Brimar

- British designation code for Receiving Valves -

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First Number: Construction
1	Half Wave Rectifiers
2	Diodes. Single
3	Triodes, Output
4	Triodes, High-mu
5	Tetrodes, Straight
6	Tetrodes, Vari-mu
7	Pentodes, Power and Video
8	Pentodes, R.F. Straight
9	Pentodes, R.F. Vari-mu
10	Diodes, Double
11	Triodes with Double Diode
12	Pentodes, A.F. with Double Diode
13	Triodes Double, High-mu
14	Triodes Double, Class B Output
15	Heptodes
16	Triodes Output, D.C. Coupled
17	Pentodes R.F. with Double Diode
18	Pentodes with Triode
19
20	Hexode/Heptode with Triode

CV numbers

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Format is CV followed by up to 5 digits, e.g. CV4004. 

Used by the British Military to codify vacuum tubes, gas tubes, and latterly, some semiconductor devices.

CV numbers can be directly mapped onto NATO numbers, as 5960-XX-000-YYYY where XX is the country code, and YYYY is the CV number. 

For example, 5960-99-000-4004 = CV4004 (12AX7WA), with 99 indicating country of origin as UK.

European (Mullard/Philips)
 - after 1934 -
(pro-electron)

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First letter: Filament

0

 tubes without filament

A

 4V

B

 0.18A (series)

C

 0.2A (series)

D 

 1.4V (series/parallel)

E

 6.3V (series/parallel)

F

 12.6V

G

 5V (parallel)

H

 0.15A (series)

I

 20V AC/DC parallel connection

K

 2V battery

L

 0.45A (series)

O

 0.15A AC/DC series connection

P

 0.3A (series)

U

 0.1A (series)

V

 0.05A (series)

X

 0.6A (series)

Y

 0.45A (series)

Second and subsequent letters: tube systems, construction

A

  Diode - single detection diode (excluding rectifiers)

B

 Double detection diode (excluding rectifiers)

C

 Triode (small-signal, not power)

D

 Power output triode

E

 Tetrode (small-signal, not power)

F

 Pentode (small-signal, not power)

L

 Power output tetrode or pentode

H

 Hexode or heptode (of the hexode type)

K

 Octode or heptode (of the octode type)

M

 Tuning indicator

N

 Thyratron

Q

 Nonode. Enneode

W

 Diode single gasfilled rectifier

X

 Diode double gasfilled rectifier

Y

 Half wave rectifier

Z

 Full wave rectifier

First digit: Basing, Socket

1

 Miscellaneous

2

 Miniature 10 pin.

3

 International octal

4

 Miniature 8-pin (B8A)

5

 Magnoval (B9D)

6

 Subminiature

7

 Subminor 8p

8

 Noval (B9A)

9

 Miniature (B7G)

Remaining digits: sequence number

*   Note that signal pentodes and tetrodes which end in even numbers are sharp cutoff tubes. 

Those ending in odd numbers, are remote cutoff tubes.

Examples

EF86:    6.3V filament, signal pentode, Noval base, sharp cutoff.

GZ34:   5V filament, full wave rectifier, International Octal base.

PCL82: 0.3A series string filament, signal triode + power pentode, Noval base.

E88CC:  Special Quality version of the ECC88, swapping of the second and third field was commonplace to denote "SQ" tubes.

European (old Philips) -
- before 1934 -

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Frank Philipse kindly translated the Dutch text to English from:

Gegevens en schakelingen van moderne ontvang-en versterkerbuizen. (1940 Philips).

Frank says "Initially type coding of Philips tubes was done in the following way: The type number consisted of a capital letter, followed by 3 or 4 digits. The capital letter indicated the heater current as follows:"

First letter: Heater current

A

 0.06 to 0.10A

B

 0.10 to 0.20A

C

 0.20 to 0.40A

D

 0.40 to 0.70A

E

 0.70 to 1.25A

F

 1.25A and higher

The first digit, or in case of a four digit number the first two digits,
indicated the heater voltage. For triodes, the last two digits
indicated the amplification factor in it's working point. For tubes
with more grids the last two digits had the following meaning:

41, 51, etc. were tetrodes with space charge grid. (dual grid tubes)
42, 52, etc. were H.F. screen grid tubes. (tetrode)
43, 53, etc. were output pentodes.
44, 54, etc. were binodes. (this was a combined diode-triode or diode-tetrode)
45, 55, etc. were H.F. tetrodes-selectodes (tetrode with variable gain)
46, 56, etc. were H.F. pentodes.
47, 57, etc. were H.F. pentodes-selectodes. (pentode with variable gain)
48, 58, etc. were hexode frequency changers.
49, 59, etc. were hexodes-selectodes. (hexode with variable gain)

An E499 for example is a triode. The E means that the heater
current lies between 0.4 and 1.25 A (1.0 actually). The heater
voltage is 4 Volts and the amplification factor is 99.
An E446 is a H.F. pentode with a heater current between 0.4 and
1.25 A (1.1A actually) and a heater voltage of 4 Volts.
Eventually, because of new developed tubes, this type numbering
system was no longer sufficient and a new system was developed.

Since 1934 all new tubes were coded according to the new system
which is still in use for most European tubes. (e.g. ABC1, EABC80, AD1, EL34)

Japanese (JIS)

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Japanese Encoding System

First number: Approximate Filament Voltage

First letter: base

D

 super miniature

G

 octal

L

 loctal

M

 miniature 7 pin

N

 nuvistor

R

 miniature 9 pin

X

 4 pin

Y

 5 pin

Z

 6 pin

W

 7 pin

T

 large 7 pin

B

 other

First letter after hyphen: class of valve, construction

L

 small triode with mu < 30

H

 small triode with mu > 30

A

 power triode

R

 sharp cutoff RF tetrode or pentode

V

 remote cutoff RF tetrode or pentode

B

 power beam tetrode

P

 power pentode

D

 detector diode

K

 kenotron

G

 gas rectifier

E

 magic eye

Final number: Indicates differences in characteristics.
For rectifiers - even digit for full wave rectifier, odd digit for half wave rectifier.

French (Mazda)

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Mazda Signal valves

First number: Filament

1

 1.4V (parallel or series)

6

 6.3V (parallel or series)

10

 0.1A (series)

20

 0.2A (series)

30

 0.3A (series)

Letters: class of valve, construction

C

 Frequency changer with special oscillator section.

D

 Signal diode(s).

F

 Voltage amplifier tetrode or pentode.

FD

 Voltage amplifier tetrode or pentode with diodes.

FL

 Voltage amplifier tetrode or pentode with voltage amplifier triode.

K

 Small gas triode or tetrode.

L

 Voltage amplifier triode or double triode, including oscillator triode.

LD

 Voltage amplifier triode with diode(s).

M

 Tuning indicator

P

 Power amplifier tetrode or pentode

PL

 Power amplifier tetrode or pentode with voltage amplifier triode.

Final number: sequence number, distinguishes between different valves in the same class

Examples:

6F22: 

 6.3V filament, Voltage amplifier tetrode or pentode.  (6F22 = EF86).

6P15: 

 6.3V filament, Power amplifier tetrode or pentode.   (6P15 = EL84 / 6BQ5).

30PL12: 

 0.3A series string filament, Power amplifier tetrode or pentode with voltage amplifier triode.  (30PL12 = PCL82 / 16A8).

Mazda Power rectifiers

Letters indicate rectifier type:

U

 High vacuum half-wave.

UU

 High vacuum full-wave.

Final number: sequence number, distinguishes between different valves in the same class

Russian/Soviet (GOST 5461-59)

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The regulation GOST 5461-59 applies to tubes manufactured from 1959 and semiconductors manufactured from 1959 to 1963. Every symbol consists of four elements. If an element is absent, a dash should be written at its place. Note: There are exceptions. Not all Russian tubes are numbered according this system.     First element: function Generator (transmitting) tubes up to 25 MHz UKF generator (transmitting) tubes 25-600 MHz Centimeter range generation (transmitting) tubes Modulator tubes High power rectifiers Voltage stabilizers Current stabilizers Thyratrons Gas-discharge rectifiers Mercury gas-discharge rectifiers , Photocells and photomultipliers Approximate (usually full volts) heater voltage Receiving tubes Screen diameter in cm Picture tubes Semiconductor diodes Transistors     Second element: class of valve, construction Diodes Double diodes Triodes Tetrodes Power pentodes and tetrodes Remote cut-off pentodes Sharp cut-off pentodes Secondary emission pentodes Mixing tubes with two signal grids Pentodes with one or two diodes Triodes with one or more diodes Tuning indicators Triodes with hexode, heptode or octode Double triodes Double tetrodes and pentodes Triode - pentodes Rectifiers Type sequence number Gas-discharge tubes, power rectifiers Gas thyratrons Mercury thyratrons Cold cathode thyratrons Oscillograph tubes with electrostatic deflection Oscillograph tubes with electromagnetic deflection TV picture tubes Type sequence number Semiconductors     Third element: tube systems, construction Receiving, picture, generator and modulator tubes voltage and current stabilizers photocells and photomultipliers cold cathode thyratrons Type sequence number Semiconductors Subtype, group (letter)     Fourth element: bulb, base none Metal Receiving tubes; voltage and current stabilizers Glass HF glass, side leads Ceramic Miniature glass 19 and 22.5 mm Subminiature > 10 mm Subminiature 10mm Subminiature 4 mm Subminiature 6 mm lock-in-key Disc leads Water cooled Generator (transmitting) and modulator tubes Air cooled letter Luminophor color code Picture tubes X/Y X = average current in A Y = reverse voltage amplitude in kV Thyratrons (non cold cathode), Gas-discharge, power rectifiers   After symbol dash and postfix of one or more letters may be added: high reliability tube (military) long life tube (military) designed for impulse operation vibration resistant   Current Regulators [A][V1-V2] [A] is the current (in amperes) and [V1-V2] is voltage range More Information about Russian and Soviet Tubes from Klaus and Klausmobile      http://www.klausmobile.narod.ru/td/gost_e.htm 1959 standard tube coding Pre-WW2 soviet tube coding consisted of 2 or 3 cyrillic letters followed by 3-digit code. Well before WW2 began, FDR was fueling Joe Stalin, pumping tubes and tube-building machinery by shiploads. In 1939 (according to post-WW2 sources), this new machinery already produced the American tube types under their original names, in such amounts that in the same 1939 the American convention, transcripted to Greek letters, became a codified standard, so far coexisting with indigenous naming convention. Military hardware received from the U.S. in 1941-45 was equipped mostly with VT-coded tubes. VT-tubes left very scarce traces in Soviet tube annals, just like British tubes. Original octal tube codes like 6J5, 6N7 etc. that were reproduced as is in the Soviet Union received names directly transcribed from originals. For simplicity, only one 'middle' letter was left out. The trailing number lost it's 'pin count' designation, becoming simply a serial number. Eventually, the code table was extended to accomodate nearly all new tube types, and as the pre-WW2 designs were retired, became a unified standard (1959). Some older tubes survived until the end of tube era with original non-standard names (i.e. G807). Gas regulators usually follow the 1959 standard, omitting heater voltage code. In the English-speaking audio world, Soviet single and dual triodes names are frequently written as 6H** or 6C** in Latin letters.  To separate Soviet and American (and Chinese) tubes with the same Latin names, American originals are written with a #prefix (#6F6, #6SN7). Notes: The second element (Device Type) is sometimes inconsistent. For example, TV damper diodes can be found under both *Z** (Rectifiers) and *D** (Single Signal Diode) coding. They all can be used as AC power rectifiers within their I/V ratings. Similar confusion with 6F6S - it's a pentode and not a Triode+Pentode as letter *F** suggests. But it's a glass incarnation of #6F6. The fourth element (Package Type) refers to package size (by diameter), not by base type. For example, 6V3S tetrode has the same 9-pin miniature base as 6N1P, but it is listed under ***S package type because of it's oversized (24.5mm diameter) glass shell. It is NOT an octal tube as it may seem at first glance Some very rugged tube types do not carry 'advanced' suffixes, because they were originally designed to advanced specs. Some other rugged tubes exist only as -E or -EV grades (i.e., 6S45P-E and 6N30P-EV exist, but no one ever heard of plain 6S45P or 6N30P). Anyway, the curves and nominal PIV for regular and improved versions are identical, with very few exceptions. Eexception are 6S3P and 6S3PE, that are two different tubes. Transmitter/Modulator tube coding Although a few transmitting tubes were named within 1959 standard framework, most transmitter tubes follow a different name convention that goes back to the 1930-s. These names consist of two or three letters followed by number and a suffix letter: GK - Oscillator, Shortwave, up to 25MHz GU - Oscillator, 25-600MHz GS - Oscillator, above 600MHz GI - Pulsed Oscillator GM - Oscillator Modulator GMI - Pulsed Oscillator Modulator Suffix stands for: A - Forced Liquid Cooling, B - Forced Air Cooling, no suffix: Convection Cooling.

BASES

 More Information: http://www.tubedata.org/bases.html

OCTAL

OCTAL

K8A, A08, International Octal (IEC 67-I-5a)      EXAMPLES: 6L6, EL34

UX4, G

UX4, G

SMALL 4-pin BASE                                 EXAMPLES: 2A3, 5Z3

UX5, N

UX5, N

SMALL 5-pin BASE                                 EXAMPLES: 24A, 76, 807

BASES

 More Information: http://www.tubedata.org/bases.html

BULB SHAPES

ST12

small

5-pin

base

                                                                                                  84

ST12

small

6-pin

base

with Cap

                                                                                                     79

ST14

medium

4-pin

5-pin

7-pin

base

                                                                                            49, 80, 6A6

ST16

UX4 base

                                                                                                  2A3, 801

ST16

UX5 base

with Cap

                                                                                                 807, 1625

ST19

                                                                                          300B, 4300B (UX4 base)

ST19

with Cap

                                                                                             811A_RCA

T5-3

RETMA 5-3

                                                                                                 6AQ5A

T6-2

EIA6-2

                                                                                         ECC83, 12AX7A_RCA

T6-4

EIA6-4

                                                                                               EL84, 6BQ5

T8

small

4-pin

base

                                                                                                                                 UX-120                           

T9

RETMA 9-11

or

RETMA 9-41

                                                                        7591S                             7591A

T9

9-pin base

JEDEC E9-75

                                                                                                    7868

T11

                                                                                                    6384

T12

                                                                                                     5931

T12

metric

                                                                                         EL34 (Octal Base)

T12

EIA 12-15

                                                                                               5881

T12

with small Cap C1-1

Octal base

                                                                                                6146

T16

Octal

base

two small caps

                                                                                                815

T16

JEDEC 7-2 base

                                                                                              829B

T18

50 Watt base

                                                                                                    838, 845B

T20

JETEC A7-17

Medium Metal Shell base

Giant 7-pin with Bayonet

medium cap JETEC C1-5

                                                                                                       813

Absolute Maximum Ratings  

Absolute Maximum Ratings must not be exceeded under any circumstances. Power supply voltage variations, component tolerances, etc. should be carefully taken into account. Exceeding only one of the maximum ratings can seriously damage the tube.

Bias

Bias is a voltage (usually negative) applied to a tube's control grid, to set the amount of idle current the tube draws. It is important to bias a tube that its Absolute Maximum Ratings are not exceeded..

Continuous Commercial Service

(CCS)

CCS is defined as that type of service in which long life and reliability of performance under continuous operating conditions are the primary considerations.

Class

The operating conditions of amplifiers are classified by letters A, B, C, and D. More specific and derivated operating classes are A1, A2, AB, AB1, AB2, B1, and B2. Subscript 1 indicates that grid-No.1 current does not flow during any part of the input cycle, while subscript 2 indicates that grid-No.1 current flows during some part of input cycle.

Class A

Class A means that the power tube conducts the same amount of current all the time, whether idling or producing full power.

There are single-ended (SE), class-A, amplifiers and Push-pull class-A amplifiers.

There are two kinds of class-A operation, which can apply to single-ended or push-pull.

--Class A1 means that the grid voltage is always more negative than the cathode voltage.

This gives the greatest possible linearity and is used with triodes.

--Class A2 means that the grid is driven MORE POSITIVE than the cathode for part or all of the waveform. This means the grid will draw current from the cathode and dissipate power.

Class AB

Class AB applies only to push-pull amplifiers. It means that when one tube's grid is driven until its plate current cuts off completely, the other tube takes over and handles the power output. This gives greater efficiency than Class A and results in crossover distortion.

There are class-AB1 and class-AB2 amplifiers; the differences are the same as explained above.

Class B

Class B applies only to push-pull audio amplifiers. It operates like Class AB, except that the tubes idle at or near zero current. That results in greater efficiency than Class AB but also in increased crossover distortion .

Controlled Heater Warm-up Characteristic

A controlled heater warm-up time ensures dependable performance in equipment employing series-connected heater strings, like in television receivers. Heater warm-up time is measured in a circuit as follows: The heater is placed in series with a resistance having a value 3 times the nominal heater operating resistance (R = 3 x Uf / If). A voltage having a value 4 times the rated heater voltage is then applied. The warm-up time is the time required for the voltage across the heater to reach 80% of the rated value (U = 0.8 x Uf). The average value has been fixed to 11 seconds in the USA and to 14.5 (11 ... 18) seconds in Europe.

Conversion Trans-

conductance

gc

or

Sc

This characteristic gc (or Sc) is associated with the mixer function of tubes (frequency converter) and is defined as the limiting value of the quotient of the intermediate-frequency (IF) current in the primary of the IF transformer divided by the applied radio-frequency (RF) voltage producing it, as the RF voltage and IF current approach zero. When the performance of a frequency converter is determined, conversion transconductance is used in the same way as control-grid to plate transconductance in single-frequency amplifier computations.

Design

Centre

Ratings

 Design Centre Ratings are given for receiving tubes to design equipment where nominal component values can be used, and normal supply voltage variations of ±10% do not affect proper tube-circuit functioning.

Design

Maximum

Ratings

Design Maximum Ratings must not be exceeded using a tube with the indicated ratings (bogey tube) under worst case operating conditions. Circuit designs based on these ratings should consider all component tolerances, supply voltage fluctuations, and signal variations which are to occur while operating the circuitry.

Dynamic

Plate

Resistance

Dynamic Plate Resistance  -  The resistance ra of the path between cathode and plate to the flow of alternating current can be derived from the tube characteristics (plate-characteristic family of curves) and is defined as the quotient of a small change in plate voltage divided by the corresponding change in plate current, under the condition that the control-grid voltage remains unchanged (partial derivative):

ra = dUa / dIa while Ug = constant

There is a relationship between transconductance, plate resistance, and amplification factor:

ra = µ / S

Effective

Plate-Supply

Impedance

Ra

Effective Plate-Supply Impedance - Ra is effective in rectifier circuits limiting the peak plate current. Topologies with filter-input capacitor mostly need an additional protective series resistor in the plate circuit, by which the effective plate-supply impedance consists of the protective resistor Rs, the DC resistance R2 of the transformer secondary, and the transformed resistance R1 of the primary:

Ra = Rs + R2 + (n2/n1)² x R1

The minimum effective plate-supply impedance Ra is generally specified by the tube manufacturer at particular operating conditions (AC supply voltage, load capacitance), whereas an eventually necessary protective series resistor may be determined in consideration of the inductive components' DC resistance. Generally higher values of capacitance than indicated may be used, but the effective plate-supply impedance may have to be increased to prevent exceeding the maximum rating for peak plate current.
Using a choke-input filter, normally no protective resistor is required, since a reasonably high inductance in series with the filter capacitor prevents large current peaks. In addition the choke is providing significant DC resistance. This results in an effective plate-supply impedance consisting of the choke's DC resistance RsL, the DC resistance R2 of the transformer secondary, and the transformed resistance R1 of the primary:

Ra = RsL + R2 + (n2/n1)² x R1  

Efficiency Plate-Efficiency

eta

(lower-case greek letter)

Efficiency Plate-Efficiency of a power amplifier tube is the ratio of the AC power output Pout to the product of the average DC plate-supply voltage Ub and DC plate current Ia at full signal, or:

eta(in %) = Pout / (Ub x Ia) x 100

EIA

  Electronic Industries Association

Getter

Very small leaks can appear in a tube envelope (e.g. at the socket). Also the tube may not be fully "degassed" at the factory, that there is still some air remaining inside. The "getter" is designed to remove stray gas.

In most modern glass tubes, the getter metal is barium, which oxidizes VERY easily when it is pure.

When the tube is pumped out and sealed, the final step in processing is to "fire" the getter, producing a "getter flash" inside the tube envelope.

That is the silvery patch you see on the inside of a glass tube.

Intermittent

Commercial

and

Amateur Service

(ICAS) 

 -  is defined to include the many applications where the transmitter design factors of minimum size, light weight and considerably increased power output are more important than long tube life. In this service, life expectancy may be one-half that obtained in Continuous Commercial Service.
Under the ICAS classification are such applications as the use of tubes in amateur transmitters, and the use of tubes in equipment where transmissions are of intermittent nature. Intermittent operation implies that no operating or 'on' period exceeds 5 minutes and every 'on' period is followed by an 'off' or standby period of at least the same or longer duration.

Intermittent

Mobile Service

(IMS)  

Intermittent Mobile Service (IMS) is defined to include those applications, such as aircraft, where the transmitter design factors of minimum size, light weight and exceedingly high power output for short intervals are the primary requirements even though the average life expectancy of tubes used in such transmitters is reduced. Tube ratings for IMS service are established on the basis that the transmissions have maximum 'on' periods of 15 seconds followed by 'off' periods of at least 60 seconds, except that it is permissible to make equipment tests with maximum 'on' periods of 5 minutes followed by 'off' periods of at least 5 minutes provided the total 'on' time of such periods does not exceed 10 hours during the life of any tube.
Although the use of tubes under IMS ratings involve great reduction in tube life, such use can be justified as economical practice in applications where high power is intermittently desired from small tubes.

OTL Amplifier

-:-

output-transformer-less

OTL (output-transformerless) , amplifiers are special high-end products. Because it is expensive and difficult to wind an output transformer for a tube amplifier to achieve the best possible performance, some designers have chosen to eliminate the transformer.

Tubes with large cathodes and high peak emission capability are used in parallel push-pull pairs to provide enough current for the low impedance speaker.

Parallel Feed

-:-

Shunt Feed

A choke is used to load the power tube (usually one, in SE mode), while the output transformer is coupled to the plate of the tube through a capacitor. So, the plate current of the tube does not flow through the output transformer. This can be a very expensive technique to implement, since the choke must be as carefully wound as the output transformer. It does offer a possible performance improvement.

Power

Output

 -  The power output Pout of a tube at proper impedance matching, if necessary by tuning and neutralization of radio-frequency stages, is given as the difference of the plate input Pba and the plate dissipation Pa. The actually available output is reduced by losses in the output transformer or the output resonating circuit.

RETMA

  Radio Electronics Television Manufacturers Association

SRPP

and

mu-follower

SRPP circuits and mu-follower circuits are special designs which use a lower tube (for gain), and an upper tube which serves as the plate load for the lower tube. The upper tube also acts as both a cathode follower and as a constant-current source for the lower tube. If properly designed, either circuit can offer improved performance over an ordinary resistor-loaded tube stage. These circuits are used only in preamp stages and in the driver stages of power amps, usually SE types, in high-end audio.

Static Amplification Factor

The amplification factor µ can be derived from the tube characteristics (plate-characteristic family of curves) and is defined as the ratio of the change in plate voltage to a change in control-grid voltage in the opposite direction, under the condition that the plate current remains unchanged (partial derivative):

µ = –(dUa / dUg) while Ia = constant

There is a relationship between transconductance, plate resistance, and amplification factor:

µ = S x ra

The static amplification factor represents the best-case voltage gain that could be obtained if the ratio of load resistance to the tube's plate resistance can be increased to any large number. This means virtually a load resistance line running horizontally on the plate-characteristic family of curves, corresponding to an ideal constant-current source. Using a practical load resistance, a voltage gain only less than µ is available that can be calculated as V.

Transconductance

or

Mutual Conductance

(gm)

The control-grid to plate transconductance gm (or S) can be derived from the tube characteristics (transfer-characteristic curve) and is defined as the quotient of a small change in plate current divided by the change in the control-grid voltage producing it, under the condition that the plate voltage remains unchanged (partial derivative):

S = dIa / dUg while Ua = constant

There is a relationship between transconductance, plate resistance, and amplification factor:

S = µ / ra

The unit of transconductance for commonly used tubes can be described simply as milliamperes per volt (mA/V). The American literature is using the unit of conductance "mho", named by spelling "ohm" backwards. For convenience, a millionth of a mho, or a micromho (µmho) is used to express transconductance.

Ultralinear operation

Ultralinear operation was invented by David Hafler and Herbert Keroes in 1951. It is used in audio power-amplifiers with beam tetrodes or pentodes. The screen grids of the output tubes are driven with part of the output signal using taps on the primary windings of the output transformer.

This lowers distortion considerably.

Voltage Amplification

Gain

V

V - is the ratio of the voltage variation (AC) produced in the load resistance to the input signal voltage and is expressed by the following convenient formula using the amplification factor µ, plate resistance ra, and load resistance RaL:

V = µ x RaL / (ra + RaL)

With pentodes it is better to use the product of transconductance S and the plate resistance ra in parallel with the load resistance RaL:

V = S x (ra x RaL) / (ra + RaL)

From the first formula, it can be seen that the gain actually obtainable from the tube is less than the tube's amplification factor but that the gain approaches the amplification factor when the load resistance is large compared to the tube's plate resistance.