The "on board" version of the SE amplifier using a 6S33S tube

Grzegorz "gsmok" Makarewicz, This email address is being protected from spambots. You need JavaScript enabled to view it. (slightly completed description from 2006)

For a long time I planned to make an amplifier on the famous "little devil" - the 6S33S tube. I have heard many contradictory opinions about this tube. The main disadvantage is the instability of its parameters over time. Changing the value of the anode current during the work of an unheated vacuum tube is a constant topic of almost every discussion forum dedicated to vacuum tubes. With all its chimericity, this vacuum tube mysteriously attracts designers. Why? I do not know. Probably everyone who did something on it had their own reasons. As for me, there are several reasons. I will mention one - this tube looks great, and these three devilish horns - great !!!

Some time ago I heard from a friend that his friend saw in the living room of his friend a simple amplifier on a 6S33S tube. Apparently it sounded great. This friend of mine decided to make himself the same amplifier and for this purpose acquired the schematic of the amplifier circuit. The schematic diagram lasted two years, until it fell into my hands. There are no revelations in the amplifier's system design. There are many such schemes on the Internet. However, this is not a disadvantage for someone who would like to build an amplifier according to a proven electronic system.

Slightly under the pressure of a friend already mentioned twice, I decided to listen to the amplifier on the 6S33S tube in person. It took me some time to gather the necessary elements, but finally I started to work.

I begin this description from the moment I was able to start such an amplifier made in the form of a prototype on a piece of plywood. So there is a chance that it won't end with a description of the struggle with the amplifier, but a good one to present its final version suitable for putting on the shelf.

As the construction progresses, the description will be successively updated. I will also modify the descriptions already provided. For this reason, I recommend from time to time those interested not only to familiarize themselves with new fragments of the text but also to check whether significant changes have been made to previous texts.

Everyone interested in the description is invited to the Triode Forum. I initiated a thread there regarding this construction. It is available at:

General notes about the amplifier design

I decided to assemble the prototype of the amplifier on plywood. Each amplifier channel needs two separate bases - one for the power supply and one for the amplifier itself. Together, this gives a considerable number of four plywood stands. Although this is only a prototype, I decided to assemble it as neatly as possible. First of all, it will be used for a lot of research and experiments, secondly, the amplifier system has high voltages and should be assembled in the right way for safety purposes. Details on the implementation of mounting bases are illustrated below with relevant photographs.

You can choose wood or plywood as the material for the mounting base. I chose plywood, because with smaller thickness (about 10mm) it provides better rigidity. This is not insignificant considering the fact that really heavy elements will be screwed to it. So that the base can be easily moved around the table, I decided to equip it with wheels.

I used the cheapest wheels available in "Praktiker". They are small but solid. Their only drawback is the fact that they protrude from the side from which they are screwed to the base. Therefore, they cannot be attached directly to the plywood, as this would inhibit them and would not fulfill their function.

But what are the tools for. Using a visible mini drill equipped with a small roller with sandpaper, I "milled" holes for wheels.

This is the fixed wheel. Fastening is done by screwing with two wood screws. Here you can see the advantage of easily attaching various elements to the base. If you make a mistake in setting the element, just move it and screw the screws in a different place. Quick and easy.

And here is the whole base ready to mount the amplifier or power supply.

Now it's time to prepare a way of fixing electronic components. Commonly available terminal blocks can be used for this purpose. I chose ring terminals and terminal blocks matching the so-called car plugs.

The advantage of connectors and the method of mounting using screws is that, depending on your needs, you can make soldering points with as many loops as we need at present.

It is also possible to connect different types of terminals in one point by screwing. There are several examples in the photo.

Power Supply

The schematic diagram of the amplifier power supply is shown in the figure. As a maniac of electron tubes I regret to admit that semiconductor components are used in the power supply.

Schematic diagram of the power supply

As you can see, the power supply is the main component of the amplifier in terms of the number of components used. Apart from functional considerations, it can even be said that the amplifier is only a small addition to the extended power supply. But seriously now. Although it is a prototype system, it is worth spending a little effort so that the elements used to connect the power supply to the mains voltage are fixed reliably and securely. This applies to the socket, fuse and switch. I really can't fix these elements with the 'spider' method. The ability to quickly turn off the amplifier, without the nervous search, where is the switch is not to be overestimated.

The socket for connecting the power cable with the integrated fuse [3] and the power switch [2] will be mounted on a special aluminum bracket [1]. I used a 1mm thick sheet with properly cut holes.

Bend the metal sheet (e.g. in a vice) in such a way that a bracket is created that allows it to be screwed to the base.

And this is what the finished bracket looks like with a wall outlet with fuse and power switch.

I started the proper assembly of the power supply with the input elements of the power supply, i.e. the transformer and its 'environment'. These elements are shown in the schematic diagram below.

Transformer and protective elements

I used a toroidal transformer in the power supply, which I simply screwed with a large wood screw. I used standard (supplied with the transformer) fixing elements in the form of rubber pads and metal plates pressing the transformer to the base.

And now the so-called 'soft start' system. Although the toroidal transformer has a relatively small power, but considering that when using the power supply system it will be often turned on and off, I decided to add this system.

Time for the next item. I screwed to the base the previously described bracket with a socket, a fuse and a power switch.

Here is a view of the transformer and its associated components from the side from which the soft-start system leads can be seen. Here you can also see how firmly the bracket with the switch and the socket are screwed (three screws with wide heads)

Time to wire the attached components. After connecting the power switch and the soft start system with the transformer, I also connected all the transformer secondary windings to the solder lugs. Because it is not known how long the winding leads will be needed in the future, I did not cut them to size but I bent it in a loop (hence the tastefully bent colorful leads).

After completing this stage of wiring, I carried out control measurements of the secondary winding voltages (numbering according to the schematic diagram):

  • Winding I: 8.65V
  • Winding II: 13.3V
  • Winding III: 82V
  • Winding IV: 335V
  • Winding V: 196V.

Here are the wiring details for the power outlet, switch, and soft start system. I used a connector for pins, which are additionally protected by insulating covers. In this way, it is not possible to accidentally touch a live wire with your hand.
Security above all !!!

Here is a photo showing enlarged how the secondary winding wires of the power transformer have been soldered.

Now it's time for the last heavy element of the power supply - the choke [DL1]. In this way, on the assembly base, I already have all parts that require other time-consuming assembly operations than just soldering before mounting.

At this point I have to justify myself in relation to professionals with a lot of practice, who using chassis mounting recommend screwing heavy elements at the very end. In this case, all other elements will be mounted from the top of the base (not from the bottom as it is on a traditional chassis) and additionally my base has wheels.

Before installing individual power supply sections, I installed all rectifier bridges. They are marked according to the schematic diagram as M1, M2, M3 and M4.

The voltages measured at the bridge outputs (without load and without filtering capacities) are:

  • M1: 10.3V
  • M2: 73V
  • M3: 316V
  • M4: 186V.

The photo above shows the details of the M1 and M2 bridge assembly.

And here are shown the diodes forming the M3 and M4 bridges.

Time to install individual PSU sections. We start with a DC power supply designed to power the filament of the tube of the amplifier input stage. The schematic diagram of the power supply is shown in the figure below.

Schematic diagram of the 6N1P filament power supply

List of elements:

  • M1 - KBL04 bridge
  • U1 - type 7806 stabilizer
  • C1 - 4700µF / 16V
  • C2 - 1µF / 63V
  • C3 - 4700µF / 16V
  • C4 - 1µF / 63V

The photo shows all the elements of the power supply generating U1 filament voltage at the output. Designations of elements are in accordance with the presented schematic diagram.

Measurement of no-load voltages has shown that the voltage across the capacitor C1 is 10.3V, while the voltage U1 is 6.08V.

Turn for the next section, i.e. the filament voltage for the 6S33S power tube. The filament voltage is taken directly from the winding number II of the power transformer. It was connected to the U2 output socket with a twisted double insulated wire. In order for the cable to not get stuck on the plywood, it was glued to the base at two points [P1] and [P2] with an adhesive gun.

The third section of the power supply is the voltage used for the initial negative polarization of the 6S33S tube grid. The power supply diagram is shown in the figure below.

Diagram of a negative voltage power supply system for 6S33S electron tube grid polarization

List of elements:

  • M2 - any bridge with a voltage of 200V / 1A
  • T1 - BD244
  • DZ1 - Zener diode for 90V
  • R1 - 4K7
  • R2 - 4K7
  • R3 - 4K7
  • PR1 - 4K7
  • C5 - 100µF / 160V
  • C6 - 10µF / 160V
  • C7 - 10µF / 100V
  • C8 - 1µF / 100V

Here's how the power supply was assembled. Individual elements have been marked in the same way as on the schematic diagram.

Time to take a breath. This is the base on which we already have three voltages (U1, U2 and U3) necessary to power the amplifier circuit.


The fourth section of the power supply is a system that provides anode voltage to the 6N1P tube. The anode power supply diagram is shown in the figure below.

Schematic diagram of the 6N1P anode power supply

List of elements:

  • M3 - four BY55 diodes shunted by 200pF capacitors
  • T2 - MJE13005
  • D1 - 1N4007
  • DZ2 ... DZ5 - Zener diode 100V / 1.3W
  • R4 - 20 / 5W
  • R5 - 10K / 2W
  • R6 - 4K7
  • C9 - 220µF / 500V
  • C10 - 100nF / 1000V
  • C11 - 220µF / 500V
  • C12 - 100nF / 1000V
  • C13 - 22µF / 450V
  • C14 ... C18 - 100pF

This is the power supply assembled on the base. The designations of the elements are in accordance with the schematic diagram.

The C9 and C11 electrolytic capacitors were glued to the base with an adhesive gun. The next photo shows more assembly details.

The fifth and last section of the power supply is the system supplying the anode voltage of the 6S33S tube. The schematic diagram of the power supply is shown in the figure below.

Schematic diagram of the 6S33S anode power supply

List of elements:

  • M3 - four BY55 diodes shunted by 200pF capacitors
  • Dl1 - choke 10H / 300mA
  • C19 - 680µF / 350V
  • C20 - 0.1µF / 1000V
  • C21 - 680µF / 350V
  • C22 - 0.1µF / 1000V

This is the 6S33S anode power supply. Capacitors C19 and C21 are composed of several (4 for each capacity) electrolytes connected in parallel.

After starting all sections of the power supply, connect their grounds to one common point. This will allow you to easily connect the ground point of the entire power supply with a single ground point of the amplifier. I advise against using many combined masses at various points (this approach often leads to hum problems). The method of connecting the ground of the PSU section is shown in the photo. The masses are carried out with a cable in a green insulation jacket.

After tests with the amplifier, I made three small changes to the described PSU design. They are not necessary, but for a full description I will briefly present them.

Modification 1
Because I used a rather small heat sink to reduce the power loss on the heater voltage stabilizer of the 6N1P-EW electron tube between the rectifier bridge and the stabilizer, I put a resistor of 1 ohm - in the photo it is marked with the symbol R

Modification 2
In the Triode discussion forum there was a thread about the positive (from a safety point of view) role of a shunt resistor electrolytic capacitors in high voltage power supplies. I knew it, but I needed a severe 'kick' to appreciate the fact that voltage across capacitors can be dangerous even after a long time after turning off the power supply. So I shunted the 6N1P-EW anode voltage power supply capacitors with a 200K resistor (element marked as R)

Modification 3
For the same reasons, I bypassed the electrolytic capacitors in the 6S33S-W anode voltage power supply (100K resistor R).
I did not use any additional shunting in the power supply of the negative voltage of the tube grid because it plays such a role sufficiently effectively: the PR1 mounting potentiometer and the R3 resistor shown earlier in the schematic diagram of this power section.


The schematic diagram of the amplifier is shown in the figure. It is a 'minimalist' construction containing only two tubes.

Schematic diagram of the amplifier

Values of used elements. During the amplifier's research, some values have changed and this is still the case today. These are not big differences, but I would like the person building such an amplifier to be aware of this. I try to make the values presented in the table below as current as possible.

For higher power resistors, I provided in brackets [] the approximate voltage value that is deposited on their terminals. This allows you to easily determine the required power.

Symbol Value Symbol Value
C1 0,1uF/1000V (MKP) R4 470KΩ
C2 0,1uF/1000V (MKP) R5 560Ω
C3 0,1uF/1000V (MKP) R6 28KΩ (2x56KΩ parallel) [155V]
C4 0,1uF/1000V (MKP) R7 1KΩ
C5 100uF/400V (elekctr.) R8 130KΩ
C6 0,1uF/1000V (MKP) R9 220KΩ
C7 100uF/400V (electr.) R10 not necessary
R1 470KΩ R11 110K [130V]
R2 2K2 RP 1Ω/5W non-inductive
R3 100KΩ [120V] P1 10KΩ (potentiometer)

Installation of the amplifier is best to start by planning and attaching the largest components. In this case it is a speaker transformer [TG1], a socket for the 6S33S [V2] tube and a socket for the 6N1P [V1] tube. In addition (as you can see in the photo below) I mounted the input socket [WE], output sockets [WY], resistor [Rp] used to measure the anode current of the 6S33S tube, and solder terminals, to which the amplifier supply voltage will be connected [U1] , [U2], [U3], [U4] and [U5]


This is the base of the amplifier from the bottom after mounting the sockets, transformer and tube sockets. Four wheels are also visible, enabling it to be moved over the surface without lifting.

In the following photos I presented the details of mounting the previously mentioned elements.
Next to it you can see the input socket [WE] and the tube base [V1]

The tube socket [V2] is attached to such aluminum spacers.

Here's how the input sockets [WE] and the speaker transformer [TG1] are screwed to the base

After checking the correctness of mounting the base [V2], I unscrewed it and soldered all the elements coming to the 6S33S tube socket's terminals. They are:
[A] - anode voltage lead
[Ż] - [Ż] - filament wires
[K] - cathode connection cable
[R7] - mesh resistor

After soldering the elements to the terminals of the 6S33S tube socket, I screwed it back on. Now the assembly of the other components cooperating with the tube (e.g. connection to the cathode of the Rp measuring resistor) is possible even though the base is screwed on.

I used a similar procedure for the 6N1P tube. Before screwing to a wooden plate, I soldered all elements and connecting cables to the feet of the noval socket. The photo shows the bottom view. The designations of the elements are in accordance with the schematic diagram.

After screwing the sockets, I started the final wiring of the amplifier. The photo (below) presents all the details necessary for its implementation. in addition to the elements mounted on the tube sockets, the elements of the anode power supply filter of the input stage of the amplifier (C4, C5, C6, C7 and R11) are placed on the plywood. The global negative feedback resistor (R9) is not connected to the secondary winding of the speaker transformer. This operation should be performed when starting the amplifier.


And this is how the whole amplifier looks before inserting the tubes into the sockets ...

... and after installing the tubes.

Before starting the amplifier, it remains only to connect it to the power supply. This is how the completely assembled one channel (power supply on the left, amplifier on the right) looks.

Sample measurement results

Here I gradually put the results of the amplifier parameters measurements. For now, these are only results. I will try to add descriptions and opinions to them after playing with measurements.

Amplitude and phase characteristics in the frequency range 0-25.6kHz. Readings at 20kHz. (system without correction and feedback, input signal - 10mV white noise, anode current of the 6S33S tube Ia = 155mA)

Amplitude and phase characteristics in the 0-50 Hz frequency range. Readings for 13 Hz. (system without correction and feedback, input signal - 10mV white noise, anode current of the 6S33S tube Ia = 155mA)

Nyquist chart (system without correction and feedback, input signal - 10mV white noise, 6S33S tube anode current Ia = 155mA)

Signal at input (upper graph) and output (lower graph), (system without correction and feedback, sinusoidal input signal 1kHz, P1 shorted / approx. 0K, anode current of the 6S33S tube Ia = 155mA)

Signal at input (upper graph) and output (lower graph), (system without correction and feedback, sinusoidal input signal 1kHz, P1 about 10K, anode current of the 6S33S tube Ia = 155mA)

Harmonic distortion (system without correction and feedback, sinusoidal input signal 1kHz 10mV, output signal 140mV, P1 about 5K, anode current of the tube 6S33S Ia = 155mA)

Harmonic distortion (system without correction and feedback, sinusoidal input signal 1kHz 62mV, output signal 700mV, P1 about 5K, anode current of the tube 6S33S Ia = 155mA)

Harmonic distortion (system without correction and feedback, sinusoidal input signal 1kHz 128mV, output signal 1.41V, P1 about 5K, anode current of the tube 6S33S Ia = 155mA)

Harmonic distortion. 1kHz input signal

Prototype on a steel chassis


Text elaboration: Grzegorz "gsmok" Makarewicz,