TRIODE SPIRIT 300B LUXE - NEW POWER SUPPLY

The famous 300B was always honoured as much as possible on the range of J.C.Verdier's amplifiers.
It is really a difficult exercise to obtain the best possible musicality with a triode amplifier, because all must be perfect around the tube.
Now we introduce a new improvement in the circuits of the regulated power supply.
It concerns a mounting with 3 additive tubes which tracks all remaining hum, for that reason we call it "hum hunting circuit". And more this circuit reduced the internal impedance to quasi nothing.
This improvement adds to the large number of those existent, it leads the Triode Spirit Amplifier 300B Luxe to a degree of musicality never yet reached.

The amplifier Triode Spirit 300B Luxe is fitted with 9 tubes used in the following function :

2 X 300B direct heating power triode
2X 5U4 direct heating power rectifier
2 X 6AS7/6080 twin power triode balast regulator
1 X 6SL7 voltage amplifier
1 X 6SN7 cathode follower/driver
1 X 6SL7 Error amplifier and "hum hunting circuit".

TRIODE SPIRIT 845

The amplifier Triode Spirit 845 luxe is fitted with 9 tubes used in the following fonctions :

1 X 845 Direct heating power triode
2 X 5R4 Direct heating power rectifier
2 X 6550 Tetrode/triode connected balast regulator
2 X 6EJ7/EF184 Error amplifier "hum hunting circuit"
2 X 6BQ5/EL84 Voltage amplifier

845  "LUXE"

TRIODE SPIRIT
THE GREAT FAMILY OF SINGLE ENDED AMPLIFIERS
300B - 845

The attraction carried on by the great audio triodes with direct heating of the filament is growing more and more.

Now, it is not possible to be ignorant of their qualities without being out of the High Fidelity world. Actually , 2 types are readily available covering virtually all requirements. They are the 300B and the 845.

Understand that they were created long time ago for the first talking movies , we must respect them in order to arrive at the musical perfection they are able to provide.
Those triodes being tyrannical with the technicians who are faced in designing an amplifier. There should be no compromise . The design around the tube must be perfect or some default will emerge and deteriorate the listening result, mainly by hum, and /or a deficiency of the bass and treble .
If the cost needs to be lowered it is easier to use the symmetrical mounting ( Push Pull ) which does not require the same care.

Now, let us take a close look at the obstacle we must clear to obtain perfection ( yes it is possible ).

First of all the hum

Generally they come from the power supply circuits. Each model of the TRIODE SPIRIT line has a regulated power supply

It provides a constant voltage in all circumstances on all the amplifier stages. The regulated power supply, expensive to realize for the 300B models becomes a real technical challenge for the 845, as it needs a plate voltage of 820 Volts.

This power supply is in addition intricate to design ( see the article in Sound Practices issue 14 ) and it needs the use of 3 tubes ( in the 300B amplifiers) or 4 tubes ( in the 845 amplifier ) to work properly.

Response in low frequencies

A detractor of Mono Triode amplifiers will say : " Yes it's musical but there is no bass. This comment might be true if the amplifier is made to save money with a cheaply built output transformer .

The size and consequently the weight of the transformer is decisive because it conditions the value of the primary inductance which must be as high as possible. ( See the technical note on this subject ) The output transformer as a vital part must according be designed at its best , taking in account the finest technology, while respecting the rules of art.

Another challenge, the treble response.

Generally, the major problem comes from the dynamic input capacitance ( about 50 to 100pF ) of the power triode. This requires a preamplifier stage with low output impedance - practically a power stage - able to provide current in high frequencies . In a sort forcing the output triode to accept the modulation against its will.

This feature is valuable because it passes on an ultimate profit: it does not change the original sound of the output triode by an
inopportune compensation effect. This feature does not alter the special sound of each triode and allows - possibly for the first time - to make auditory comparisons really significant between tubes of different origin.

For this last reason, we named this line of amplifiers "TRIODE SPIRIT" hopping to show the devoted care, the quasi religious character of our reverence for the great triodes of the past.

TECHNICAL NOTE
THE CURIOUS DEVELOPMENT OF SINGLE ENDED POWER STAGE

At the origins of low frequency tube amplifiers, the mounting of a single tube was reserved to devices using simple and cheap materials .
By opposition the symmetrical mounting was meaning of perfection and therefore found only on performing and expensive equipment. It is interesting to note that this tendency is now quite opposite. This surprising result has various reasons , two of them are described below.

1 - Poetical reasons
During the 70's Japanese enthusiasts - passionate of music and technique - have discovered the qualities of an old "WESTERN ELECTRIC" apparatus. This talking picture amplifier designed for small movie theaters was fitted in its output stage with a single tube 300B (WE 91A).
The amplifier was not perfect but gave surprising musical results. The dynamics coming from this amplifier seems more potent than its modest rated power let suppose. Working on the matter by improving the power supply to reduce the " hum "and using a bigger output transformer, our Japanese enthusiasts gave birth to a new range of High Fidelity single tube amplifier. We have to recognize that this amazing result was ,it seems,a real snook to the technical world.

2 - Technical reasons
As we know, technical explanations given their value to the musical results have never been approached .
For this reason, I ask the reader to take my conclusions only as assumptions.
First, we have to evoke a specific aspect: The dynamics . Let us suppose that a musical ( sinuso‹dal) signal modulates the grid of a tube in a positive direction: the plate current will increase and become maximum. This current flowing in the primary of an output transformer creates a maximum instant voltage at the secondary corresponding to a maximum instant power.
When the signal modulates the grid of a tube in a negative direction: the plate current will decrease and arrive to break when the grid is at the cut-off point, the current inside the primary of the output transformer ,then ,becomes zero. At the secondary we recover a maximum instantaneous power, there is no more current.

Where is this power coming from?

Of course there is the Lenz law which " works " by providing a magnetic field opposed to the disappearance of the current. This natural effect works well as much as the primary inductance of the output transformer is enough to store the energy; and there is the fundamental importance of the design of a generously sized output transformer if we wish to have a good reproduction of the low frequencies.

The energy stored in the primary is:

For example in a 300B stage, the idle current is generally 75mA when a good transformer has a 20 Henrys primary inductance.
In this case :

Let us calculate the distribution of this power according to the frequency we have to reproduce:
At 20Hz the time necessary for the inductance to give back energy is 0,025 seconds.
During this time the inductance is able to give back a power of :

It is not much and explains the fall of the power in the bass extremities.
At 100Hz the time of return is 0,005 seconds. The corresponding available power is:

This power is widely appropriate and the respone at 100Hz is practically perfect .
The same calculation for the frequency 1000Hz reveals now why Mono Triodes amplifiers have a musical power reserve so amazing. At 1000Hz the time of restitution is 0,005 seconds that gives a disposable power of:

Here we ought to compare with what happens in a push pull amplifier, it is easy because of their principle . The electricity stocked in the primary of the output transformer is always non-existent, and therefore the profitable phenomenon described above is not present.

In the future, I will comment about the reserves written at the beginning of this technical note. My assumptions are not yet confirmed by well-know scientific studies and I am waiting for the confirmation of a mathematician .

 I added to my "Triode Spirit" amplifiers range a "luxe" version of the 845 model. The great success of that presentation, more attractive, in spite of its high price, leaded me to apply the same formula to my other amplifiers of the "'Triode Spirit" range :

Please notice that the models are equiped with the same important transformers and  are made on a brushed anodic and aluminium chassis that is Champagne colored of 10 mm thick.

Moreover, all the technical aspects of the models are still the same and musical performances have not been modified by these aesthetic improvements.