A study for the Implementation of
Super Triode Connection (STC) Amplifiers

http://www2u.biglobe.ne.jp/~hu_amp/amput3e.htm
2000/05/08 (Rev3.8) ,07/22 (Rev3.9) ,2001/1/31 (rev3.10), 4/30 (rev3.11) Hiroshi Uda

Table of Contents

1. Preface

1.1 Why the STC Amplifier?
1.2 Evolution of this Article
1.3 Definition of the STC Version 1 Amplifier ---- 4/30(3.11) Rev. applied.
1.4 Principle of the circuit operation in Detail
1.5 Stopping Diode and Linearizer
1.6 Variations of the STC Amplifier and Relationship Between the Speaker System
1.7 Reproducibility and Stability

2. Progress of Experiments, Results, and Consideration

2.1 Experimental Building
2.2 Evaluation of the Results
2.3 Application to the (Beam) Tetrode and Pentode
2.4 Application to the Triode

3. Adjustment Process and Associated Issues

3.1 Adjustment of the Bias Voltage of the Final Stage
3.2 Issues Associated with the Circuit
3.3 Consideration for High Bias Tubes
3.4 The Combination of 1st Stage Amplifier, Voltage-feedback-tube, and Plate to Cathode NFB
3.5 Discussion Regarding Application to Triodes


1. Preface

Prior to entering the body of this article, I would like to express my appreciation to Mr. Paul Cambie (Melbourne, Australia) and Mr. Hideo Torii (Chiba, Japan) for their guidance and advice in the preparation of the English transcription of this article from the original Japanese version.
Hiroshi Uda, author.

1.1 Why the STC Amplifier?

The author came across articles on the Super-Triode-Connection (STC) audio power amplifier in issues of MJ Magazine 1991/5, 1992/10, and 1993/2, written by Mr. Shin-ichi Kamijo, the inventer of the STC circuit.

Based on those articles, an 807 STC amplifier of Kamijo's version 3 type was built for experimental purposes, and evaluated. The sound quality showed a distinct improvement over that of conventional circuits. Afterward several STC amplifiers of Kamijo's version 1 type, (plate to grid NFB based), were tried, as the circuit is simpler than that of the version 3 (plate to cathode NFB based).

Later prototype circuits built by the author have all been the version 1. Version 1 and version 3 are refered to as V1 and V3 in the following text. This article is based partially on Mr. Kamijo's V1. The differences between the V1 and V3 are detailed at Mr. Kamijo's own home page (at http://www.ne.jp/asahi/evo/amp/guide.htm).

Recently, Mr. Kamijo defined the new name "Voltage-Feedback-Tube" which is identical to auther defined name "Driving-Triode",
the auther updated related portion of this article on 1999/8/31 (Rev3.3).


1.2 Evolution of this Article

The intention of the auther in the very beginning was to write a "Prototype Introduction" to STC amplifiers.

However, experiments went further, until it was evident that any kind of power tube can be or should be applicable to the STC circuit. So, many kinds of power tubes were tried, either examples to hand or bought for the purpose of trialing. Now new trials have just started, using TV horizontal amplifier tubes and some kinds of transmitter tubes.

Data in this article is based on such tubes in the author's own stock; some brand new, some others second hand. It also is to be noted that operating voltages and other parameters are not precisely those specified for the tubes used; tubes may not be operated at precisely optimum condition.
But allowing for slight deviations of individual tubes from specification, the author is confident that these tubes are operated with reasonable and safe margins.
The operation was checked by playing and listening to music and confirming that no odd or bad sounds were heard.

It is recommended that the constructor try variation of operating parameters when building and tuning an STC amplifier.

The adjustment procedure is described in "3.1 Adjustment of Bias Voltage".


1.3 Definition of the STC Version 1 Amplifier

The auther extends Mr. Kamijo's definition of the STC V1 amplifier, and specifys the following four key features, seen in the Figure 1 circuit schematic.

Figure 1; Schematic of a 6BM8 Super Triode Connection Amplifier

(1) A voltage-feedback-tube for high level plate to grid local NFB, controlled by a constant current source. Insertion of this non-linear element in the NFB is a very significant element of the topology.

(2) No overall NFB.

(3) Direct coupling.

(4) Insertion of a "Stopping Diode" and "Linearizer".

1.3.1 Functional Allocation --- from a standard STC circuit

As seen in the schematic, the STC consists of three tubes, (or equivalent active elements), and several capacitors, resistors and diodes. The function of each tube and diode is as follows.

(A) First stage amplifier; the 6AK5 forms the voltage amplifier and constant current source.

(B) Driving-triode; the triode portion of the 6BM8 makes up the voltage converter, the driver for the final stage amplifier tube and constitutes the non-linear NFB element.

(C) Final stage amplifier; the pentode portion of the 6BM8.

(D) Stopping diodes; inserted in the B+ power supply side of the plate and/or screen grid circuit in the pentode portion of the 6BM8.

1.3.2 Implimentation of the Four Key Features

The truly unique characteristic of the STC amplifier comes from the first of the four key features. The operating DC voltage for the triode portion of the 6BM8 is supplied from the pentode portion of the same tube. The triode portion provides a non-linear characteristic in the plate to grid NFB (P-G NFB) of the pentode portion, and makes the NFB 100% in voltage.

The second of the four features "No overall NFB." and the third item "Direct coupling" are already applied in conventional amplifiers.

The fourth item, the "Stopping Diode" and the "Linearizer" are new additions. The following subsections explain each of these four defining items in detail.

1.3.3@Another sample circuit in principle --- P-G NFB'ed cathode follower drive

The auther tried to explain the difficult STC V1 circuit directly by Rev3.10 update stage, the auther found a another sample that seems to be easier way to explain the STC circuit. The circuit, the auther tried "P-G NFB'ed cathode follower drive" circuit , that is a kind of semi-STC, and present the "STC effect" to some extent.

1.3.3.1@Even Semi-STC circuit, present effective STC sound.
The auther continued to try STC-rization of large amplitude requiring triode, such like UX2A3, applied P-G NFB'ed cathode follower drive circuit as follows.

2a3sch.gif

6js6cstc.gif

In above circuit, from the power supply voltage issue, hire a C/R coupling circuit. If a direct coupling circuit would be applied, about 150V or more of bulk voltage should be stacked up.

As for P-G NFB, (1) The internal impedance of cathode follower tube "Ri", will be round 10k ohms or more. (2) The current regulated diode (CRD) and resistor inserted to the cathode "Rk", will be round 100k ohms or more. So, by the relation of Ri < Rk, signal generated from the plate of final stage tube will be returned round 90 pecent to the grid of final stage tube. This situation makes final stage amp deep P-G NFB condition and voltage gain by "Mu" of final stage tube will be suppressed to nearly 1. However, the current component generated by the mutual conductance "Gm" is kept untouched.

1.3.3.2@STC circuit may be configured by driver stage and final stage
Thus, the deep P-G NFB conditioned final stage amp is a prototype of STC version 1 amplifier. However, enough amplitude of input signal should be supplied to the grid-ground of the cathode follower tube.
In case large amplitude requiring triode, SRPP pre-driver with high Mu voltage amplifying triode would be required. In case tetrode/pentode, simple high Mu voltage amplifying triode would be enough to drive.

1.3.3.3@STC V1 circuit
The circuit STC V1, insures the voltage gain by signal input between the grid-cathode of cathode follower tube. Then the "voltage feedback tube" acts not only for the NFB, but also acts as a voltage amplifier also. Load of amplifier is the CRD and resistor inserted to the cathode, that is the first stage amplifier by voltage amplifying pentode or FET/bipolar-transistor. The naming "voltage feedback and amplifying tube" of that tube would be more suitable. And also in the STC V1 circuit, signal input to final stage is done from cathode of voltage feedback tube. This connection eliminates the effect of grid current from final tube and makes stable the whole circuit. (2001/4 Rev3.11)


1.4 Principle of the circuit operation in Detail

1.4.1 The Basic Circuit Schematic

Mr. Kamijo explains the basic idea as follows; "Inserting a non-linear element (i.e. a triode) in the negative feedback circuit in an operational amplifier causes the dynamic of the total circuit to take on a triode-like characteristic." (ref. Figure 2)

Figure 2; Basic STC Circuit Schematic

The basic circuit of the STC consists of the following three functional elements.

Name of tubes and releavant circuits Formerly, the naming of the driver stage was not specifically determined in Mr. Kamijo's article; the auther defined the simple name as "Driving-Triode".
Afterwards, Mr. Kamijo defined the name as "Voltage-Feedback-Tube", the auther update this article according to the new deffinition.

To the first stage amplifier, which acts as a V/I convertor and constant current source, the auther gave the simple name "1st stage".

As for the final stage amplifier; "final stage (amplifier)".

Through this paragraph, some readers may feel some difficulty understanding the operation fully.
So the most critical points are clarified through the following explanation. In order to understand the following section, it is necessary to know the "Three constants of the vaccum tube".

Consider how the "Mu"= 1 operation described later is realized, and how the constant current source works.

Constant current source
In a circuit which has a constant current source (here-in-after called CCS) character, when a given voltage is varied, the current does not vary (much). For example, a component called a "Constand current diode" has such a character. And also pentodes, bipolar-transistors, and FET's have the same character. The reason to utilize the constant current source, will be explained later. Refer to the figure "Figure 3 Character of a Constant Current Source etc.".

Constant current source by pentode
Refer Figure 3(1), The figure explaines how the Character of a CCS can be obtained from the Eb-Ib characteristic of a pentode, with its high internal resistance character.

V/I conversion by pentode
Refer Figure 3(2), The figure explaines how the function of V/I conversion can be obtained from the Eb-Ib characteristic of a pentode with smaller load.

Voltage amplification by pentode
Refer Figure 3(2), The figure explaines how the function of amplification can be 0btained from the Eb-Ib characteristic of a pentode with larger load.

Co-existence of CCS function and V/I conversion of function by pentode
Refer Figure 3(2), The figure explaines the co-existence ob both CCS and V/I conversion of a pentode with smaller load.

Characteristic of triode
Refer Figure 3(3), The figure explaines the low internal resistance character of triode.

Figure 3 Character of a Constant Current Source etc.

1.4.2 Principle of Operation of the V1 STC

Now, return to the main subject;

Mr. Kamijo further explains;

"(1) The amplification factor of the final stage will be unity, where 100% NFB is applied". This expression means that if the final stage has full amplitude NFB applied, then the final stage tube doesn't work as a voltage amplifier, just as a V/I converter (Figure 4.1/4.2). The current conversion performance is simply determined by the mutual conductance, gm, of the output pentode. Also in his article, Mr. Kamijo says;

"(2) The total gain of the voltage-feedback-tube (here-in-after called VFT) and the final tube combination is approximately equal to the amplification factor, "Mu", (which is written with the same Greek symbol as "Micro"), of the VFT".

Tracing the circuit, the following points become clear.

1.4.2.1 Theoretical circuit/ Grid input

Refer Figure 4.1(1). This circuit style coprresponds to the "Inverted Mu follower with P-G NFB".
The input signal is that between the cathode of the VFT and CCS "ed".
To regard the CCS as the ground, VFT amplifies the signal with the load of CCS.

1.4.2.2 Theoretical circuit/ Cathode input

Refer Figure 4.1(2). The grid of the VFT is grounded to the CCS, and so the VFT acts as a grounded grid (so-called GG) amplifier. other portion of the circuit will act same as figure 4.1(1), i.e. Grid input circuit.

In the normal amplification circuit, the sequence of element will be arranged as ground - amplification element - load - power supply - and return to the ground.
In above two circiuits, the sequence is invereted as ground - load - amplification element - power supply - and return to the ground. However the operation of amplification suffers no effect by this inversion. Actually such inversion can be oftenly seen in the SEPP/OTL circuits.
The output transformer (here-in-after called OPT) and the B+ power supply are in the signal loop. However the impedances of these intervening elements are far smaller than that of the constant current source. The function of the CCS is to keep the degree of plate to grid NFB at maximum.

Figure 4.1; Theoretical and Practical Circuits of the V1 STC.

Figure 4.2; Implementation of Theoretical and Practical Circuits of the V1 STC.

1.4.2.3 Practical Circuit/ Input from 1st stage

The 1st stage tube works rather as a V/I conversion than voltage amplification, and send to VFT current based signal.

1st stage tube convert input voltage signal "ei" to current signal "id" with its mutual conductance "Gm1";
id = ei*Gm1 (where * means mltiplication operator )

The resistance located in the cathode of VFT called "Rd" convert "id" to "ed" by the multiplication;
ed = Rd*id

After this I/V conversion being done, further circuit operation is identical to the Theoretical circuit/ Cathode input descrobed on paragraph 1.4.2.2.

However, in above described operation, some undesired effect will be occured in the VFT. That is the incurrence of reversed phase signal generated from internal impedance of VFT(Ztp) with "id" I/V conversion;
Ztp*id.

Precisely the total amplitude of VFT should be expressed as;
Mu*ed - Ztp*id = Mu*Rd*id - Ztp*id

The signal, amplified by the VFT, appears as (Mu-1) in amplitude, between the grid and cathode of the final stage.

The meaning of the -1 term is as follows.
The signal is fed from the cathode of the VFT, and the source signal to the VFT is that between the cathode of the VFT and ground; i.e. the CCS; this signal is reversed in phase with respect to the amplified signal.

However, in practice, the mu value of a typical triode will be greater than 10, and so the effect of the -1 term will typically be negligible.

Finally, the driving voltage for the final stage "Efdr" is;

@@@Efdr = *Rd*id - Ztp*id - Rd*id = id *{Rd*(-1) - Ztp}

Refer Figure 4.3 Operation of Practical Circuit.

Figure 4.3; Operation of Practical Circuit

1.4.2.4 The reason for doing V/I conversion/ I/V conversion

In the Practical Circuit, 1st stage tube acts as a load of VFT and the voltage distribution element for the P-G NFB, that is described in next paragraph, and also has to handle the input signal.
In abpve reason, 1st stage has no way without handling the signal with V/I conversion mode. Consequently, VFT should make I/V conversion.

1.4.3 Principle of Operation - High P-G NFB

The principle "Mu"= 1 means current conversion operation
The basic operational circuit may be broken down, on a signal handling basis, to help explain the operation, as follows;

Figure 5 The Movement of 100% NFB

Fig 5(1) shows the amplification operation of the VFT.
The signal "ed" input to the cathode, is amplified, and the output signal appears as "Mu*ed" in amplitude, reversed in phase.

Fig 5(2) shows the elements, and their approximate impedance values, that are involved in the amplification of the VFT.
Zcc: Impedance of the constant current source
Zed: Impedance of the signal source
Ztp: Impedance of the triode i.e. internal resistance
Zot: Impedance of the output transformer
Zbp: Impedance of the B power supply

Fig 5(3) shows nearly 100% NFB, that is a "Mu"= 1 operational mode. Strcitly speaking, the triode has five load impedances to be considered. However the dominating impedance will be Zcc.
Then the signal appearing between Zcc and ground, is input to the final stage. The operation of the final stage is as follows.
The amplified signal voltage appearing on the plate, will returned to its grid.

Zcc/(Ztp+Zcc)....Nearly equal to 1, whereas Zcc is far greater than Ztp.

This operation means nearly 100% voltage NFB, and it also means no voltage amplification being done by the final stage.

Fig 5(4) shows the principle of the current conversion function. Then the circuit might be calleded a {Current conversion amplifier} or a {Gm amplifier}, and these would be more suitable than "STC".

(C) The need for the constant current source (CCS)
At first glance, the reader will ask "Why do they use the CCS? Can't the CCS be simply replaced by a capacitor or resister?" Using a CCS is most important. The operation of the CCS is as follows. In the low voltage area (see Figure 3), current climbs with a steep slope, but after the early rise, the slope changes to a more gradual one.

This character of the CCS is very convenient for realizing a constant current character with a low supply voltage. To realize such a circuit characteristic with just a high valued resister, and to make a similar gradual V-I slope, would require a high value minus power supply, to pull down the other end of the resister. So the CCS is an essential element to facilitate voltage distribution and signal handling functions together, in the STC-V1 direct-coupled circuit environment.

1.4.4 Review of Practical V1 STC Circuit

The practical V1 STC circuit is configured as follows;

(1) The input signal controls the CCS, which is the first stage.

(2) The plate current is "modulated" by the input signal as "id".

(3) The resistor inserted in the cathode of the VFT converts the signal-modulated current to a voltage,

(4) The VFT amplifies the voltage signal using a grounded grid amplifier.

At first glance, this circuit looks like an SRPP. However, using plate to grid NFB (P-G NFB) as well, changes the operation of the VFT to the following combination of functions;

(1) An amplifier performing current to voltage conversion of the input signal.

(2) Non-linear element inserted in the plate to grid NFB (P-G NFB).

(3) Cathode follower driver for the final stage, providing low impedance drive and protect malfunction of CCS when final stage tube begin to flow grid current with over swing.

The actual element of the 1st stage tube, which has the constant current source function as well, can be a voltage-amplifier pentode, a BJT, or a FET [where BJT Bipolar Junction Transistor, FET = Field Effect Transistor]. This element voltage-amplifies the input signal and converts it to current amplitude. So, from the viewpoint of circuit operation, the first stage amplifier and the circuit of VFT of the V1 STC circuit obviously differ from a conventional SRPP circuit.

1.4.5 Distribution of DC voltage

1st stage tube has high internal resistance (impedance) and low ohmic resistance, so it is easy to realize direct coupling to final stage with one B power supply. Contrary to !st stage, VFT has low internal resistance (impedance) and high ohmic resistance.
The suite of 1st stage tube and VFT realizes multi-function direct coupled signal handling, P-G NFB signal distribution, and DC voltage regulation for final stage with 2 fixed resistor, 1 variable resistor and 1 capacitor.
The simple configuration of STC V1 circuit conribute to the reliability, stability and quality of the amplifier.

1.4.6 Difference from conventional SRPP circuit

As explained in above paragraphs, STC V1 circuit is a "Inverted Mu follower with P-G NFB". It is not equal to SRPP driven direct coupled amplifier with P-G NFB.

1.4.7 Final Stage Power Amplifier

The tube for the final stage may be a pentode, a beam-tetrode, or a pentode/tetrode in triode connection, or a pure triode, and parallel connection of those tubes may be employed too.

1.4.8 As for the output power of STC V1 amplifier

As explained in above paragraphs, by the principle of operation of STC V1 circuit, the voltage component in output power is supressed, so the scaled output power is far less than conventional power amplifier. However, the motion of speaker unit is retained by the current component, and the current component is fully kept by "Gm" based operation, STC V1 amplifier can drive most speaker system with a little power down.

1.4.9 Non overall NFB

It is impossible to avoid phase delay in a signal taken from the secondary of the output transformer. So to utilize such a signal for overall NFB requires that this intrinsic problem be solved. Also, operation into a loudspeaker load, under transient signal conditions, causes NFB problems too, and these are similarly avoided by not using overall NFB. In this sense the V1 STC circuit might be termed a "Neo-Classic" circuit.

1.4.10 Direct Coupling

Direct coupling is also a traditional feature of many amplifier circuits. By direct coupling the signal input to the final stage amplifier, it is possible to avoid the colouration of a coupling capacitor, and so improve transient response. From the STC viewpoint, a 1st stage amplifier having a constant current source characteristic, should not be shunted with any impedance that reduces the effect of the CCS. Removing the coupling capacitor and output tube's grid-leak resistor maintains the sonic improvement of the STC topology.

1.5 Stopping Diode and Linearizer

The addition and operation of these two elements may appear curious or novel to audio amplifier constructors.
Both the SD and LNR are effective in improving fidelity in a listening sense.
This is the first introduction of the Stopping Diode (SD) concept. The measurements which back up the phenomena, are still in the experimental and analytical stage. The Linearizer circuit is however already utilized in flat amplifiers, RF-linear amplifiers and so on.

1.5.1 Stopping Diode (SD)

The SD may potentially be applied in both a voltage amplifier stage and in a power amplifier stage. The following discussion is in regard to the power amplifier application; experiments have not yet revealed any effectiveness of the SD in voltage amplifier applications.

(1) Effect of the SD

Inserting a diode between the B+ terminal of the output transformer and the power supply itself, in the typical STC amplifier, "stiffens" the sound of low frequency music content.
This phenomenon implies the existence of some inter-relationship between this type of amplifier and the loudspeaker system. The SD becomes more effective when a small output transformer is used, rather than a large one.

The effect of the SD is not merely specific to the STC amplifier however. It is also effective in other non-NFB or local NFB amplifiers. In the case of overall loop NFB amplifiers, the effect does not appear at all or is reduced. Supposedly, the reason is that overall loop NFB amplifier acts to amend the shape of signal waveforms passing through, by NFB action.

(2) Supposed Operation of the Diode in an Ideal Power Amplifier

The action of the diode appears similar to the operation of a "diode-switch", which acts as a signal path when forward biased with some DC current.

In an ideal amplifier, the final stage tube acts as a signal source and the output transformer acts as a load. The power supply should ideally have zero internal impedance. These three circuit elements constitute a DC loop, although the power supply itself is not involved with the signal path, as such.

Just the signal source and the load on the DC loop should share the signal power. The DC loop is essential however, to convey the signal power. There will be some impedance mismatching between the signal source and the load. The mismatch means that power is partially "pushed" through the output transformer, with a remaining portion absorbed and dissipated by the signal source, i.e. the final stage tube.

(3) Approach to an Ideal Power Amplifier

The ideal power amplifier would be supplied pure DC power ONLY from the power supply.
The SD acts to make the power supply act as a pure power supply, i.e. never handling or affecting the signal. In another words, the diode causes the signal power to be restricted to the signal power source and the load. By virtue of such assumed operation, the diode is termed a "Stopping Diode".

Further, the B+ power supply should contain enough energy storage capacity to support the amplifier's transient power requirements. It should contain about 5 or more times the "micro F" of capacitance as the nominal B+ current in "mA". For example, an amplifier that requires about 200mA DC at no-signal, should preferably have a capacity of (5 x 200 ) 1,000micro F or more. This requirement for large capacitance yields a power supply with less internal DC resistance; that is to say, one providing close to the performance of an ideal power supply.

(4) SD Effectiveness is Related to Tube Type

Through listening tests, the effectiveness of the SD varies; pentode > (beam) tetrode > triode.
This tendency implies a relation with the internal resistance of each type of power tube.

(5) SD Effectiveness for the Screen Grid

While the screen grid is normally connected to B+, the screen grid current is affected by the input signal. Inserting an SD in series with the screen grid produces a detectable sonic improvement, heard in listening tests. The operation of the SD here is similar to that of a plate circuit SD.

(6) SD Effectiveness for the Push-Pull Amplifier

When an SD is used with a center-tapped output transformer for a push-pull amplifier, inserted between the center-tap and the B+ power supply, the sound of low frequency music content "stiffens". The assumed principle of operation is that the SD pushes back or shuts off unbalanced signal power which appears on the center tap of the output transformer. In some cases trialled, the low frequency content of music signals became excessively boosted. The effect appears to be dependent on the inductance of the output transformer and the extent of the unbalance in the signal components.

(7) SD Effect Generating Elements

As a general rule, all kind of diodes, such as silicon diodes, diode-connected tubes, and diode-connected transistors etc. can be used for the SD. However, any element high in internal resistance, (for example more than 50 Ohms in the dynamic state), is unsuitable as an SD for the plate circuit. The internal resistance limits the output power, and "loosens" the sound of low frequency music content, etc. From this viewpoint, a power silicon diode such as the 1000V, 1A, 1N4007 is most suitable. A paralleled 6CA4/EZ81 or diode connection of the 6AS7G, 6AS7GA, 6080, 5998, 5998A would be considered second. Most general rectifier tubes or damper tubes are not suitable, except where inserted into the screen grid circuit.

1.5.2 The Linearizer

(1) Prior Examples

The lineariser (LNR) is already widely used in various circuits and applications; for example, as the load for a tube amplifier. Some kinds of linear amplifier utilize a tube as a load element in the amplifier circuit. Communication satellites are an example. In the case of a transponder utilizing FDM (Frequency Divided Multi-access) mode, a Linearizer installed in the transponder may be employed to reduce cross modulation distortion.

The operation of a Linearizer is by virtue of a serially connected amplifier element and compensating element. The former element amplifies the input signal and its output includes non-linear distortion. Then the latter (linearizing) element reforms and/or absorbs the distortion by applying the reverse of the original non-linear characteristic. For example, such circuits as the differential amplifier and/or SRPP look like having latent lineariser functions.

(2) LNR Effect Generating Element

It seems to be best to use the same kind of element that is used for the amplifier itself, in a diode-connected configuration. Examples of this type are often found in linear amplifiers. The diode connected element acts to compensate for distortion in the amplified signal by having the same distortion characteristics, and applying them in a reverse, distortion cancelling, manner.
Even if the compensating element differs from the active amplifier element, a similar effect may be expected. So the diode connection of a voltage amplifier triode such as the 12AX7, or a diode tube such as the 6AL5, may be used as an LNR for the first stage amplifier.

1.5.3 Relationship between the SD and the LNR

In an actual audio amplifier, the added diode, whether tube or semiconductor, has characteristics of both the SD and LNR. Most or all such elements have both non-linear characteristics and the diode switch characteristics, at the same time. However, the silicon diode has a steep and linear slope once conduction has started, and so, in this case, while the SD function will act fully, the LNR function will not be so pronounced.

1.6 Variations of the STC Amplifier and Relationship Between the Speaker System

1.6.1 Variations of the STC Amplifier

There are many variations of the STC amplifier.

(1) In the MJ Magazine 1992/10, Mr. Kamijo described an EL86/6CW5 STC SRPP amplifier.

(2) An associate of the author has tried an STC driven, non-NFB, SE amplifier. The author also tried this, in a CV18 direct coupled grounded grid amplifier.

(3) The same associate also tried an STC push-pull amplifier, but it didn't sound so good.

1.6.2 Relationship Between the STC Amplifier and the Speaker System

Various speaker systems were trialled with the STC amplifier, and it appears the STC amplifier is able to drive any kind of speaker system. However, a speaker unit of large M0 (mass of the moving portion), can not be driven fully. This is the same situation as for low powered conventional amplifiers. The STC amplifier is especially capable of outputting large amplitudes in the low frequency portions of music signals, tending to make excessive drive of the woofer much more likely than with the power sharing that occurs, due to increasing internal impedance at low frequencies, in a conventional amplifier. In such a case, inserting a low-cut filter into the input-line is effective.

1.7 Reproducibility and Stability

As the STC amplifier is direct-coupled, one might be concerned about the reproducibility and stability of the circuit. Experimental building and testing of practical STC circuits clearly eliminated such concerns. With regard to reproducibility, when other constructors tried to build the STC amplifier, they succeeded completely, after some Q&A by e-mail. As for stability, around 30 STC amplifier examples have been built by the author; in none of them has voltage imbalance trouble occurred. The circuit is configured to fail safety; should some resistor, for example, fail due to over-heating.


2. Progress of Experiments, Results and Consideration


2.1 Experimental Building

Experimental operation of the STC amplifier has yielded the following results. Experiments started with (beam) tetrodes and pentode power tubes and then some power triodes were tried. Details are described in section "2.3 Application to the (Beam) Tetrode and Pentode" and "2.4 Application to the Triode".

2.2 Evaluation of the Results

Having trialled the application of various types of tube to the STC circuit, and allowing for slight differences in operating point of the VFT and differences due to output transformers, it appears the sonics of the various amplifiers trialled are similar to each other. The characteristics of each individual tube look like being eliminated by high NFB. In another words, in operation the STC circuit utilizes only the Gm "character" of the output tube. Any voltage amplification due to the output tube, given my its "Mu", is reduced to unity by plate to grid NFB (P-G NFB), so any sound character coming through voltage amplification (in the output stage) is eliminated.

Anyway, in listening tests the impression of the audience is typically "Clear and powerful sound", "Delightful sound", and "There is no resistance to making the volume larger" and so on.
As for the overall effectiveness of the STC circuit, it appears (beam) tetrode and pentode power tubes give a very good result, while triode power tubes are somewhat poor.


2.3 Application to the (Beam) Tetrode and the Pentode

2.3.1 1st Stage Amplifier for the Tetrode or Pentode

Generally power tetrode and pentode tubes require only a low bias voltage, and circuit designing is easier than for power triode tubes. With appropriate countermeasures against potential oscillation in the subsequent driving circuit, various combinations of 1st stage amplifier and VFT are easily realized. With direct-coupling from the VFT, the required total gain and distribution of DC voltage are achieved with little difficulty.

(1) Combination of pentode voltage amplifier and CCS, and VFT
or
(2) Combination of BJT/FET voltage amplifier and CCS, and VFT

The first issue to be noted, to impliment these topologies, is as follows. With a pentode for the 1st stage amplifier, the supply DC voltage for the screen grid should be precisely set. If this voltage is too low, correct operation of the 1st stage amplifier, and consequently the whole amplifier, is not assured. For example, while the 6AK5/6AS6 requires only a low voltage - around 30V, correct operation for a 6AU6 requires a minimum of 45V, and so on.

The value of the resistor for the cathode of the final power stage should then be calculated as follows. The cathode voltage (typically approximately 50V~70V) equals the sum of the normal self bias (V) for the final tube, plus "bulk" voltage for the screen grid of the 1st stage tube. The circuit will work even if at higher or lower than this calculated value. However to ensure sufficient signal amplitude from the 1st stage amplifier, the voltage should err on the high side to some extent. Additional voltage is further required to ensure correct operation of the VFT.

Consideration is now given to the "balance" or distribution of the operating voltages for both the 1st stage and the VFT.

In the case of a tetrode or pentode final stage, no trouble occurs as long as sufficient supply voltage is available. In case of a triode final stage, the bias voltage is higher than that for the tetrode or pentode topology, so some manipulation of the distribution of the supply voltage or the "bulk" voltage is required.

In most cases, after construction is finished, testing can be started immediately. Check the cathode voltage of the final tube and adjust the cathode bias voltage of the 1st stage amplifier, using VR, to set the grid voltage of the final stage tube. Detailed adjustment procedures are described in the later section "3.1 Adjustment of the Bias Voltage of the Final Stage".

2.3.2 Example of Operation with a Tetrode or Pentode

"Table1: Examples of Operation of an STC V1 SE Amplifier", shows operation of various tetrode and pentode STC amplifiers. The operating voltages and the other parameters are not precisely adjusted to tube data sheet values. The values actually used were checked through practical listening tests. Although a data sheet shows single values for each parameter, there will exist some deviation from these standards due to the characteristics of each individual tube.

Table 1; Examples of Operation of an STC V1 SE Amplifier
using a Tetrode or Pentode Output Tube
where;
Ep/Esg(V) :voltage between plate and cathode, and screen grid to cathode
Eg1(V) :nominal voltage for reference, between control grid and cathode
Ek(V) :voltage between cathode and ground
p :pentode section in the multi-element tube
t :triode section in the multi-element tube

@STC operation Sample of most pentode/beam (1) Before half
Final tube
1st stage
Voltage
FB tube
Ep/Esg(V)
Eg1(V)
Ek(V)
Ik
(mA)
Rk()
Remarks
6AN52SK68A5965/2 120/120-6.74034 1200
6AQ52SK68A12AX7/2 240/200-12.56030 15606DS5 amp
6AR52SK68A12AX7/2 235/195-186533 15606DS5 amp
6AS52SK68A12AT7/2 110/110-8.54040 1000
6AU5GT 6U8A-p 12AX7/2 220/150 -20.0? 60 45 1320
6AV5GA 6U8A-p 12AX7/2 220/145 -22.5 50 61 820
6AW8A-p 6AS6 12AT7/2 210/165 180 55 14 3660
6BK5 6AK5 12AT7/2 290/280 -5 58 38 1500
6BM8-p No.1 6AK5 6BM8-t 210/210 -16 44 37 1200
6BM8-p No.2 2SC1775A 6BM8-t 220/190 -16 42 34 1200+47** 47 for Tr
6BQ5 6GH8A-p 6GH8A-t 260/230 -7.3 50 42 1200
6CL6/6197 6AS6 12AT7/2 200/150 -2 44 37 1200 6AG7
6CW5 6BX6 12AX7/2 140/140 -12.5 60 68 880
6DQ6A/B 6U8A-p 12AX7/2 240/140 -22.5 51 62 820
6DS5 2SK68A 12AX7/2 250/210 -8.5 50 32 1560
6F6G/GT/42 6GH8A-p 12AX7/2 250/210 -16.5 60 38 390+1200* 46VSG
6G-B7 6BL8-p 12AT7/2 215/120 -22.5 55 50 1100
6GW8-p 6AK5 6GW8-t 245/230 -7 50 42 1200 =ECL86
6HZ8-p 6AS6 12AT7/2 200/150 100 52 30 1680
6K6GT/41 6GH8A-p 12AX7/2 250/170 -18 60 38 390+1200* 46VSG
6L6GB/GC 6BL8-p 6BL8-t 290/270 -14 60 73 820
6LR8-p 6BL8-p 6LR8-t 210/110 -10 60 60 1000
6R-HP3-p 6AK5 6R-HP3-t 175/125 ? 60 32 2000
Remarks*F"xxVSG" is the divided supply voltage for the screen grid of the 1st stage amp.
Remarks**FFor the bias voltage for the BASE of 2SC1775A transistor.

@STC operation Sample of most pentode/beam (2) After half
Final tube
1st stage
Voltage
FB tube
Ep/Esg(V)
Eg1(V)
Ek(V)
Ik
(mA)
Rk()
Remarks
6V6G/GT No.1 6EA8-p 6EA8-t 250/250 -12.5 56 47 1200
6V6G/GT No.2 6GH8A-p 12AX7/2 250/210 -12.5 60 38 390+1200* 46VSG
6Y6G/GT 6BX6 6SL7GT/2 190/135 -14 60 60 1000
12A6 6U8A-p 12AT7/2 225/230 -12.5 74 30 680+1800* 54VSG
12BY7A 6U8A-p 12AX7/2 240/190 -2.6 46 29 1600
30A5 2SK30A-Y 12AU7/2 100/100 -4.7 38 38 1000
38 6U8A-p 12AT7/2 245/250 -25 65 26 680+1800* 48VSG
807/1625 6BL8- 12AT7/2 250/245 -14.5 65 80 820
1619 6U8A-p 12AX7/2 280/240 -10 50 42 1200
6146 6U8A-p 12AX7/2 290/170 ? 60 52 1160 =S2001/A/M
6360 para 6U8A-p 12AT7/2 190/140 -7.5 55 67 820 parallel use
6384 6U8A-p 12AT7/2 235/215 -22.5 65 74 880 =6AR6
6550C/KT88 6U8A-p 12AX7/2 290/270 -14 65 79 820
CV450 6U8A-p 12AX7/2 240/140 -22.5 50 61 820 =6CU6
EL33 6EA8-p 6EA8-t 250/250 -4 50 42 1200
EL34/6CA7 6BL8-p 6BL8-t 290/270 -14 60 73 820
EL509 6U8A-p 12AX7/2 315/185 ? 75 91 820 =6KG6
WE350B 2SK30A-Y 6N7GT 215/180 ? 35 70 500 Mr. Yamada
Remarks1F"xxVSG" is the divided supply voltage for the screen grid of the 1st stage amp.

2.4 Application to Triodes

2.4.1 Variations with Power Triodes

Several issues were revealed when trialling the application of power triodes, or triode connected tetrodes or pentodes, to the STC amplifier. The triode has many variations in performance and characteristics and needs circuit manipulation to some degree, in order to work properly. For example, there is a wide variation in amplification factor (= "Mu") of the power triode.

(a) 6AC5GT "Mu" 58; an exceptionally high "Mu" value. It is driven by a direct coupled UY56/UY76 or 6P5GT.

(b) 6BX7GT "Mu" 10; a typical modern triode.

(c) 6EM7 "Mu" 5.4; a typical TV horizontal driver.

(d) 6AS7G "Mu" 2; an exceptionally low "Mu" voltage regulator tube.

Generally, high "Mu" triodes require low grid bias voltage, and have high internal impedance.

A low "Mu" triode requires a high bias voltage, and has low internal impedance. Exceptionally, some kinds of triode, such as the 6AC5GT, require positive grid bias voltage drive and current mode drive.

In another exceptional instance, some kinds of transmitter tube, such as the 811 and CV18, have two characteristic operational domains. In the minus bias domain, they work as a normal triode, and in the positive bias domain they work with lots of grid current.These kinds of triode require low impedance driving techniques to yield full power operation.

Generally speaking, some countermeasures to allow for variations in individual tube characteristics will be required to operate a triode output tube properly in an STC amplifier. The areas of concern include the following.

(a) Large drive amplitudes are required.

(b) There is a tendency to lack sufficient drive voltage output from the 1st stage amplifier, when the circuit is configured with a pentode and VFT combination.

(c) There is difficulty adopting a direct coupled final stage topology, and operation with its high bias voltage, from a single B+ power supply.

The characteristics of a power triode tube require a multi voltage power supply, or a fixed minus bias power supply for the first stage amplifier, and/or input signal separation with an input transformer, etc. at the same time.

To implement a power triode amplifier, the required amplitude of driving voltage can be roughly estimated by calculation from the bias voltage. The required driving voltage

Vd (peak to peak) bias voltage x square root of 2.

Consider for example, the 2A3 triode, which requires 45V bias voltage. Vd would be around 63V.

It is easily imagined that such a value cannot be realized by a combination of voltage amplifier pentode and VFT or the combination of BJT/FET and VFT. In these cases, to drive the power triode fully, the following countermeasures are required.

(1) A high voltage power supply is required for the pentode and VFT combination.

(1.1) An additional power supply cascaded on the main power supply may be needed for the final stage.

(1.2) Alternately, the cathode and G1 of the voltage amplifier pentode could be taken to a negative power supply. In this case, a signal separation transformer should be inserted into the input signal line.

(2) Utilizing a conventional SRPP, to get enough drive signal amplitude.

The above points notwithstanding, listening tests with a 6EW7 V1 STC amplifier in under-powered condition, showed that enough volume was available, even in an incompletely driven amplifier.

In case a power troiode with pentode and VFT configuration applied, the screen grid voltage for first stage amp. fed from the cathode of final tube, should be "round bulk voltage" i.e. lower than corresponding bias voltage of the power triode as shown in the "Table 2; Example of Operation of a STC V1 SE Amplifier" in section 2.4.4 Example of Triode Operation.
This voltage distribution makes triode-STC operation stable. (2000/5/8)

2.4.2 Resolution by Using a SRPP

The SRPP circuit has enough driving power for driving a typical triode power tube in a non-NFB amplifier. To impliment the circuit, with plate to grid NFB (P-G NFB), the output signal of the SRPP circuit is connected to the grid of the final stage triode, with RC coupling, and the power supply with the NFB signal for the SRPP is supplied from the plate of the final stage triode.
 

With this circuit, the NFB section conforms to the V1 STC circuit. However the constant current source function of the 1st stage amplifier may not qualify. The reason is that the 1st stage amplifier is a rather pure voltage amplifier and has no constant current characteristics. Through experiment with this NFB'ed SRPP circuit, which, like an incomplete STC circuit, looks like having plate to grid NFB applied to some extent, the sound obtained appears a little similar to the sound of a standard STC V1 circuit.

Obviously there will be an accuracy difference in the operation of the circuit, between a pure SRPP directly connected to the B+ power supply and the SRPP used together with plate to grid NFB. It is assumed that the latter circuit would have some current mode driving function to the final stage. This circuit is refered to in this text as the "SRPP drive" to distinguish it from the pure STC, as the operation of the two differs.

2.4.3 Solution by Using the "Power SRPP" Circuit

As the NFB'ed SRPP circuit gives a lack of overall total gain for the amplifier, a FET aided 1st stage amplifier circuit was designed. The new circuit is refered to as the "Power SRPP" circuit.
The drain of a 2SK30A-Y JFET is inserted at the cathode of the 1st stage amplifier (a 12AU7/2 in grounded grid configuration) and configured as a cascode amplifier. By using this circuit enough gain was obtained to overcome the gain reduction due to the plate to grid NFB, and the features of the SRPP were retained. This circuit is classified as a variation circuit of the "SRPP drive", and its static operation is shown in Figure 5.

amput3t.gif
Movement status of Powered SRPP
Case
E0(V)
E1(V)
E2(V)
E3(V)
Ik(mA)
1 1.0 2 62 12 2.3
2 0.5 4 90 10 2.0
3 0.0 10 165 7 1.4
4 -0.5 15 235 4 0.8
5 -1.0 20 295 1 0.2

Figure 5: Schematic and circuit voltages of the "Power SRPP"

This "Power SRPP" circuit was applied to the (paralleled) CV18, and the 1626, 2A3, 6EM7 "SRPP driven" amplifiers.

2.4.4 Example of Triode Operation

The following "Table 2; Example of Operation of a STC V1 SE Amplifier" roughly shows the operation of various triode STC's and "SRPP driven" circuit.

Table 2; Examples of Operation of a STC V1 SE Triode Amplifier

where;
Ep(V); voltage between plate and cathode, and screen grid to cathode
Eg1(V); nominal voltage for reference, between control grid and cathode
Ek(V); voltage between cathode and ground
p; pentode section,  t; triode section,  (T); triode connection
Final tube
Eg1(V)
1st stage
Voltage
FB tube
Ep/Ip
(V/mA)
Ek(V)
Rk()
Remarks
6BQ5(T) -10 19 6U8A-p 12AX7/2 220/38 46 1200
76-6AC5GT** -13.5=76 58# 6U8A-p 12AX7/2 250/36 54 1500 #=6AC5GT's
6G-A4 -19 10 6U8A-p 12AX7/2 250/40/TD> 62 390+1200* 46VSG
6R-A8 -19 9.7 6U8A-p 12AX7/2 235/41 65 390+1200* 46VSG
12B4A -22 6.5 6AS6 12AT7/2 200/22 65 620+2700* 47VSG
6AH4GT -23 8 6U8A-p 12AX7/2 250/24 60 820+1640* 40VSG
EL34(T) -26 11 6U8A-p 12AX7/2 250/50 60 390+820* 40VSG
6EW7*** -40 6 6U8A-p 12AX7/2 200/36 80 1000+1200* 45VSG
*Remarks1F"xxVSG" is the divided supply voltage for the screen grid of the 1st stage amp.
**Remarks2FSynthetic single unit --- regarded/handled as an triode.
+++++++++ Driver tube for 6AC5GT may be 12AU7 parallel, 12BH7A/2, 6350/2 also.
***Remarks3FLarger unit of 6EW7, nearly equal to larger unit of 6EM7 or 5998(A)/2.


3. Adjustment Process and Several Issues

In the following paragraphs, the adjustment procedures and associated issues for tetrodes, pentodes and low-biased triodes, and triodes connected power output tubes, are described.

3.1 Adjustment of the Bias Voltage of the Final Stage

In an STC circuit, the grid of the final stage tube and the cathode of the VFT are directly connected. (If the amplifier has however been configured with RC coupling, this paragraph can be skipped). Initially, when experimenting with an STC amplifier, the voltage of this inter-connection point of the voltage amplifier pentode, (or BJT/FET), and the VFT, will not be particularly high. Generally the point should be set to around 50V to 70V.

It is possible to configure the whole STC amplifier by using just "one" B+ power supply. The adjusting process of the bias of the final stage tube is explained for this set-up. The following steps are required to set up an STC amplifer for proper operation.

(1) Set the bias of the first stage voltage amplifier, pentode or BJT/FET, to a low level initially, using VR2 (variable resistor). In this condition, the first stage amplifier is set to a low internal resistance state. Then the greater portion of the voltage output from the plate of the final stage tube will be carried by the VFT.

(2) In this condition, the grid of the final stage tube is at lower than normal bias voltage. Also the voltage on the screen grid of the first stage amplifier is lower than normal, and operation of the STC will be incorrect and it'll have heavy distortion.

(3) Now slowly increase the resistance of the VR2 in the cathode (emitter/source) of the 1st stage amplifier. The operation of the STC will gradually become normal. In this condition, measure the cathode voltage of the final stage tube. Calculate the cathode current using the fomula;

cathode current (Ik mA) = cathode voltage (Vk Volt) / cathode resistance (Rk Ohms)

Ensure that the operation of the final stage tube is within the tube's data sheet specification limitations.

(4) Then find the most powerful or most favourable operating point of the final tube, within its specification limits.

(5) If the adjustment failed, check and vary the cathode resistors of the VFT and the final stage tube. Find a proper operational point, by repeating steps (1)~(4) above.

In a very few cases this step (5) adjustment was required in the author's early experimental stages. In such cases, the voltage of the cathode of the final stage tube was insufficient. If a BJT/FET is employed as the 1st stage amplifier, this adjustment will be somewhat critical.


3.2 Issues Associated with the Circuit

The bias voltage of the final stage of the STC V1 circuit has to be set by controlling the cathode voltage. That is to say, the output tube's bias voltage is governed by its cathode current. The circuit is also controlled by the plate (collector/drain) current of the 1st stage amplifier. In another words, even if the bias of the final stage is set correctly, it is not then automatically assured that the bias of first stage will be correct. Even if the bias of the 1st stage is set correctly, it is similarly not assured that the bias of the final stage will be correct.

Essentially the operation point of the first stage and the operation point of the final stage should be independent. However, the STC V1 circuit is adjusted by only one variable resistor in the 1st stage, for convenience. So a circuit adjusted for the best point of the final stage doesn't mean the setting of the 1st stage is at the best point.

If the 1st stage can tolerate a wide latitude of operating voltages, then the 1st stage can produce proper operation even though its setup voltages are not precisely suitable.

In most examples of the author's designs, using newly tested tubes, the cathode current of the final stage was initially determined and then the cathode voltage, giving the required value of cathode resistance. After doing this design step, the voltage will, in practice, be within plus or minus 10% of this calculated value. The voltage supplied to the 1st stage may be varied from the best point, as its operation is not as critical as that of the final stage.

In most cases, the STC V1 amplifier can be adjusted to approximately the correct operating point. In a few worst cases, the author had to change and adjust the cathode resistance of the final stage, to give correct operation of the 1st stage.

3.2.1 1st Stage using a Pentode

As described above, (3.2 Issues Associated with the Circuit), adjust the cathode current of the final tube to set it within the plate power dissipation limit and the screen grid power dissipation limits. That's all that is required. Check these points and there will be no problem operating the amplifier. In most cases the author adjusted to the position that gave the most powerful or most favourable sound. In this condition, the bias of the final tube will be near the most suitable position.

3.2.2 1st Stage using a BJT/FET

While fundamentally the same as the pentode case, BJT's and FET's have no screen grid as with the pentode, and the adjustment positions are somewhat critical. In case of a BJT, operation with varied collector voltage seems to be not so critical.

In the case of FET's the author experienced troubles to some extent. Varying the drain operating voltage had some subtle impact on the sound. Depending on the actual type of FET used, the sound quality varies, depending on the drain voltage it's operated at. The latitude of favourable sonics is generally narrow with some kinds and wide with others. In the worst case the cathode resistance was changed to get the optimum "bulk" voltage for the FET.

From this viewpoint, the pentode is most favourable with regard to set-up and adjustment.

3.2.3 Power Maximization

The method of achieving power maximization is the same as for a conventional amplifier. With an oscilloscope, check the amplitude and the wave-shape balance for several operational points at the plate and grid of each stage, using a high amplitude input. Sometimes it is nessesary to change the bias resistor of the final stage or VFT.

However, the clipping will begin from the cut-off side of the final tube. Never set the operational point to exceed the maximum plate passivation power limit, even if the clipping balance is in asymetric. (2000/7/22)


3.3 Consideration for High Bias Tubes

For most tetrode or pentode STC V1 amplifiers, the screen grid power supply for the 1st stage amplifier is supplied from the cathode voltage of the final stage. Generally the plate voltage is lower than the screen voltage, and operation of the 1st stage presents no problems. In case of a final stage in which the bias voltage is low enough (up to approximately -15V), this method is applicable. However in cases of higher bias (approximately over -15V), the gap between screen grid and plate voltage gets larger. Here a lower screen voltage would be preferable.

The way to get a lower voltage is very simple. By dividing the cathode bias resistor of the final stage, an adequate screen grid voltage will be obtained. Then it is possible to realize both high bias levels for the final stage and proper screen grid voltage at the same time.

In Table 1; Example of Operation of an STC V1 SE Amplifier using a Tetrode or Pentode Output Tube and Table2; Example of Operation of an STC V1 SE Triode Amplifier, the divided cathode resistor for the screen grid is applied in several examples.


3.4 The Combination of 1st Stage Amplifier and VFT, and Plate to Cathode NFB

3.4.1 Voltage Distribution - Sound Quality Issues

As described in section 3.2 Issues Associated with the Circuit and paragraph 3.2.3 Power Maximization, the relationship between the total supply voltage and the effect on the sound has not yet been clarified systematically. Neither has the relationship between the kind of 1st stage amplifier and VFT circuit used and the effect on the sound. So the following are merely the author's experience based rules of thumb, for getting favourable sonics. For example suitable supply voltage or the following combinations appears to be:

6AS6 - 12AT; up to around 250V
6U8/6BL8 - 12AT7; ditto
6AK5 - 6AT6 (triode of 6BM8); ditto
6AU6 - any voltage amplifier triode; ditto, but the voltage for the screen grid should be over 45V
Any kind of voltage amplifier pentode - 12AX7; 250V or more

The supply of the 1st tube's screen voltage by the cathode bias of the final tube makes the issue somewhat complicated. However the parts count is reduced by this method. A systematic investigation is required of this area in the future.

3.4.2 Plate to Cathode NFB

To achieve the total required gain in a standard V1 STC amplifier, a parallel capacitor was connected across the bias resistor of the 1st stage voltage amplifier. It appeared that this connection might be killing the effect of current NFB in the 1st stage amplifier. The V3 STC circuit, configured with plate to cathode NFB (P-K NFB) with a non-linear element inserted, having no plate to grid NFB (P-G NFB) factor, has a different beauty in the sound. So then combining the V1 and V3 circuits was tried (ref. Figure 5). Currently the author is tialling and checking several combined V1 and V3 STC amplifiers.

Figure 5; STC V1 Plus V3 Circuit


3.5 Application to Triodes

A tendency was found when trying the STC amplifier with power triodes or triode connected tubes. It felt like the low frequency portion of the sound lacked breakup effects, when comparing to STC amplifiers using a tetrode or pentode output tube. It appears that the difference is due to the difference in current conversion mechanism and the value of amplification factor, "Mu", between triode and tetrode or pentode. The single exception is a 6AC5GT STC amplifier, in which the character of the final tube somewhat resembles that of a BJT, and which generates sound similar to a pentode STC V1 amplifier.


End of this article