening grid 24 is connected through a value. In each example described it will 65
30 resistance 25 to the source of high tension also be appreciated that the current which
current and is also connected in the man flows in the screening grid also contrilmtes
ner shown through a decoupling condenser to the useful output of the circuit.
~6 to a tapping point on the primary wind
mg 22 of the output transformer. If Dated this 4th da~- of June, 1937.
35 desired, in Fig. 2, instead of connecting F. 'V. CACKETT,
the screen grid 24 to a tapping point on Chartered Patent Agent.
COMPLETE SPECIFICATION
Improvements in or relating to Thermionic Valve Amplifying
Circuits
I, ALAN DOWER BLUMLEIN, of 37, rrhe than the ratio of the steadv D.C. voltag'e
70 Ridings, Ealing, London, W.5, a British to the steady D.C. current. Consequentl,\-,
L
Subject, do hereby declare the nature of ~n order to obtain 0. good power output the
this invention, and in what manner the Impedance of the load must he very much
same is to be performed, to be particularly less than the internal impedar.ce of the 95
described and ascertained in and by the valve. Furthermore, particularlY in the
75 following statement:- . case of either a. pentode or a. power tetrode
This invention relates to thermionic valve, which is coupled to its load through
valve amplifying circuits, and is p~rti~u­ a transformer, it is known that if the load
larly concerned with power amphfymg is removed then for relatively moderate 100
circuits. variations in potential of the input grid,
.80 It is usual in such circuits to employ excessively high output voltag'es will he
as the last valve of the stage a pentode produced which may be sufficient to cause
valve or a power tetrode. These disruption of the valve or of the output
valves are provided with screening elec transformer. 105
trodes or grids which are usual1y main- It is the object' of the present invention
8f1 tained at a constant. positive potential to provide an improwd circuit in which
with respect to the cathode. Valves of the internal impedance of valves of the
this kind give a greater output for a given kind referred to can be more nearilny
anode potential than triodes, but they matched to their optimum loads and 110
suffer from the disadvantage that. their which the danger of an excessive increase
90 internal impedance is very IDl1Ch greater of voltage on removal of the load may be
496,888 3
sUbstantially avoided. that in a triode the anode impedance" is
According to one feature of the inven less than the ratio oJ tbie steady D.O.
tion a thermionic valve amplifying tir anode voltage to the steady D.O. anode
cuit is provided comprising a valve having current. Hence, by choosing a suitable
I; at least a cathode, control electrode, an tapping point for the screening electrode 70
anode and ru screening electrode adapted the internal impedance of the anode and
to form an electrostatic screen between the screening electrode effectively combined
control electrode and anode, and a rela can be given any desired value between
tively low impedance load connected to the impedance of a pentode or a tetrode
1() said valve, wherein instead of maintain and the impedanCe of a triode. A"suitable 76
ing said screening electrode at the usual tapping point, in practice, may be found
constant positive potential less than the to be such that the voltage swing on the
anode potential which would cause said screening electrode is between about a
valve to have a high impedance, it is so quarter and a half the voltage swing on
15 connec.ted to the anode circuit of said: ihe anode. 80
valve that in operation the potential of In order that the invention may be
said screening electrode will vary in clearly understood and readily carried
phase) but at a lower ~amplitude with. into effect it will now be more fully
respect to the potential variations OT said described with reference to the drawings'
20 anode so as to reduce the impedance of accompanying the Provisional Specifica- Sf)
the valve· whereby it is more nearly tion in which:-
matched to its low impedance load. Fig. 1 illustrates the application of the
According to another feature of the invention to a circuit embodying a pen
invention a thermionic valve amplifying tode valve, and
25 circuit is provided comprising a valve Fig. 2 illustrates the application of the 90
having at least a cathode, control elec invention to a tetrode valve.
trode, an anode and a screening electrode As shown in Fig, 1, signals to be
adapted to form an electrostatic screen amplified are applied between the control
between the control electrode and anode, g'rid 8 and cathode 4 of a pentode valve
80 and a relatively low impedance load con through a coupling condenser 5 and leak 95
nected to said valve, wherein instead of resistance 6, the cathode of the valve
maintaining said screening electrode at being biassed by a dropper resistance 7
the usual constant positive potential less shunted by a decoupling condenser 8.
than the anode potential which would ':Phe suppressor grid. 9 is connected to the
35 ca"use said valve to have a high imped cathode in the usual ma.nner. The anode ] 00
ance, it is so connected with the output 10 is connected through the primary
circuit of said valve that in operation the "'-inding 11. of an output transformer to
potential of said screening electrode will the positive terminal of a suitable source
vary in phase) but at a lower amplitude of high tension (jurrent the secondary
40 with respect to the potential variations of winding 12 of the transformer being con- 105
said anode so as to reduce the impedance nected to a load not shown. rrhe SCl'een-
of the valve whereby it is more nearly ing grid 18 is connected a'l shown to a
matched to its low impedance load. The tapping point on the primary winding 11
desired result may be achieved by con- so that the screening grid 13 varies in
45 necting the screening electrode to a tap potential in direct propOl,tion to or in 110
ping point On the primary winding of an phase with rthe potential 'Variations on
output transformer, or alternatively, the the anode 10. As stated !1bove, the tap
screening electrode may be connected to a ping point may be arranged so that the
source of potential through a resistance voltage swin!!' on the screen grid 13 is
50 and connected through a by-pass con between about a quarter and a haH the 115
denser to a tapping point on the primary voltage swing on the anode 10.
winding of the output transformer. Other The arrangement shown in Fig. 2 of
alternative connections for causing the the drawings illustrates the invention as
potential of the screening electrode to vary applied to a tetrode va.lve and, in this
55 in proportion to the potential of the anode case, signals to be amplified are applied l~O
may be employed. For example, the between the control grid 14 and cathode .
screening electrode may be connected to 15 through an input transformer 16 the
a tapping point in a choke connecting the cathode 15 being biassed by a dropper
anode of the valve to the source of anode resistance 17 shunted by a decoupling
60 current. condenser 18. The anode 19 is connected 125
The effect of Musing the screening elec to the positive terminal of a source or
tf{)de to vary in potential proportional to anode current through a choke 20 and is
the variations in anode potential is to connected through a by-pass condenser 21
cause the -valve .to .function somewhat to the primary winding 22. of an output
65 after the manner of a triode. It is known transformer the secondary winding 23 of 180
4
·W(l.S~3
which is connected to a load, not shown. purpose above-mentioned and the iact
In this example of the invention the that the screening properties of the screen
screening grid 24 is connected through a ing electrode are reduced is relatively im
resistance 25 to the source of high tension material particularly in low frequency
5 eurrent and is also connected in the man amplifiers. 70
ner shown through a by-pass condenser 26 Having now particularly described and
to a tapping point on the primary wind ascertained the nature of my said inven
ing 22 of the output transformer. H tion, and in what manner the same is to
desired, in Fig. 2, instead of connecting be performed, I declare that what I claim
10 the screen grid 24 to a tapping point on is:- 75
the primary winding 22, it maj: be con 1. A thermionic valve amplifying cir
nected to a suitable tapping point on the cuit. comprising a valve having at least a
choke 20. Many other alternative con cathode,· control electrode, an anode and
nections are possible. a screening electrode adapted to form an
15 The cir~uits described also have the electrostatic screen between the control 80
advantage that if the loads are removed electrode and anode. . and a relatively low
the output voltage obtained for relatively impedance load connected to said valve,
small potential variations of the control wherein instead of maintaining said
- grids is not as great as with the arrange- screening electrode at the usual constant
20 ments usually employed, since the screen positive potential less than the anode 85
ing grid which is partially effective in potential which would cause said valY£' to
controlling the cathode current will swing have a high impedance, it is so connected
in opposite phase to the grid and so pre to the anode circuit of said valve that in
vent or reduee an excessive output voltage. operation the potential of said sl'reening
25 In some cases the decoupling condensers eleetrode will vary in phase, but at a lower 90
8 and 18 of Figs. 1 and 2 respectively amplitude with respect to the potential
may not be employed, in which cases variations of said anode so as to reduce the
neg'ative feedback will occur tending to impedance of the valve whereby it is more
reduce the second harmonic output. The nearly matched to its low impedance load.
80 effect of such feedback will, 11Owever, be 2. A thermionic valve amplifying cir- 95
to increase the impedance of the valves euit comprising a valve having at least a
so that in cases where it is desired to cathode, control electrode, an anode and
reduce the second harmonic output it a. screening electrode adapted to form an
should be arranged that the potential electrostatic screen between the control
:15 variations of the screen should be greater electrode and anode, and a relativel~' low 100
compared with the cases in which t.he; impedance load connected to said valve,
decoupling condensers are employed in wherein instead of maintaining said
order to maintain the anode impedance at screening electrode at the usual constant
the required value. In each example positive potential less than the anode
40 described it will also be appreciated that potential which would cause said valve to 105
the current which flows in the screening have a high impedance, it is so connected
grid also contributes to the useful output with the output circuit of said valve that
of the circuit. in operation the potential of said screen-
With the arrangements described the ing electrode will vary in phase, but at n
45 internal impedance o-f the valves is reduced lower amplitude with respect to the 110
thus enabling the valves to be matched potential variations of said anode so as
more nearly to their low impedance loaas to reduce the impedance of the valve
such as are normally employed with power whereby it is more nearly matched to its
output valves. The effect of causing the low impedance load. .
50 screening electrode 13 (Fig. 1) or 24 (Fig. 3. A thermionic valve amplifying cir- 115
2) to vary in phase with the potential cuit according to claim 1 or 2, wherein
variations of the anodes of their respective the screening electrode is connected to a
valves sacrifices however some of the tapping point on an impedance connected
screening properties of the electrodes 13 between the anode of the valve and the
65 and 24 compared with the case where source of anode current. 120
these electrodes are maintained at con ~. A the.rmionic :alve amplifying eir
stant potentials. Sbreening action does CUlt accordIng to clalm 1 or 2, wherein the
however to some extent occur since the screening electrode is connected through a
potential differences between the screen- resistance to a source of potential and
60 ing electrode and the control electrode are through a b:r-pass condenser to a tapping 125
less than the potential differences between point on an impedance, across which the
the anode and control electrode. By voltage variations occurring at the anode
causing the potential of the screening of the valve are set up.
electrode to vary as above described the 5. A thermionic valve amplifying cir
65 impedance of the valve is reduced for the cuit substantially as described herein with 130
496,883
reference to Figure 1 or 2 or the drawings Dated this 3rd day of March, 1938.
accompanying the Provisional Speci£.ca~ F. W. CACKETT,
tion. Chartered Patent Agent.
TJeamingt{)n Spa: Printed for His Majesty's Stationery Office, by the Courier Press.-1939.
496883 PROVISIONAL SPECIFICATION i SHEET
H.T.+
Fig. I.
ItT. +
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:J1Ib
z.
Fig.
Malby &- Sons, Photo -lIth,.
UNITED STATES PATENT OFFICE
2,218,902
THERMIONIC VALVE AMPLIFYING ClRCUITS
Alan Dower Blumlein, London, England, assignor
to Electric & Musical Industries Limited, Hayes,
2,218,902 Middlesex, England, a corporation of Great
Britain
Patented Oct. 22, 1940 Application Ma.y 18, 1938, Seria.l No. 208,619
In Grea.t Britain June 5, 193'1
4 Claims. (CI.l79-171)
This invention relates to thermionic valve am that in a triode the anode impedance is less tha·n
of
plifying circuits and is particularly concerned the ratio the steady D. C. anode voltage to the
with power amplifYing circuits. steady D. C.' anode current. Hence, by choosing
It. is usual in such circUits to employ as the last a suitable tapping point for the screening grid
5 valve of the stage a pento.de valve or a power the characteristic impedance of the anode and -5
tetrode. These valves are provided with Screen screen effectively combined can be given any de
ing grids which are usually maintained at a con sired value between the impedance of a pentode
stant positive potential with respect to. the cath or tetrode and the impedance of a triode. A
ode. Valves of this kind give a greater output for sUitable tapping POint, in practice, may be found
10 a given anode potential than triodes, but they to be such that the voltage swing on the screen lO
suffer from the disadvantage that their char is·between about a quarter-and a-half the voltage
acteristic impedance to the anode is very much swing-on the anode.
greater than the ratio. of the steady D. C. voltage In order that the invention may be clearly un
to the steady D. C. current., 'Consequently, in derstood and readily carried into effect it will
\5: order to obtain a good power output the imped now be more fully described with reference to l5
ance of the load must be very much less than the the accompanying drawing in which:
characteristic ,impedance of the valve. Further Fig. 1 illustrates the application of the inyen
more, particularly in the case of either a pentode tion to a circUit embodying a pentode valve; and,
or a power tetrode valve, which is coupled to its Fig. 2 illustrates the application of the inven-
20, load through a transformer,- it -is known that if tion to a tetrode valve. 20
the load is rem'oved, then for relatively moderate As shown in Fig. 1, signals to be amplified are
variations in potential of the input grid,exces applied between the control grid 3 and cathode 4
sively high output voltages will be produced which of a pentode valve through a coupling condenser
may be suflicient:to cause disruption of the valve 5 and leak resistance 6, the cathode of the valve
25 or of the output transformer. being biased by a dropper resistance 1 shunted by 25
It is the chief object of the'present invention a decoupling condenser B. The suppressor grid
to provide an improved circuit in which the char 9 is connected to the cathode in the usual manner.
acteristic impedance' of valves of, the 'kind re The anode (0 is connected through the primary
ferred to can' he more nearly' matched to their winding II of an output transformer to the posi
~O, optimum· loads and in which the danger of an tive terminal of a suitable source of high tension 30
excessive increase of voltage on removal of the current the secondary winding 12 of the trans
load may be substantially avoided. ·former being. connected to a load not shown.
According to the invention a thermionic valve The screening grid 13 is connected as shown to a
amplifying circuit is provided employing a pen- tapping point on the primary winding (t -so that
tode or tetrode valve in which the screening grid the screening grid (3 varies in potential in pro- 35
:Ji)
of the valve is so connected that the potential portion to the potential variations on the anode
thereof varies in proportion to the A. C. potential I D. As stated above, the tapping point may be
variations of the anode of the valve. This re arranged so that the voltage swing on the screen
sult may be achieved by connecting the screening grid 13 is between about a quarter and a half the
.10 grid to a tapping point on the primary winding, voltage swing on the anode 10. . 40
of an output transformer, or alternatively, the The arrangement shown in Fig. 2 of the draw
screening grid may be connected to a source of ing illustrates the- invention as applied to a tet
potential through a resistance and connected rode valve and, in this case, signals to be ampli
through a by-pass condenser to a tapping point on fied are applied between the control grid 14 and
'1:3 the primary winding of the output transformer. cathode (5 through an input transformer 16 the 45
Other alternative connections for causing the po cathode (5 being biased by a dropper resistance
tential of the screening grid to vary in proportion 11 shunted by a decoupling condenser lB. ' The
to the potential of the anode may be employed. anode 19 is connected to the positive terminal of
For example, the screening grid may be connected a source of anode current through a choke 20 and
:it! to a tapping point in a choke connecting the an is connected through a by-pass condenser 21 to 50
ode of the valve to the source of anode current. the primary winding 22 of an output transformer
The effect of causing the screen to vary in po the secondary winding 23 of which is connected
tential proportional to the variations in anode to a load, not shown.
potential is to cause the valve to function some- In this example of the invention the screening
5,) what after the manner of a triode. It is known grid 24 is connected through a resistance 25 to 55
2
2,218,902
the source of high tension current and is also 2. In an amplifying circuit, a thermionic valve
connected in the manner shown through a by having an anode, a cathode, a grid electrode and
paBS condenser 26 to a tapping point on the pri a. screening electrode, an input circuit for said
mary winding 22 of the output transformer. If valve connected between the cathode and grid
5 desired, in Fig. 2, instead of connecting the screen electrode thereof, said input circuit including 5
grid 24 to a tapping point on the primary wind means for connecting to a source of signal volt
ing 22, it may be connected to a suitable tap age, an output circuit for the valve including a
ping point on the choke 20. Many other alterna load impedance across which is developed the
tive connections are possible. output voltage, and means for connecting the
10 The circuits described also-have the advantage screening electrode to an intermediate point of 10
that if the loads are removed the output volt said load impedance device to thereby produce
age obtained for relatively small potential varia a voltage swing on the screening electrode whioh
tions of the control grids is not as great as with is an amplitude reduced image of the voltage
the arrangements usually employed, since the SWing produced on the anode due to signal energy
15 screening grid which is partially effective in con impressed upon the input circuit, the load im- 15
trolling the cathode current will swing in oppo pedance comprising a transformer having a pri
site phase to the grid and so prevent or reduce mary winding connected in the output circuit of
an excessive output voltage. the valve and a secondary winding adapted to
In some cases the decoupling condensers 8 and he connected to a utilizing device, said screening
20 .It of Figs. 1 and 2 respectively may not be em electrode being connected to a point of said pri- 20
ployed, in which cases negative feedback will mary winding.
occur tending to reduce the second harmonic 3. In an amplifying circmt a thermionic tUbe
output. The effect of such feedback will, how having an anode, a cathode, a grid electrode and
ever, be to increase the impedance of the valves an auxiliary electrode, an input circuit for said
25 so that in cases where it is desired to reduce the tube connected between the grid electrode and 25
second harmonic output it should be arranged the cathode thereof and including means for con
that the potential variations of the screen should nection to a source of signal voltage, a source of
be greater compared with the cases in whi-ch the anode potential for said tube including a high
decoupling condensers are employed in order to tension terminal, a connection including a choke
30 maintain the anode impedance at the required coil between the anode of the tube and said high 30
value. In each example described it will also be tension terminal, an output circuit for said tube
appreciated that the current which flows in the including a load impedance connected between
screening grid also contributes to the useful out the anode and cathode of the tube, means in
put of the circuit. cluding an impedance device for connecting the
35 r claim: auxiliary electrode to said high tension tenninal 35
1. In an amplifying circuit, a thermionic valve and a by-pass condenser co-nnected between said
having an anode, a cathode, a grid electrode and auxiliary electrode and an intermediate point
a screening electrode, an input circuit for said of said load impedance whereby there is im
valve connected between the cathode and grid pressed upon the auXiliary electrode an ampli
40 electrode thereof, said input circuit including tude reduced image of the voltage swing produced 40
means for connecting to a source of signal volt on the anode due to signal energy impressed upon
age, an output circuit for the valve including a the input circuit.
load impedance across which is developed the 4. An arrangement as described in the next
output voltage, and means for connecting the preceding claim chal'acteIized by that the out
4;3 screening electrode to an intermediate point of put impedance comprises a transformer having 45
said load impedance device to thereby produce a a primary winding connected in the output cir
voltage swing on the screening electrode which cuit of the tube and a secondary winding pro
is an amplitude reduced image of the voltage vided with means for connection to a utilizing
swing produced on the anode due to signal energy device, said auxiliary electrode being connected
[i0 impressed upon the input circuit, the point of through the by-pass condenser to an intermediate 50
connection of the screening electrode to the load point of said primary winding.
impedance being so chosen that the voltage swing
on the screen is of the order of Va the voltage ALAN DOWER BLUMLEIN.
swing on the anode.