Tube amplifier "Mikrus PCL86"

Details: Written by: Grzegorz Makarewicz; Category: Tube equipment; Last Updated: 15 December 2025

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

The PCL86 vacuum tube belongs to the group of so-called "television tubes", i.e. tubes used in TV sets produced as the last models of tube TVs. In principle, models produced in the late 70s and early 80s of the last century are already "hybrid" transistor-tube designs. A classic example of such a TV was "Libra" produced in the Warsaw Television Plant (WZT). I have sentimental memories associated with this design, because I belonged to a group of so-called "assemblers" who, as the name suggests, were involved in assembling these TVs from parts obtained from "Bomis" (younger Internet users will have to search the web a bit to find out what it is).

My neighbors and closest family members were equipped with Libra brand televisions. They were happy to have devices that were scarce at that time, while I had a nice financial source to support my modest student budget. I have to admit with some embarrassment that the television tubes used in Libras were a necessary evil for me at that time, which I had to accept, being dependent on the ideas of the designers from WZT. Like other assemblers, I immediately gave up the "compulsion" to live with them as soon as fully transistor Vela, Telefunken, etc. televisions appeared on the horizon (read in the offer of "Bomis" stores).

PCL86 tubes had real bad luck when it came to their appreciation by audio equipment designers. The reason for this bad luck was ECL86 tubes, i.e. tubes with analogous parameters, but adapted to 6.3V filament voltage. They were the basis for the construction of small low-frequency amplifiers for many years. Their "television equivalent", the PCL86 tube, was treated even as a so-called second-class tube, not suitable for use in a tube audio amplifier.

Recently, after many years of "humiliation", PCL86 tubes are starting to be noticed by the tube sound enthusiasts. What is the reason for this? Could it be that their mysterious sonic features have been revealed? Nothing of the sort. The stocks of ECL86 tubes have simply been practically exhausted, their prices have skyrocketed and, as the Polish saying goes, "the goat has come to the cart", the designers have apologized for the previously unwanted "television product".

In the literature and generally available Internet resources, you can find many diagrams of low-frequency amplifiers based on the PCL86 (ECL86) tube. The amplifier described here (in two implementation versions) is based on the classic configuration recommended by tube manufacturers shown in Fig. 1. The diagram shows one amplifier channel (right - R), the second channel is of course identical. The elements related to the left channel are marked with the letter "L" (e.g. C4L, R4L), the elements related to the right channel are marked with the letter R (e.g. C4R, R4R), while the elements common to both channels do not have an additional marking (e.g. C13, R13, LED1).

Additionally, I included in the diagram all the elements that enable building the amplifier in the version published in the Polish magazine "Elektronika Praktyczna" issue 2/2005. If you want to assemble the amplifier in a form identical to that published in EP2/2005, you should omit resistors R8L, R8R, R9L, R9R, R10L, R10R, R13, capacitors C10L and C10R and diode LED1, short-circuit the screen grids of the electron tubes with the positive pole of the anode voltage power supply and use jumpers in place of the CLL and CLR chokes.

Fig. 1

Fig. 2 shows a diagram of the power supply system including a power supply for the filament voltage of the electron tubes and two power supplies supplying the anode voltage to both channels.

Fig. 2

I made the amplifier in two versions, differing in mechanical construction. Both models are shown in Fig. 3.

Fig. 3

On the left side you can see an amplifier in the form of a "tower", on the right side an amplifier made in a traditional form on a small aluminum chassis.

Amplifier version 1

The tower-shaped amplifier is shown in Fig. 4.

Fig. 4

For the first amplifier, I designed a single printed circuit board shown in Fig. 5. As you can see, it is a single-sided board that does not require soldering any jumpers.

Fig. 5

Fig. 6 shows the outline of the amplifier components.

Fig. 6

The printed circuit board has been designed in such a way that it can be used to mount not only electronic components, but also all additional components including input sockets, volume control potentiometer and power switch. Particular attention should be paid to the input sockets used, so that they enable proper connection of the signal and ground inputs. If necessary (lack of matching sockets), the paths on the board inputs should be corrected/swapped.

The installation of additional elements is of course optional. In the case of a housing that does not allow for the insertion of a board with these elements, the sockets, potentiometer and switch should be mounted on the housing and connected to the board with wires. The view of the printed circuit board with the elements installed is shown in Fig. 7.

Fig. 7

The printed circuit board has labels with component markings that match the markings on the schematic diagram. Before soldering in the component, it is worth making sure that it has been installed in the right place. This way, we will avoid having to desolder it - which is not a pleasure. It is best to start the assembly with the smallest components (resistors R1L, R1R, R2L, R2R, R3L, R3R, R5L, R5R, R7L and R7R). Next, we mount slightly larger components (R4L, R4R, R8L, R8R, R12, R9L, R9R, R13, D1, R6L, R6R, R10L, R10R, C8L, C8R, B1, B2, CLL, CLR, etc.).

In my experience, ARK connectors have a negative feature in that some pins can cause cold solder joints, so you should pay special attention to the effect of their soldering and it is best to degrease them and not touch them with your fingers before soldering.

Resistors R11L and R11R heat up a lot during amplifier operation. They should be soldered in such a way that they do not touch the printed circuit board. It should be ensured that resistor R11L is as far away from electrolytic capacitor C12R as possible. If this resistor is soldered on longer legs above the board, it may happen that during assembly it will be accidentally bent and touch the casing of capacitor C12R - this is unacceptable. This resistor should be bent away from this capacitor as much as possible. If the amplifier design allows it, it is recommended to mount this resistor from the printed circuit side.

Radiators often have rivets installed by the manufacturer to attach them to the printed circuit board. To mount this radiator in a "traditional" way, these rivets must be removed and the radiator can then be screwed on, for example, using sheet metal screws (it is then not necessary to thread the holes in the radiator). Make sure that the screw heads do not touch the tracks - this can happen if screws that are too large are used. Just in case, I recommend using insulating washers under the screws.

RCA input sockets are subject to high stress when connecting and disconnecting cables. That is why I designed the holes in the printed circuit board in such a way that frequent connection of cables does not damage (break) the paths to which the sockets are soldered. Each socket, in addition to the signal terminals, has two protrusions that provide additional mechanical stiffening. These protrusions have a square cross-section, and the diameters of the holes in the board are such that a significant amount of pressure must be used to properly lock the sockets on the printed circuit board. If you have trouble inserting the socket, you can use a file to slightly round off the protrusions. Remember that the more you file the protrusions, the easier it is to press the sockets, but the less mechanically stable they are.

The following drawings show the construction details of the amplifier assembled in the form of a "tower".

Fig. 8

Fig. 9

Fig. 10

Amplifier version 2

The view of the amplifier assembled on an aluminum chassis is shown in Fig. 11.

Fig. 11

The electrical system has been slightly changed compared to the first version. The amplifier diagram is shown in Fig. 12.

Fig. 12

The power supply diagram is shown in Fig. 13.

Fig. 13

The amplifier components are assembled on two printed circuit boards. Figure 14 and Figure 15 show a view of the board and the amplifier components from Figure 12.

Fig. 14

Fig. 15

Fig. 16 and Fig. 17 show a view of the board and components of the power supply from Fig. 13.

Rys. 16

Fig. 17

Fig. 18 shows a photograph of a partially assembled board containing the amplifier section. The ARK connectors are not yet mounted. As you can see, the tube sockets and thus the electron tubes are placed on the print side.