Polski (PL) 
English (United Kingdom) 
Grzegorz 'gsmok' Makarewicz,
The PRIBOJ amplifier (original name: Прибой, meaning coastal wave) is one of the few tube audio amplifiers available on the aftermarket manufactured beyond our eastern border. Like most of its brethren, it is characterized by a hopeless design and terrible build quality that makes your teeth ache. These shortcomings are offset by a well-thought-out circuit board and above-average sonic performance. In short, it's a princess dressed in a beggar's rough clothes.
The cats said they would lie down peacefully if I let them listen to a tube amplifier.
The described amplifier, marked "Прибой-50УМ-204С", has a power of 50W (a 75W version was also produced, marked "Прибой-75УМ-204С"). According to the manufacturer, the amplifier has the following parameters:
- input sensitivity 600mV ±150mV (quite "frivolous" tolerance),
- nominal frequency response 20Hz...20kHz,
- load resistance 4Ω/8Ω,
- harmonic distortion in the rated band 1%,
- intermodulation distortion 2%,
- channel separation (crosstalk) in the 250Hz...10kHz band not less than 40dB,
- signal-to-noise ratio 86dB,
- input resistance 33KΩ,
- power consumption 250 ±30W (I wonder what this ±30W depends on),
- dimensions 430mm x 400mm x 163mm,
- weight 18.5kg.
Here's what the amplifier looks like. It's a box reminiscent of a typical 1980s transistor amplifier. I'm not a fan of such enclosures used in tube circuits. I prefer the traditional approach with exposed transformers and tubes. But that's a matter of taste. This example is silver, but apparently there are also black-painted enclosures.
The characteristic features of the mentioned design line are: the power switch in the form of an ISOSTAT button (such switches either break after some time or lose their buttons, which tend to "shoot out" and disappear in some dark corner of the listening room)...
...and slider potentiometers. What kind of "sliders" are used in this amplifier? That's a surprise, described a little later.
Now the view from the back.
A few details of the rear panel. Here are the connection sockets, fuse, and the hole through which the power cable passes. In short:
- The speaker connectors are the worst. I don't think I've ever seen such junk in any other device. The material is flimsy, sloppily machined. The machining tolerances are measured not even in millimeters but in fractions of centimeters. For example, the hole depths allow a banana plug to be fully inserted into some sockets, while in others, it's missing by as much as half a centimeter.
- Old-style signal socket (DIN). Unfortunately, it won't work without an adapter.
- The fuse socket is very solid, although replacing the fuse insert requires pressure similar to unscrewing a well-sucked jam jar.
- Unfortunately, the network cable outlet does not meet the normal evaluation criteria.
The amplifier uses an unusual form of socket labeling. The lettering is embossed in raised letters. The labels will never "wear out," but despite their three-dimensionality, reading the socket functions is very difficult and requires appropriate backlighting. And worst of all, the socket panel bends dangerously when plugs are inserted or removed, giving the impression that it's undergoing this process for the last time before spectacularly falling to pieces.
The panel with connection sockets is topped by two symmetrically arranged boxes with grilles for ventilation. They look unsightly, but they perform their functions well.
View from above.
A very interestingly displayed warning on the top cover. Unlike other manufacturers, the amplifier manufacturer doesn't prohibit anything, only recommends it (true democracy). It's a nod to the amplifier's owner, intended to help them overcome the shock of the build quality.
And finally, of course, the view from below.
We've examined the amplifier from all sides. Now we can begin disassembly. I'm starting with the top cover. It's attached to the chassis with four anemic screws. This is what the removed top cover looks like. It's made of metal. The quality of the fit is beyond reproach.
Now the bottom cover has been removed. It's secured with seven screws, just as weak as the top cover. Unlike the top cover, it's made of plastic. Its strength parameters are just as weak as the screws mentioned above. As you can see, the amplifier, weighing over 18 kg, wouldn't be able to stand on such a base. Therefore, the amplifier's feet are screwed directly to the chassis, not to the cover. The cover has holes through which the amplifier's feet pass – quite cleverly.
Before looking inside, it's worth checking out the amplifier's schematic diagram. Here's a diagram of the basic amplifier block. The input stage and phase inverter utilize 6Ż32P (EF86) and 6N6P tubes. As you can see, while the topology is typical, the output tubes aren't typical of audio amplifiers. The circuit utilizes 6R3S dual beam tetrodes, which, while the manufacturer intended for use in wideband, low-frequency power amplifiers, haven't found their place in the otherwise large group of tube amplifiers available on the market. As far as I'm concerned, the "Priboj" is, to date, the only amplifier using such tubes that I've had the opportunity to service.
A diagram of the power supply block is shown below.
The interior of the amplifier seen from above (as can be seen from the carpet showing through the amplifier, both covers are removed).
The electronic components are mounted on three printed circuit boards: the left and right sides are the left and right channel amplifiers, and the center is the power supply board. A paper laminate is used. It has superior electrical parameters to glass-epoxy laminates. It does not absorb moisture, which is crucial for high-voltage systems. Its drawback is its poor mechanical properties (it simply crumbles). The laminate used in this particular unit is characterized by extremely poor temperature resistance. Any soldering operations should be carried out with extreme caution – soldering points and connection paths come loose at the mere sight of the soldering tip.
Now for some details of the design, seen from above. All the tubes are mounted in sockets with special springs. It's an excellent idea. Springs lack visual appeal and look awful in amplifiers with exposed tubes. Here, they're perfectly appropriate.
Mains transformer. Huge, made with a wound core, solidly twisted. Zero humming noise. Even when touched with your fingers, you can't feel the slightest vibration. It's absolutely wonderful.
Two output transformers. Made with wound cores, of a decent size (the dimensions seem to be even oversized), and no "playing" during amplifier operation. Solid workmanship. As with the mains transformer – superb!
The power supply components are placed (actually crammed) between the mains transformer and the output transformers. This "cramming" of components is due to the large dimensions of the electrolytic capacitors. Capacitors of this design were manufactured exclusively beyond our eastern border. In this example, they survived in excellent condition. The capacitors filtering the anode voltage of the output tubes are additionally covered with "tasteful plastic jackets."
Here you can see the amazing "caps" placed over the output tube anode leads. I'm personally impressed by their understated charm. How strikingly different they are from the modern tube amplifiers with electrodes leading out from the top of the tube.
"Caps" for the second time.
This photo shows the considerable effort required to mechanically secure the electrolytic capacitors. The white element on the right is the power switch cover.
The previously shown "Caps" once again - this time side portraits.
Before I move on to the bottom views, it's time to take a closer look at the solution used to adjust the input signal level. But first, a small digression. Is there anyone on this earth who has seen indelible markings on a slider potentiometer? I'm referring, of course, to the markings on the front panel of a device that has been in regular use. I've never seen such a device. The amplifier in question is no exception. I wonder if any manufacturer has ever conducted any research leading to the production of a device with markings that are resistant to abrasion through proper use.
So much for the complaints. What's hidden behind these less-than-aesthetic slider potentiometers?
I peer inside and am astonished to see this interesting audiophile component: a resistor ladder disguised as a slider potentiometer. Illustrating the supply difficulties of those years (the amplifier dates back to 1990), the factory supplier was unable to assemble the ladders from the same type of resistors (they used 0.25W and 0.125W MŁT metal film resistors – the reddish and light green ones – and, one rare thing – a dark green carbon resistor). Furthermore, the left channel ladder has different resistor types than the right channel ladder. Fortunately, at least the resistance values are the same.
This is what the mechanical implementation of a resistor ladder looks like.
For those eager for detail, here's another photo showing how the resistor switching (or rather, "sliding") was implemented. For me, it's brilliant. The only drawback is that the slider controls themselves, compared to the amplifier's rotary switches, are not very user-friendly. Regardless, it's a big plus for an idea implemented at a time when the audio market hadn't yet reached the heights of audiophilism it has today.
Here are a few shots of the amplifier, taken from the bottom. As you can see, the ceramic sockets for the output tubes are wired (not soldered to the board).
Mains transformer and power supply board.
Power supply board.
The power switch and one of the amplifier's feet. The feet are riveted to the chassis rather than screwed in. This solution is nice on the one hand, as there's no need to screw or tighten anything. On the other hand, it's not nice, as there's nothing worse than a loose rivet and a wobbly foot that simply can't be tightened.
Here you can see some details of sloppy workmanship.
The amplifier's printed circuit boards are easy to disassemble. The entire electrical structure is based on several multi-pin sockets, shown below. Here's the socket on the power supply board...
...and these are the sockets on one of the amplifier boards. The top one connects the amplifier to the power supply, while the bottom one is for connecting the input level control.
We slide out the plugs, and two, the amplifier boards are out. Here they are.
After removing the boards, the amplifier becomes empty.
Now some photos of the printed circuit boards.
Amplifier board from the print side.
Amplifier board from the components side.
Power supply board from the print side.
After replacing the output tubes, the correct operating points (cathode currents) must be set. I haven't found manufacturer-recommended values. According to them, the grid bias voltages should simply be set to -50V. Nothing could be further from the truth. I checked the tubes I already have. Online, I found information from users of these amplifiers that they set the currents in the range of 50mA to 65mA. Two potentiometers located on the power supply board are used to adjust the negative voltages on the output tube grids independently for the left and right channels.
Before turning on the amplifier for the first time after replacing the output tubes, it's a good idea to turn them all the way to the right (clockwise). In this position, the currents flowing through the tubes are minimal (at least in this amplifier).
Each amplifier board contains an additional potentiometer used to balance (equalize) the currents flowing through the tubes. Initially, it's a good idea to set these potentiometers to their mid-position.
Adjusting the output tube quiescent currents is a real pleasure. It's worth doing it for all tubes simultaneously.
The "Priboj" amplifier is a truly cool design. The build quality, unfortunately, resembles the Meloman amplifier described in the Gallery. The mechanical flaws are compensated by the electrical solutions used: no quirks, proven topology, excellent output transformers, and a decent mains transformer. The amplifier is service-friendly. Finally, the listening impressions are very positive. The amplifier is quiet, with virtually no hum. It sounds a bit "hard," but it's not a triode, after all. In any case, I liked it.
Fun fact:
According to the dictionary, PRIBOJ/Прибой means a coastal wave, a tide, while PRIBOI/Прибои means a snowman. It's worth keeping this in mind when Googling information about amplifiers in the Tolstoyan language (as I learned the hard way).
Prepared by: Grzegorz "gsmok" Makarewicz,