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Why 6C33C ? Here is an article by Ari Polisois 2009.
There, you will find more information on how the two novel types of OPTs, the Self-compensated ( SC ) and the Split Core - Stereo Common Circuit ( SC-SCC ), work and what their limits, as well as advantages, are. Kindly note that the concepts, in the present paper, refer mainly to the Single Ended amplifiers’ layout.
The Queen of these valves is, by far, the 6C33C-B, the Russian tough and generous triode whose internal resistance can go as low as 80 ( eighty ) ohms.
But in the western production range, there are also some remarkable types, such as the 6AS7, 6336, 6080, and several others, all designed to be used in voltage or current stabilizing applications. The average internal resistance of these valves is 300 ohms per section, but all of them are double triodes, so the resulting resistance, when paralleled, is one half, that is 150 ohms, which is not bad.
Why do I consider this kind very precious, is what I will explain in the text.
When I retired, after many years spent as an export executive, I dedicated my free time to my preferred hobby, namely audio-electronics, starting with a brush-up of my basic knowledge. Morgan Jones and Menno van der Veen’s books ( respectively “Valve amplifiers” and “Valve amplifiers from 10 to 100 Watts” ) were a fundamental step.
From them and some other sources, I became aware of the importance of the frequency range in the reproduced sound. Menno has dedicated many years of his life to extend the frequency range of his wonderful output transformers and he convinced me in an absolute way.
Why is it important to preserve the frequency range with its rich bouquet of harmonics?
This requires many efforts and a strong will, focused to refuse any avoidable compromise. It also means that we do not have to add anything new to the original sound ( relatively original because it is just what we get from a CD and a CD player or a vinyle and its associated peripherals ).
Is the result worth the effort ? Definitely yes .
As far as we are concerned ( valve amplifiers’ designers and builders ) it is a must not to deteriorate further the quality of the source‘s output, if we are really aiming at the best possible sound.
In order to achieve this, we must first know where and how the frequency range is affected.
Generally speaking, leakage inductance, low power-handling ability of the output transformer and stray capacitances are responsible of bending the -0 dB straight line that should go, unaffected, from few hertz to several kilohertz.
I will start with the latter cause.
Stray capacitances and their effects on the hi frequency side of the audio range.
Where do these stray capacitances build up ? Everywhere, from the input to the output, including the RCA jacks, the connecting wires and the output transformer.
Any path that shifts the alternate current signal to ground restricts its amplitude. If it is a capacitance, it affects particularly the higher frequencies. . The same result appears if the signal is “trapped” by a non productive inductance.
The major capacitances are found between :-
Winding and winding - Winding and core - Layers - Turns.
The capacitances that act in parallel and the leakage inductance in series are both harmful.
Figure 1 shows that a capacitor can be assimilated to a load, whose resistance is part of a voltage divider. The value of this resistance changes with the frequency, as per the formula Zc = 2πfC , ( Fo )
where f is the frequency in Hertz, C is the capacity in Farads and Zc the reactance ( or ac resistance ) expressed in ohms.
In the case of the presence of several capacitors, these can be strictly in parallel or separated by one or more elements, each with its own resistance or reactance. If all capacitances are strictly in parallel, the effect is more pronounced (see Fig.2)
What to say ? If one can succeed to reach the second goal, why shouldn’t he?
A telephone voice or music, whose frequency range is kept below 3 kHz, is far from conveying the same impressions as a hi-end wide frequency range amplifier, such as those used by singers and orchestras.
In addition to the stray capacitances, another factor deteriorates significantly the high frequency range extension : the Leakage inductance of the output transformer. This subject will be dealt with later in the text, in the chapter dedicated to the OPT’s construction.
The low frequencies contribution.
If a loss of bass occurs, the sound is “ill” and the overall wave appears to have lost a lot of energy .
The presence of a consistent bass range basement supports the soundstage and keeps the listener’s attention focused.
The bass contributes to generate satisfaction, whilst the mid range and treble notes define the scene.
Most of the power generated by the output valves is used by the low frequencies. These require heavier magnetic core transformers, heavier loudspeakers and so on.
The sound traffic is composed by different stations and vehicles, that operate and behave in different ways. What matters is to let them transfer to destination every item, without delays or losses.
Possible accidents are caused by the capacitors, that discriminate between high and low frequencies as well as by the most critical component of an amplifier : the output transformer. All the other components have, of course, their specific importance, but their behavior is much more predictable than the OPT’s one.
The OPT’s task must be backed by the support of suitable output valves, as everybody knows.
The interpretation of this statement is very wide. In the present paper, I will concentrate on few important aspects.
Matching the valves to the output transformer.
It would have been more accurate to say: matching the output transformer to the valves.
It all depends of what you want to achieve.
We have already set a complex goal : very low frequency response and very extended high frequency range.
The valves are what they are, with their average characteristics and tolerances well known, so we rather concentrate our efforts on the OPT.
1. We know that we must reduce the stray capacitance to a minimum, in this critical component.
Here are the requirements to minimize the capacitance:-
1.1.1. - increase the dielectric thickness ( that we will designate with the letter “d” )
1.1.2. - reduce the winding width ( letter “a” ) 
1.1.3. - increase the number of layers ( xL1...2...3 etc.)
1.1.4. - provide a large potential difference between the windings
1.1.5. - do not bifilar wind
1.1.6. - it is suggested to use a Faraday shield ( electrostatic) , but I do not understand why. Kindly refer to Fig. 3 , for the stated references.
The parts of the windings that are not intimately coupled together generate a leakage inductance. This includes a bad winding geometry.
Considering the primary of an audio transformer, the flux set up by it, which, for some reason, does not link its secondary(ies), produces a leakage inductance, due to the corresponding loss of mutual coupling. How to reduce the leakage inductance ?
Here below the most common ways :-
2.1.1 - Keep the number of turns to a minimum
2.1.2 - Minimize the build of the coil
2.1.3 - Increase the width of the winding
2.1.4 - Reduce the insulation thickness between the windings
2.1.5 - Adopt twin-wire winding ( bifilar ).
2.1.6 - Interleave the secondary windings with the primary ones.
At this point, we observe some contradictory advices concerning the ways to minimize the leakage inductance and stray capacitance. The situation is shown in the following table:-
- reduce build of coil, including depth
- increase the winding width
- minimize insulation thickness between windings - use bifilar winding
- the opposite (increase the nbr of layers) - reduce the winding width
- the opposite
- the opposite
In order to understand what to do best, we should examine the formula that calculates the lowest possible bass response at -3dB .
f-3L= (Za*Kb)/2*π*Lp (F1)
Finally, Lp is the inductance of the primary, in Henry.
The formula to calculate the inductance is :-
Where N is the number of turns of the primary winding, Ac is the area of the core in square meters,
Lm is the length of the magnetic path of the iron core. (meters) and μr is the relative permeability of the core. Regarding the latter parameter, we will consider a low value that represents the departure for a higher permeability taking place at stronger magnetic fields’ levels. The figure chosen is 500, and this is good enough for our consideration.
To express a value that corresponds to a real example, besides μr = 500 , we will take a transformer with an area core of 10 sq. cm ( 0,001 sq.m in the formula) and a magnetic length of 0,4 meters.
If we choose to have a primary of 1000 turns, the inductance will be :-
However, our analysis will be limited to just some of the parameters that we listed in the former paragraphs.
I want to underline how important the internal resistance of a valve is to match most of the requirements set forth above, in order to obtain a significantly extended frequency range, both at the low frequency and at the high frequency ends.
I selected 5 popular valves having different characteristics, as far as the anode resistances and the suggested loads are concerned.
The following table shows their basic characteristics on which we will focus.
EL34 (triode mode) EL84 (triode mode)
(<>) this factor will be used for the following calculations.
Lp = (Za*Kb) / (6,28*f-3L) ( F4 )
You must have noticed the big differences in the primary’s inductances required to reach the low frequency of 20 Hz, according to the internal resistances of the valves, listed in the preceding table.
The second gives the primary’s number of turns if you want to fix your -3dB limit at 10 Hz .
Note : To obtain the number of turns, I used the formula
Now, going back to the former chapters, we re-discover that, with a reduced number of primary turns, we can satisfy many of the conditions to reach a higher frequency response, still getting a lot of bass.