pentode straped triode
TRANSCRIPT
7/23/2019 Pentode Straped Triode
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by
Tom Schlangen
Pentodesconnected as Triodes
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Tom Schlangen – ETF 06
Pentodesconnected as Triodes
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Born 1962 in Cologne / Germany
Studied mechanical engineering at RWTH Aachen / Germany
Employments as „safety engineering“ specialist and CIO / IT-headin middle-sized companies, now owning and running an IT-consultant business aimed at middle-sized companies
Hobby: Electron valve technology in audio
Private homepage: www.tubes.mynetcologne.de
Private email address: [email protected]
About the author
Tom Schlangen
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many pentodes, especially small signal radio/TV ones, are stillavailable from huge stock cheap as dirt, because nobody caresabout them (especially “TV”-valves),
some of them, connected as triodes, can rival even the best realtriodes for linearity,
some of them, connected as triodes, show interesting
characteristics regarding µ, gm and anode resistance, that have noexpression among readily available “real” triodes,
because it is fun to try and find out.
Reasons for connecting and using pentodes as triodes
Why using pentodes as triodes at all?
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All additional grids serve a certain purpose and function – they wereadded to a basic triode system to improve the system behaviour incertain ways, for example efficiency.
We must “disable” the functions of those additional grids in adefined and controlled manner to regain triode characteristics.
Just letting them “dangle in vacuum unconnected” will not work – they would charge up uncontrolled in the electron stream, leading to
unpredictable behaviour.
We must look somewhat closer at each grid´s function, to find outhow to reverse their functionality back to triode operation in the bestway.
How to make a triode out of a tetrode or pentode again?
Or, what to do with the “superfluous” grids?
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One of the principle problems of the triode is interelectrodecapacitance, especially between anode and control grid.
Ca-g is a path transferring energy back from the anode (output) tothe control grid (input), thus reducing efficiency. This effect is mostly
dependent on Ca-g value and stage gain and is known as “Millereffect”.
The screen grid was invented to electrostatically shield (or screen)the anode from the control grid, reducing Ca-g massively.
It also can be used as an additional control grid.
An electron tube with four electrodes is called tetrode.
Function of the screen grid
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Tetrode characteristics – fixed screen voltage
T e t r o d e k
i n k a r e a
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“Tetrode kink” due to secondary emission
electrons from the anode, attracted by thescreen at Vg2 < Va.
Unstable operation at Vg2 < Va. Usable
operation conditions are limited to Va > Vg2.
Suboptimal efficiency, but still much betterthan a triode, especially at high frequency.
Tetrode problems
A third grid, thesuppressor grid,was introduced.
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Very wide meshed and negative to anode and screen grid, it hasvery few influence on fast travelling electrons on their way fromcathode to anode – they just “pierce” through the weak electrostaticfield.
But its weak electrostatic field is strong enough to repel slowmoving, low energetic secondary emission electrons to the plate,blocking their way to the screen grid, thus preventing negative
effects (KTxx: “Kinkless Tube”) further enhancing efficiency.
It also can be used as an additional control grid.
An electron tube with five electrodes is called pentode.
Function of the suppressor grid
Another grid we must care for…
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Pentode characteristics – fixed screen grid voltage
Va>Vg2Va<Vg2
R e d u c e d
k i n k a r e a
E n l a r g e d o p e r a t i o n a r e a
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Beam forming plates are used instead of a suppressor grid. Theywork by forming a concentrated electron beam, which also preventssecondary emission electrons to travel from anode to screen grid.
To maintain their function, beam forming plates must be at cathodelevel. Usually they are connected to the cathode within the valveinternally – no separately accessible pin.
Thus, beam forming plates cannot be used as an additional control“grid”.
Beam power valves
A special case to circumvent the pentode patent
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Where to connect what?
…. to get triode characteristics, again!
???
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Possible screen
grid connections
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Special case of tetrode operation:Vg2 just lowered to cathode level
Combined g1 + g2 control action is ofmultiplicative character
Valve is almost shut down
Possible screen gridconnections
Screen grid connected to cathode
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Screen grid connected to cathode(Grid family characteristics shown)
Vg2 = 0Vhardly
useful! V
g 2 = 2 5 0 V
: W i d e u s a b l
e r a n g
e
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Again, combined g1 + g2 control actionis of multiplicative character
Extreme µ, but also extreme internalresistance
”Current trickler”, only useful for fewvery special applications
Possible screen gridconnections
Screen grid connected to control grid
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Screen grid voltage is not fixedanymore, but swings with anode
Screen/shielding function of g2is disabled
Screen grid area effectively addedto anode area
Possible screen gridconnections
Screen grid connected to anode
Preferred methodto connect g2!
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Usually, a low valued „grid stopper“resistor is added to prevent parasiticoscillations
Common misconceptions:
The purpose of this resistor is not tolimit screen current (it hardly does,except when blowing like a fuse)
The purpose of this resistor is not tokeep Vg2 below Va (there is no reason
to do so – wouldn´t work in a linearmanner using just a resistor)
Possible screen gridconnections
Screen grid connected to anode
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Possible suppressor
grid connections
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Normal way the suppressor grid isconnected / operated in a pentode
Does not do harm in triode mode
For beam power valves, usually thereis no choice to use another connection,since g3 is permanently connected to
the cathode internally
Possible suppressor gridconnections
Suppressor grid connected to cathode
Preferred methodto connect g3!
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Combined g1 + g2 + g3 control actionis of multiplicative character
Extreme µ, but also extreme internalresistance
”Current trickler”, only useful for few
very special applications
Possible suppressor gridconnections
Suppressor grid connected to control
grid and screen grid
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Screen and suppressor grids swingwith anode
Normal functions of g2 and g3are disabled
Screen and suppressor grid areaeffectively added to anode area
Alternate viable method to connectthe suppressor grid (if it isaccessible)
Possible suppressor gridconnections
Suppressor grid and screen grid
connected to anode
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Comparing suppressor grid connection influence
Ca. 10-15%
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Screen grid to anode
Suppressor grid to cathode (or anode)
Summary:
Preferred grid connections
to get triode characteristicsfrom a pentode are:
Preferred methodto connect g2+g3!
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What happens when you triode-strap a remote cutoff(variable µ/gm) pentode like EF92/6CQ6?
You get a variable µ/gm triode, of course ☺☺☺☺
S t i l l h a s u s e f u l
l i n e a r r a n g e !
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Sometimes manufacturers of pentodes also published triode
connected curves and often they are used this way. Famousexamples include: EF86 (extremely linear), EL34 (everybodyknows it can power a decent triode SE or PP amp).
But – are there “sleeper tubes”? Cheap pentodes nobody uses
in triode connection because there are no anode curvesavailable for them? Maybe even from (gasp!) TV set heritage?And, can they compete to real triodes?
Some practical examples
Yes, yes and yes!
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EF184 / 6EJ7 frame grid pentode strapped as triode
OP
R L = 3 0 k
102V100V
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„ECC88 section on steroids!“
+ Va(max) = 250V+ Ik(max) = 40mA
ECC88section!
~~
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Example: PCL84 compound valve based headphone amp
Pentode sections connected as triodesin push-pull operation
100% class A triode(d) push-pulloperation
About 1W/channel Po into 8 Ohms
Due to internal dummy load, it candrive headphones from <32 Ohms upto whatever impedance
Also can drive small bookshelfs
Automatic disconnect of speakers onheadphone plug insertion
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Example: PCL84 TV compound valve based headphone amp
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Poor Man´s 300B?R L = 5 k
260V 220V
P t d
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Example: SE amp circuit using “TV duty” valvesTrioded EF184 used as µ-follower driver
Trioded EL36 used as SE power stage
Circuit courtesy of Gregg van der Sluys
P t d
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Thank you very much for your attention!
Next lecture:
Mr. Morgan Jones
14:00, library room