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Solid State 12AX7 Tube compared with Traditional 12AX7 Vacuum Tube
27 Mar

Solid State 12AX7 Tube compared with Traditional 12AX7 Vacuum Tube

Times Read: 273 Comments: 0

SOLID STATE TRIODE series AMT “WARM STONE”

Article researched and written By Kampf V.A.



INTRODUCTION


Tube sounding almost initially became a kind of standard guitar sound, and it is so to this

day. The most frequently used in guitar amplifiers, low power dual triodes such as EСС83

(12AX7). Despite the undoubted musical merits of this triode, it does have some drawbacks

common to all vacuum tubes (fragility, low efficiency, limited resources, deterioration of character-

istics over time, etc.), therefore has long led development engineers to think about creating

devices which will replace the tubes, but devoid of the above-mentioned drawbacks.


Back in the 70 years of the twentieth century in the music industry have been carried out first

attempts to use low-power semiconductor tube replacements. In some models of company

Mesa-Boogie, for example, in the first stage instead of the usual triode 12AX7 was used the

components of the company Fetron. It should be noted that the Fetron's components was

intended primarily for telephone exchanges, where they had been used since the late 60's

in order to save electricity. Substitutes of tubes from FETRON were built in a cascоde

schematic, which provided the ability to operate at high voltages, but they have a pentode

output characteristics, which is clearly visible in the graphs the output current-voltage char-

acteristics [1, 2], therefore, a direct replacement for triode devices with pentode's CVC led

to significant differences in the sound. This circumstance, of course, played a considerable

role in that how cold enough the musical community met such innovation in guitar amplifiers

and, as time has shown, substitutes of tubes from FETRON not taken root in the world of

music.


Some time ago the other companies have also implemented a significant try to the “tube

heights”. Unfortunately, the manufacturers have not accompanied its products with sufficient

technical documentation, so we have no right to speak about conformity of triode current-voltage

characteristics (CVC), or other aspects of the triode vacuum behavior emulation, especially

when dealing with high signal levels. We’re just stating a fact of issue some semiconductor

tubes, leaving to the music community itself the right to form an opinion about the sound

properties of these devices.


AMT Electronics has its own tradition of building tube replacements. In 2007, the company's

engineers used their version of the cascode switching-on JFET and NPN bipolar transistor

in the SS-20 preamp, allowing to accelerate the signal without limitation to several hundred

volts with a minimum of noise floor. Then, were introduced circuits into the cascades that

simulate the grid limitation, and the shape of the signal became very close to that of the sig-

nal on the 12AX7. In 2007, on the basis of article [4] the company started the development

of low-voltage cascades that emulate the behavior of triode at the restrictions. Despite the

use of field-effect transistors with a pentode characteristics, engineers managed to get the

waveforms of signals which are similar to restrictions of a vacuum triode and in 2008, the

world saw the first series of preamps LegendAmps (LA). In 2011, as a result of hard work

on the modernization of low-voltage cascades, a series LA2 were born, which continues the

tradition of using field-effect transistors in guitar preamps.


By 2012, the AMT engineers had accumulated a lot of experience in the development of

semiconductor stages, which are in one way or another substituting the vacuum triodes,

and in early 2012 the company AMT began intensive work on creating a complete functional

semiconductor analogs of popular tubes used in guitar amplifiers.


In this article, we introduce the reader to the main points that characterize the sound of the

vacuum triode in guitar amplifiers and their implementation in the solid-state triode

AMT WARM STONE. Description of high-power solid-state penthodes and tetrodes

expected in the near future in a separate article.


Basic principle operation of the semiconductor triode.


1. "Triode character" of sound of the vacuum triode in the linear region is largely

determined by the shape of the output characteristics, namely, by the strong dependence

of the anode current Ia from anode voltage Ua at a fixed potential of the control grid Ug =

const (pic. 1).


                                                       Picture 1. The anode current-voltage characteristics of the 12AX7, ECC83


In triode the change of anode potential UA under the influence of the signal on the grid UG

led to a change in electric field EA between the anode and the cathode, which is summed

up with a field of the control grid EG in such a manner that prevents the change of the grid

field under the influence of UG (Pic. 2), that determines the presence of the inner negative

feedback connection in the triode - parallel to NFB by voltage [4].



                          Picture 2. NFB of vacuum triode and organization of management of the field in triode by AMT Electronics.


                                              Picture 3. The scheme of the triode with field control.



In the shown base scheme (Pic. 3) T1 - MOS transistor depleted type, ie the device with

pentode CVC.

Node of CVC correction is implemented in the cascade at T3. The required non-linearity of

the cascade for accurately reproduce the vacuum triode CVC determined by the presence

diode D3 and the nonlinearity of resistance of the channel T2, included as an repeater.

Transistor T2, controllable by a cascade at T3, maintains the necessary capacity on source

T1, providing the required CVC. Attitude R1 and (R2 + g), where g - is the resistance

channel T2, determines the extent of fan-shapes of the current-voltage characteristics, as

well as in conjunction with a divider R3R4 sets the internal gain of the triode - μ (Mu). D3

forms the CVC at low anode voltage in region of the voltage on the grid, close to zero.

Diode D2 - is dividing. It provides a cutoff of the anode current at any negative voltages

below the voltage closing of the "Tube". [6] Moreover, D2 is involved in the formation of a

significant in the sound a smooth bend of CVC at the region of low anode current.

                                                                       Picture 4. Triode and pentode regions of anode CVC. Triode with field control.


The boundary of transition into the pentode region depends on the parameters of the

applied components and in triode AMT-12AX7-WS is not less than 500V.

2. The second important feature of the vacuum triode, largely determines the sound, espe-

cially in the overloaded guitar amps, is the presence of current cathode-grid at close to zero

voltage on the grid relative to the cathode [3].




                                                     Picture 5. Generalized dependence of the current grid from the voltage grid-cathode.


At pic. 5 seen that in the vacuum triode even at zero voltage on the grid relative to the

cathode the electron current flows from the cathode into the grid. This effect is due to the

fact that the fastest electrons overcome the potential well of an electron cloud space charge

near the cathode and attracted to the grid. For 12AX7 the full cutoff of the grid current

occurs only when applying a locking negative voltage of the order of -1.5 V and below.

In the development of a series triodes WarmStone special attention was paid to reproduce

the cutoff levels of the grid current and the preserving the dependence of voltage - current,

at the low voltages region in accordance with the analogical characteristics of the vacuum

12AX7. The node forming of the grid current is realized from source of negative voltage B2

and from chain of semiconductor diodes D1 (Pic. 3), jointly implementing the necessary

current-voltage characteristic of input diode grid-cathode. Selecting the type and number of

diodes, as well as the bias voltage in the grid circuit AMT 12AX7WS carried out on the

basis of the criterion of the most exact match of dependence current - voltage to the vacuum prototype (Picture 6).



                  Picture 6. Dependence of the grid current from the voltage grid-cathode for vacuum and semiconductor triodes 12AX7



As seen from Fig.6 in region -1.5 ... +0.7V dependences practically coincide. At voltages

above +0.7V the observed discrepancy does not affect the adequacy of the semiconductor

triode functioning, because the value of the grid currents in real circuits of 12AX7 is almost

always limited to the value of 0.5 ... 1 mA.

The saturation a current of the grid at the level of 2 mA is the result of a specific circuit

design AMT-12AX7-WS and indicated to account for this feature in the case of special

applications with significant currents of the grid.

Note! Characteristics of vacuum devices from different manufacturers, and characteristics

inside the series even at one and the same manufacturer sometimes vary considerably.

In the process of design of AMT 12AX7WS, AMT company focused primarily on the

standard passport data of 12AX7 (EСС83) and exemplars of tubes of different

manufacturers that best meets industry standards for 12AX7.


CURRENT-VOLTAGE CHARACTERISTICS



                                                                              Picture 7. The output characteristics of Ia=f(Ua) (Ug=0…-5V)


                                          Picture 8. The initial region of the output characteristics of Ia=f(Ua) (Ug=0…-1.5V

                                            


                                                                               Picture 9. Transfer characteristics AMT 12AX7WS



LITERARY SOURCES:

1. FETRON, solid state vacuum tube replacement. Teledyne semiconductor. June

1973.

2. World’s only all FETRON RADIO & the OMEGA device. Dr. H. Holden. Dec 2011.

http://www.worldphaco.com/uploads/WORLDFETRON.pdf

3. Kampf V.A., JFET - emulation of triode restriction.

http://www.sugardas.lt/~igoramps/article68/article.htm

4. Grishin V.A., Internal feedback in electrical vacuum triode.

http://www.audioworld.ru/Books/Tubes/tub_02.html

5. H. Stockman, S.D. Inherent Feedback in Triodes. Wireless Engineer, April

1953. Page 29.

http://www.next-tube.com/articles/Stockman/Stockman.pdf

6. Kryukov M.G., Some of the nuances of using MOSFET as repeaters in guitar tube

amplifiers.

http://milas.spb.ru/~kmg/irf.html





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