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Priboj (priboi) 50, small problem (SOVIET TUBE AMP)

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Hi folks, I recently got the Priboj 50 which is a back in the day soviet made but rather nice sounding tube amplifier.
So I cleaned it out since it was left standing in an old shed for years.
Everything works except one channel has this distortion thing going on.It's not big but it's audible and the weird thing is that on lower frequencies like on bass kicks etc that channel moves the speakers weirdly.Like the speaker cone moves in but doesn't come the other way out, like it would amplify only one half period leaving the other one out.

this is my first hands on tube amplifier.I've only built and repaired semiconductor stuff.I do know the background in physics.
I checked the old electrolytic capacitors with a capacitance meter and they all hold up pretty much ok.
I also swapped the tubes from the other fine working channel and still the same problem persists for that channel , so it's not tube related , maybe it's the capacitors , I will change them later on anyway.
what else could it be any thoughts?

Not that much parts on each channel anyway. Maybe i should swap the caps to see whether that has something to do with it.

Joined 2015
Paid Member
Have you replaced the coupling capacitors between the driver and the output tubes? They may check good on the meter but they may still be leaky (most certainly they are, if the amplifier is more than 40 years old). A leaky coupling capacitor on the output tube grid will change the tube bias, distorting the sound. Replace them with standard 1000V ceramic disk or film capacitors of suitable voltage.
the amp itself , the very one i have is actually made in 1990.One of the last years of USSR and it's product lines.
I will attach some pictures were you can see the channels and what's on them , also I will have to look at the schematic to see what goes were.

the coupling caps you were referring to are probably the blue colored epoxy film ones.
the best replacements that i could get here locally would probably be wimas , maybe you can recommend some other high quality caps both electrolytic and film that i could use on this amp as it really sounds quite nice even with one channel distorting. :D

after all i mean if it's not the tubes then there really isnt much parts to blame for the distortion , it's probably the caps.
I will check resistor connections and values today.


link to pictures
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I'm back with good news , there is a resistor which has a value of 240 ohms , there's one for each output tube , on the distorting channel one of the resistors was literally hanging loose , i pulled it out with my two fingers , in other words an old and cold soldering thing.
the resistor was previously changed by someone since the original one was probably dead from too much heat as I inspected the pcb underneath there were signs of heat exposure but the resistor in there had no such signs.

someone did a poor job at soldering before that's all.

as we are talking about this , I made a green circle around the resistor in question in the schematic , could you guys please explain me a bit in depth what it does in the schematic? As I understand it it's in series with the tubes driving or accelerating grid?


  • priboi 50 schematic resistor problem.jpg
    priboi 50 schematic resistor problem.jpg
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Hey once again folks,

now that I repaired the loose resistor I listened the amp for some two hours and everything was smooth.then it started making sort of cracking and popping noises in one of it's channels.the noises were rather random and had nothing to do with the signal source nor the potentiometers , then it made one rather liud bang through that same channel then i disconnected the power plug.
I initially thought maybe the output tubes are too hot or something

when i connected the amp back to power after it had cooled down there was an immediate very loud 50hz mains hum in that same channel and the hum was on from the very instant i plugged in the power cord.it also draws more current than normally because it keeps blowing a 5A fuse after a few secs of loud hum.

Now here's my conclusions , tubes themselves cannot produce this hum , the tube filament may have been damaged and is injecting its heating ac supply into the tube but since i swapped the tubes around and nothing changed i'm sure the tubes aren't to blame.

I'm almoust fairly certain that the electrolytics have gone dead , after all this amp has stayed in a shed for many years without being used and the caps are rated for 300v so maybe under high voltage they just leak heavily and so inject AC into the output transformer which then directly induces it in the output speaker winding.
If it came through the signal path rather than power supply path it could not be heard from the moment the power is turned on because tubes only start their operation after no less than few secs when the filament heats up.

Is my analysis correct?
Joined 2015
Paid Member
Your diagnosis seems to be correct. Replace the power supply filter caps and try again. If a old vintage device is turned on for several hours without exchanging the filter caps or reconditioning them with gradual voltage increase or a few short runs, the capacitors may go bad. They may also take with them some resistors and the rectifier.
Next time you hear loud crackling noises and/or increasing hum from a unrestored vintage tube radio or amplifier, turn the power off immediately to avoid bigger damages. Your amplifier is comparatively modern and has a working fuse. Older devices have insufficient protection (or a previous owner tampered with the fuses), and the transformer may be damaged. I usually don't even try to turn on a vintage device if it has been in storage for more than a decade. Some hi-spec electrolytic caps may last longer, but they all degrade with time. Your 300V capacitors seems to have been underspecced from the start.
Also check all the fuses, just to be sure that they haven't been exchanged with a improper type.
About your previous questions: yes, the blue colored caps are the ones I was referring to. I found pictures on this page: Tube amplifier PRIBOJ
I am not a expert of soviet-era electronics, but they don't look hi-quality to my eyes, and standard Wima replacements would probably be a safer choice. The resistor you resoldered before is there to limit the screen current of the tetrode. It also seems to have been under specs, I would exchange it with another one with same value and higher dissipation.
thansk guys for your answers , well since my experience with soviet electronics is much higher due to location I can safely say that apart from old electrolytics there isnt much in this amp that can be broken or can brake itself, the transformer is a tank it can run shorted for a while and don't care they used similar ones in tv's and some even made small welders out of them.
as for the resistor , it's a 1w rated resistor the bigger ones are 2w rated but they can take more , i have used them in many places and they have ran so hot you can boil water on them with no problems for long time , mines in this amp dont even get that hot.so the resistors are fine , it's mostly the electrolytics in civil soviet equipment that fail the scene often.

also theres not much parts in there as you can already see so changing the caps to good brand's and specs should do some tricks and then theres the tube upgrade thing, it seems people go two ways one way is to use the existing circuit and sockets and insert analog but better specced tubes , the other route seems to be modifying the circuit of the channels and go for the soviet military fighter jet tubes that were robust as hell and quite high quality like the 6S33S (russian 6C33C)

for starters i might just upgrade the existing circuit with better tubes but firstly i have to get the caps done and see it it sounds again.
the tube thing would probably ask for another new topic , i already found some info that the gain tube is good and high quality but the input tube could be changed to western EF86german made analog, and so the output tetrodes , only i havent found replacement tubes yet.
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