SE amplifier on 6S33S tube - plywood construction

Details: Written by: Grzegorz Makarewicz; Category: Tube equipment; Last Updated: 04 August 2026; Hits: 1901

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

For a long time I have been thinking about making an amplifier using the famous soviet military electron tube 6S33S commonly called the "little devil". I have heard many contradictory opinions about this tube. The main disadvantage is the instability of its parameters over time. The floating of the anode current during the operation of an "unrun-in" tube is a constant topic on almost every discussion forum devoted to electron tubes. Despite its capriciousness, this tube mysteriously attracts designers. Why? I don't know. Probably everyone who has tinkered with something on it had their own reasons. As for me, there are several reasons. I will mention one - this tube looks great, and those devil horns on a glass bulb - awesome!!!

Some time ago I heard from a friend that his friend had seen a simple amplifier on a 6S33S tube at his friend's. Apparently it sounded great. My friend decided to make himself the same amplifier and for this purpose obtained a schematic of the amplifier circuit. The schematic was lying around for two years until it fell into my hands. There are no revelations in the circuit design of the amplifier. You can find many such schematics on the Internet. However, in my opinion this is not a disadvantage for someone who would like to build an amplifier according to a proven circuit.

A bit under pressure from my friend, who I mentioned twice, I decided to listen to the 6S33S amplifier in person. To do this, I had to build such an amplifier in some form. It took me some time to collect the necessary elements, but I finally started working.

I am starting this description from the moment when I managed to start up such an amplifier made in the form of a prototype on a piece of plywood. So there is a chance that it will not end with a description of the struggles with the amplifier, but I will get to presenting its final version, suitable for putting on the shelf.

As the construction progresses, the description will be gradually supplemented. I will also modify the materials already made available. For this reason, I recommend that interested people from time to time read not only new fragments of the text but also check whether any significant changes have been made to the previous texts.

General notes on construction

I decided to assemble the prototype circuit on plywood. Each amplifier channel needs two separate bases - one for the power supply and the other for the amplifier. Together, this gives a considerable number of four plywood bases. Although it is only a prototype, I decided to assemble it as properly as possible. Firstly, it is to serve me for many tests and experiments, secondly, we are dealing with high voltages in the circuit and for safety purposes it would not be a problem to assemble it properly. I have illustrated the details regarding the construction of the mounting bases with appropriate photographs below.

As a material for the mounting base, you can choose wood or plywood. I chose plywood because with a smaller thickness (about 10 mm) it provides better rigidity. This is not without significance considering the fact that really heavy elements will be screwed to it. In order to easily move the base around the table, I decided to equip it with wheels.

I used the cheapest wheels available in the "Praktiker" chain of stores. They are small but solid. Their only drawback is that they stick out from the side from which they are screwed to the base. They cannot therefore be attached directly to the plywood, because it would slow them down and they would not fulfill their function.

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

This is what one attached wheel looks like. It is attached by screwing in two wood screws. Here we see for the first time the advantage of easy attachment of various elements to the base. If you make a mistake in positioning an element, simply move it and screw in the screws in a different place. Quick and easy.

And here is the entire base ready for mounting the amplifier/power supply.

Now it's time to prepare the technology for attaching electronic components. For this purpose, you can use generally available connectors. I chose eyelet connectors and connectors that fit so-called car plugs.

The advantage of the connectors and the adopted method of assembly using screws is that, depending on the needs, it is possible to make soldering points containing as many eyelets as we currently need.

You can also connect different types of terminals within one point by screwing in one screw. The photos show several examples.

Amplifier power supply

The schematic diagram of the amplifier power supply is shown in the figure. A version enlarged to a size that allows for free analysis of the circuit can be opened in a separate browser window by tapping the mouse on the surface of the reduced version of the diagram. I regret to admit that the power supply of the tube circuit also uses semiconductor elements.

Power supply diagram

As you can see, the power supply is the main component of the amplifier in terms of the number of elements used. Leaving aside functional considerations, you could even say that the amplifier is just a small addition to the extended power supply. But now seriously. Although this is a prototype system, it is worth putting in some effort to ensure that the elements used to connect the power supply to the mains voltage are mounted solidly and safely. This applies to the mains socket, fuse and switch. I really do not recommend mounting these elements in a makeshift manner. The ability to quickly turn off the amplifier, without nervously searching for the switch, is really invaluable.

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

We bend the sheet metal (e.g. in a vice) in such a way as to create a support that allows it to be screwed to the base.

This is what the finished bracket looks like with a mounted power socket with a fuse and a power switch.

The actual assembly of the power supply began with the input elements of the power supply, i.e. the transformer and its 'surroundings'. These elements are shown in the diagram.

Transformer and safety components

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

It's time for the so-called 'soft start' circuit. Although the toroidal transformer has relatively little power, but considering that it will be switched on frequently when using the power supply circuit, I decided to ease its "luck" a bit and mount this circuit.

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

Here is a view of the transformer and its cooperating components from the side where the soft start circuit leads are visible. You can also see how solidly the bracket with the switch and power socket is screwed (three screws with wide heads).

Time to wire the attached elements. After connecting the switch and soft start system to the transformer, I also connected all the transformer secondary winding leads to the soldering eyes. Since it is not known how long the winding leads will be needed in the future, I did not cut them 'to size' but bent them into a loop (hence the 'tastefully' bent colored leads).

After completing this stage of wiring, I performed control measurements of the voltages on the transformer secondary windings. They were (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 details of the wiring of the power socket, switch and soft start system. I used a slide connection, which is additionally protected by insulating sleeves. This way, there is no possibility of accidentally touching a live wire with your hand.
Safety above all!!!

Here is a photo showing, magnified, how the secondary winding wires of the mains transformer were soldered.

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

At this point I have to justify myself to the 'old pros' who, when using chassis mounting, recommend screwing in heavy elements at the very end. In this case, all the remaining elements will be attached from the top of the base (not from the bottom as is the case with a traditional chassis) and, in addition, my base has wheels. That's it!!!

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

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

M1: 10,3V
M2: 73V
M3: 316V
M4:186V.

This photo shows details of the assembly of the M1 and M2 rectifier bridges.

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

Time to assemble the individual sections of the power supply. We start with a DC power supply designed to heat the amplifier's input stage tube. The schematic diagram of the power supply is shown in the figure below.

Schematic diagram of the power supply for the filament of the 6N1P electron tube

List of elements:

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

The photo below shows all the components of the power supply that provides the filament voltage U1 at the output. The component markings are consistent with the presented schematic diagram.

The measurement of the voltage without load showed that the voltage on the capacitor C1 is 10.3V, while the voltage of U1 is 6.08V.

It's time for the next section, i.e. the filament of the 6S33S power electron tube. The filament voltage is taken directly from winding number II of the mains transformer. It was fed to the output socket U2 using a twisted double wire in insulation. In order for this wire not to 'swing' on the plywood, it was glued to the base in two points [P1] and [P2] using a glue gun.

The third power supply section is the voltage used to initially negatively bias the grid of the 6S33S electron tube. The power supply diagram is shown in the figure below.

Schematic diagram of the power supply system (so-called grid overvoltage) of the grid of a 6S33S electron tube.

List of elements:

M2 - any rectifier bridge for voltage 200V/1A
T1 - BD244
DZ1 -Zener diode for voltage 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. The individual components were marked in the same way as in the schematic diagram.

Time to take a breath. This is what the base looks like, 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 circuit that supplies the anode voltage of the 6N1P electron tube. The diagram of the anode power supply is shown in the figure below.

Schematic diagram of the anode power supply of a 6N1P electron tube (double triode)

List of electronic components:

M3 - four BY55 diodes bypassed 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 what the power supply looks like assembled on a plywood base. The component markings are consistent with the schematic diagram.

Electrolytic capacitors C9 and C11 were glued to the base using a glue gun. The next photo shows more assembly details.

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

Schematic diagram of the anode power supply of the 6S33S electron tube

List of electronic components:

M3 -four BY55 diodes bypassed by 200pF capacitors
Dl1 - choke 10H/300mA
C19 - 680µF/350V
C20 - 0,1µF/1000V
C21 - 680µF/350V
C22 - 0,1µF/1000V

This is what the anode power supply of the 6S33S vacuum tube looks like. Capacitors C19 and C21 are composed of several (four elements for each capacity) electrolytes connected in parallel.

After starting all the power supply sections, connect their grounds to one common point. This will allow you to easily connect the ground point of the entire power supply to a single ground point of the amplifier. I advise against using multiple grounds connected at different points (this approach often leads to hum problems). The method of connecting the grounds of the power supply sections is shown in the photo. The grounds are conducted using a wire in a green insulating jacket.

After tests with the amplifier, I introduced three minor changes to the described power supply design. They are not necessary, but to make the description complete, I will briefly present them.

Modification number 1

Since I used a rather small radiator to reduce the power losses in the filament regulator of the 6N1P-EW electron tube, I inserted a 1 ohm resistor between the rectifier bridge and the regulator - in the photo it is marked with the symbol R.

Modification number 2

On the Triode discussion forum there was a thread about the very beneficial (from a safety perspective) role of a resistor bypassing electrolytic capacitors in high-voltage power supplies. Although I knew about it, I needed a severe 'kick' to appreciate the fact that the voltage on the capacitors can be dangerous even after a long time after the power supply has been switched off. So I bypassed the capacitors of the anode voltage power supply of the 6N1P-EW tube with a 200K resistor (the element marked as R).

Modification number 3

For the reasons mentioned above, I bypassed the electrolytic capacitors in the 6S33S-W tube anode voltage power supply (100K resistor R). I did not use additional bypass in the negative output grid voltage power supply circuit of the tube, because this role is sufficiently effectively performed by the PR1 mounting potentiometer and R3 resistor shown earlier in the diagram of this power supply section.

Amplifier section

The amplifier schematic is shown in the drawing. It is a 'minimalist' design containing only two tubes (one of which is a double triode).

Schematic diagram of the amplifier.

Values of the electronic components used.

During the amplifier tests, some values changed and this is still the case. These are not big differences, but I would like the person building such an amplifier to be aware of this. I try to ensure that the values presented in the table below are as up-to-date as possible.

For higher power resistors, I have given the approximate value of the voltage that is applied to their terminals in square brackets. This allows for easy determination of the required power.

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

It is best to start assembling the amplifier by planning and mounting the largest elements. In this case, it is the speaker transformer [TG1], the 6S33S tube socket [V2] and the 6N1P tube socket [V1]. Additionally (as you can see in the photo below) on the base I have mounted the input socket [WE], the output sockets [WY], the resistor [Rp] used to measure the anode current of the 6S33S tube, and the soldering tips to which the amplifier supply voltages [U1], [U2], [U3], [U4] and [U5] will be connected.

This is what the amplifier base looks like from the bottom after the sockets, transformer and tube sockets are attached. You can also see four wheels that allow it to be moved around the table surface without having to be lifted.

In the following photos I showed the details of mounting the previously mentioned elements.

Next to it you can see the input socket [WE] and the electron tube socket [V1]

The socket of the electron tube [V2] is attached to such aluminum spacer sleeves.

And here is how the input sockets [WE] and the speaker transformer [TG1] are screwed to the base

After checking the correct assembly of the [V2] socket, I unscrewed it and soldered all the elements connected to the 6S33S vacuum tube to the leads. They are:
[A] - anode voltage supply wire
[Ż]-[Ż] - heater wires
[K] - cathode connection wire
[R7] - grid resistor

After soldering the components to the leads of the 6S33S vacuum tube socket, I screwed it back on. Now, the assembly of the remaining components cooperating with the tube (e.g. connecting the measuring resistor Rp to the cathode) is possible even though the ceramic socket is screwed on.

I used a similar procedure for the 6N1P vacuum tube socket. Before screwing it to the plywood, I soldered all the components and connecting wires to the socket legs. The photo shows the socket from the bottom. The component markings are consistent with the schematic diagram.

After screwing the tube sockets, I started the final wiring of the amplifier. The photo (below) shows all the details necessary for its completion. In addition to the elements mounted on the tube sockets, the plywood also contains the elements of the anode power supply filter of the amplifier's input stage (C4, C5, C6, C7 and R11). The global negative feedback resistor (R9) is not connected to the secondary winding of the speaker transformer. This operation should be performed while the amplifier is running.

And this is what the entire amplifier looks like before inserting the tubes into the sockets...

... and after installing the electron tubes.

Before you start 'firing' the amplifier, all that's left is to connect it to the power supply. This is how one channel looks when fully assembled (power supply on the left, amplifier on the right).

Example measurement results

I am posting here successively the results of the amplifier parameter measurements. For now, these are just the results. I will try to add descriptions and my opinion on them after I finish playing with the measurements.

Amplitude and phase characteristics in the frequency range of 0-25.6 kHz. Readings for a frequency of 20 kHz. (amplifier with a wound-core speaker transformer, circuit without correction and feedback, input signal - white noise 10 mV, anode current of the 6S33S tube Ia = 155 mA)

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

Nyquist diagram (amplifier with a wound-core output transformer, circuit without correction and feedback, input signal - white noise 10mV, anode current of the 6S33S tube Ia = 155mA)

Signal at the input (upper graph) and output (lower graph), (amplifier with a wound-core output transformer, circuit without correction and feedback, 1 kHz sinusoidal input signal, P1 shorted/approx. 0K, 6S33S tube anode current Ia = 155 mA)

Harmonic distortions (amplifier with a wound-core output transformer, circuit without correction and feedback, 1kHz 10mV sinusoidal input signal, 140mV output signal, P1 about 5K, 6S33S tube anode current Ia = 155mA)

Harmonic distortions (amplifier with a wound-core output transformer, circuit without correction and feedback, 1kHz 62mV sinusoidal input signal, 700mV output signal, P1 about 5K, 6S33S tube anode current Ia = 155mA)

Harmonic distortions (amplifier with a wound-core output transformer, circuit without correction and feedback, 1kHz 128mV sinusoidal input signal, 1.41V output signal, P1 about 5K, 6S33S tube anode current Ia = 155mA)