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Tube amplifier PRIBOJ


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

Tube amplifier Priboj (original russian name - Прибой or Breaker in english) is one of the few secondary market tube audio amplifiers manufactured by our neighbors across the eastern border. Like most of devices from that region it has a hopeless design line and poor craftsmanship. The counterweight to these defects is a thoughtful schematic diagram and outstanding sonic performance. In short it is a princess dressed in a beggar's rags.

Amplifier described here marked "Прибой-50УМ-204С" has the power of 50W (another version with 75W output power marked "Прибой-75УМ-204С" was also produced). According to the manufacturer amplifier has the following parameters:

  • Input sensitivity 600mV ±150mV (rather frivolous tolerance ),
  • Nominal frequency range 20Hz...20kHz,
  • Load resistance 4Ω/8Ω,
  • Harmonic distortion in nominal frequency range 1%,
  • Intermodulation distortion 2%,
  • Channel separation (crosstalk) in the band 250Hz...10kHz no less than 40dB,
  • S/N 86dB,
  • Input resistance 33KΩ,
  • Power consumption 250 ±30W (interestingly which determines the ±30W )
  • Dimensions 430mm x 400mm x 163mm,
  • Weight 18.5kg.

Here's how it looks. It is a box-like design typical for transistor amplifiers from the 80's of the twentieth century. I am not a follower of such enclosures for tube devices. I prefer the traditional approach of exposed transformers and tubes. But it's a matter of taste. This one has a silver color, but apparently there wera also black painted housings.

Characteristics of the mentioned design line are: the power switch in the form of ISOSTAT key (such switches after a certain time break or lose their pushbuttons which like "firing" and disappear in some dark corner of the listening room )...

... and sliders. What are the "sliders" used in this amplifier? It's a surprise described later on.

Now look from behind.

A few details of the rear panel. Here we have output sockets, fuse, and the hole through which the power cord is "ejected". In brief:

  1. Poor quality loudspeaker connectors. I do not think I've seen this crap in any other equipment. Carelessly treated flimsy material. Tolerance treatment measured in fractions of centimeters. For example, the depth of holes allows insertion of banana plug into three sockets to the end, in other lacks even a half an inch .
  2. Old type (DIN) terminal connection. DIN to CINCH adapter is necessary.
  3. Fuse Socket is very solid, although fuse replacement requires the load like opening a well-drawn jar of jam.
  4. Derivation of a power cord, unfortunately, does not catch the normal assessment criteria.

An unusual form of outlets descriprion is applied. They are molded in the form of convex letters. By that means descriptions are really indestructible (resistant to abrasion) but despite their 3-dimensionality, reading of their functions is very difficult and requires proper illumination. And the worst - when inserting and removing connectors, the rear panel  bends dangerously giving the impression that is subjected to this process for the last time before the spectacular disintegration into pieces.

On both sides of the junction panel there are two symmetrically arranged boxes with ventilation grilles. They look ugly but are very effective.

View from the top.

Very interestingly exposed warning on the top cover. Unlike other manufacturers, manufacturer of this amplifier does not prohibit anything but merely not recommended (true democracy). It's a tribute to the holder of an amplifier having him out of shock after a confrontation with the quality of workmanship.

And finally of course the view from the bottom.

We saw the amplifier on all sides. So we can proceed with the opening of the cover. Starting from the upper housing. It is bolted to the chassis with four anemic screws. This is the top cover removed. It is made of metal. For a fit you can not have any major objections

Now the bottom cover removed. It is fastened by means of seven as anemic as the top cover screws. In contrast to the top cover the bottom one is made of plastic. The strength parameters are as anemic as these screws. As you can see the weighing over 18kg amplifier could not even stand on the bottom cover directly. Therefore, the amplifier feet are attached not to the cover, but directly to the chassis. In the cover there are openings through which the amplifier legs pass - quite clever .

Before looking into the interior one should look at the schematic diagram. Here is the basic block diagram of the amplifier. In the initial stage and phase inverter 6Ż32P (EF86) and 6N6P tubes are applied. As you can see as far as a typical topology is used, the power tubes are not typical for audio amplifiers. The output stage uses dual beam tetrodes 6R3S that although the manufacturer recommended them to broadband low-frequency power amplifiers, have not found their place in a large group of tube amps on the market. As for me, "Priboj" is, so far, the only amp with these tubes, which I had the opportunity to service.

The following shows the schematic of the power supply.

The interior of the amplifier as seen from the top.

The electronic components are mounted on three printed circuit boards: left and right channel of the amplifier and the power supply board in the middle. "Paper" laminate is used as a PCB. It has better electrical characteristics in relation to the glass-epoxy laminates. Does not absorb moisture, which is very important for high-voltage systems. Its disadvantage is poor mechanical properties (just crumbles). The laminate used in this particular amplifier is characterized by an extremely low resistance to heat. Any operations using soldering should be performed with extreme caution - soldering points and connecting paths detach very easily .

Now some details of the construction, viewed from the top. All tubes are inserted into the sockets with special springs. This is an excellent patent. Springs look hopeless when applied to amplifiers with exposed tubes. Here (in closed box case) are the most on the spot.

Mains transformer. Huge, oversized C-core, firmly twisted. Zero of any hum. Even when you touch it with your fingers you do not sense the slightest vibration. Sensational .

Two output transformers. Huge, oversized C-core, firmly twisted and completely silent. Solid job. Sensational .

Elements of power supply are "packed" between the mains transformer and output transformers. Stuffing was necessary due to large size of electrolytic capacitors. Capacitors of this appearance were carried out only for our eastern border. This copy survived in excellent condition. The filter capacitors of anode voltage of output tubes are dressed in tasteful plastic jackets.

Here you can see an amazing "bakelite caps" imposed on the output of anodes of 6R3S tubes. I am personally impressed with their discreet charm .

"Bakelite caps" once again.

On this photograph you can see how much effort required mechanical fixation of electrolytic capacitors. The white part of the right side of the power switch cover.

"Bakelite caps" once again - this time portraits from the side.

Before turning to the view from the bottom, it's time to look more closely on the solution used to adjust the input signal levelWhat is behind these very aesthetic sliders?

I look inside and was surprised to look at the interesting audiophile part. Resistor ladder pretending sliders. Illustration of supply difficulties of those years (amplifier is dated 1990.'s) is tha factory's provider failed to make a ladder of resistors of the same type .. Fortunately, that although the resistance values are the same.

And here is the implementation of mechanical resistor ladder.

Another photograph showing the completed resistors switch. Really impressive. The only drawback is that the volume adjustment, as compared with rotary switches, is not ergonomic. Anyway, a big plus for the idea realized in the period when the audio market has not yet reached such audiophilie peaks, as is currently the case.

Now a couple of shots taken from the bottom of the amplifier. As you can see the output tubes ceramic sockets are wired (not soldered to the PCB).

 

Mains transformer and power supply board.

Power supply PCB.

Power switch and one of the amplifiers legs. The legs are not screwed but riveted to the chassis. The solution is fine - one do not need to tighten the screw - on the other hand not fine - because there is nothing worse than a loose rivet which can not be simply tighten.

Here you can see some of the details of sloppy workmanship.

Amp PCBs are easy to dismantle. All electrical design is based on a number of multipin sockets below. This is the power supply socket ...

... and a socket on one of the amplifiers channel. The upper links the power supply, the bottom is designed to connect the input signal level slider.

We put forward two plugs and amplifier PCBs are free. Here they are.

After removing the PCBs the amplifier gets empty.

Now some photos of PCB.

Bottom side of the amp PCB.

Element side of the amp PCB.

Bottom side of the power supply PCB.

After replacing output tubesit is necessary to adjust operating points (cathode currents). I have not found the value specified by the manufacturer. According to the instruction manual adjusting means simply set the grid bias voltage at -50V. Nothing could be further from the truth. On the Internet I found information of users of these amplifiers - they set the current values of 50mA to 65mA. To adjust the negative voltage on the output tubes grids independently for left and right channels there are two potentiometers located on the power supply board.

Before first powering the amplifier after replacing the output tubes potentiometers should be turned to the right (clockwise). In this position, the currents flowing through the tubes are minimal (this is at least in my copy of the amplifier).

On each amplifier PCP an additional potentiometer is used to symmetrization of the currents flowing through the tubes. The pre-set value of the potentiometer should be in the middle position.

Current adjustment is easy . It should be done for all tubes simultaneously.

 

Amplifier "Priboj" is a really cool design. Poor mechanical quality is compensated by electrical solutions: proven topology, excellent speaker transformers, good power transformer. The amplifier is friendly for service. At the end some listening experience. These are very positive. The amplifier sounds really good and there is virtually no hum.

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