Hello everybody,
i have a bass head in which two of six output transistors were blown by some person by connecting a 2 ohm load of what the amp is not capable. i myself only use a 8 ohm cabinet with it and i really need the rig in a few days so i cant wait for spare transistors. so my question: can i run the amp with only 4 of 6 transistors without the bias current raising too high or so? they are complementary pairs of toshiba 2sc5200 and 2sa1943. the rail voltage is about +/-56V and i only need about 100W rms (in original the amp delivers 300w rms).
i hope you can help me
i have a bass head in which two of six output transistors were blown by some person by connecting a 2 ohm load of what the amp is not capable. i myself only use a 8 ohm cabinet with it and i really need the rig in a few days so i cant wait for spare transistors. so my question: can i run the amp with only 4 of 6 transistors without the bias current raising too high or so? they are complementary pairs of toshiba 2sc5200 and 2sa1943. the rail voltage is about +/-56V and i only need about 100W rms (in original the amp delivers 300w rms).
i hope you can help me
Hi,
since it is already blown you have nothing to lose by trying it.
However, expect there to be much more damage than just two output devices. The other output devices may be damaged but still operating, the drivers may be damaged and not operating. Some other semiconductors may be damaged and maybe a few passives as well.
since it is already blown you have nothing to lose by trying it.
However, expect there to be much more damage than just two output devices. The other output devices may be damaged but still operating, the drivers may be damaged and not operating. Some other semiconductors may be damaged and maybe a few passives as well.
As already suggested, assuming nothing else is damaged (which is pretty unlikely), it will happily feed 8 ohms on only two pairs of transistors. If it was 300W in to 4 ohms, it should provide 150W or so in 8 ohms, even with an output pair missing.
well i already ordered the two spare transistors, so if more of them fail i will need a further order. i tested the transistors that are not blown with a transistor tester and they should be ok. i dont see many passive parts in the power stage that could fail. i havent tested the drivers so far but the last time somebody blew the transistors (i should stop giving my amp away 😀 ) only two output transistors and a fuse were blown. this time it was the same, i already replaced the fuse. my only worries were the bias current and the high supply voltage...
from a theory
point of view it should work happily for years .... though expect the sound to change and also your amp will not have the same safe operation area....
wile playing the bass that produces very low frequency there is a chance that your speaker "dives " by far lower than the 8 ohms reated Z
now that ...might be a problem
point of view it should work happily for years .... though expect the sound to change and also your amp will not have the same safe operation area....
wile playing the bass that produces very low frequency there is a chance that your speaker "dives " by far lower than the 8 ohms reated Z
now that ...might be a problem
tried it, fuse doesn't blow anymore but there is no output. only a little hiss (can't really say if it comes from the amp 😀 ) and about 0,5V offset... ill have to check the drivers
gruni said:
I would measure and match all of output beta's of the affected rail. One scenario is the 2 output devices that blew were mismatched and were doing all the heavy lifting. I would expect the driver to be shorted and the other rail to be stressed if not broken.
IMO for pro repair service ie mass device replacement with matching... , anything else is risky and time wasting.
edit> you said "about 0,5V offset... ill have to check the drivers"
looks like the driver is good... offset is from the front end.
You could probably run it ...as you know lighty.
infinia said:
i would not agree on that even though it actually sound the right thing to do ..... but let me explain for a minute
chinese are no stupid ...they know very well that their outpouts are not really quality stuff so they make sure that the use of 0.47R in the emmiter will guarantee to cover almost most diference between the beta ....also base resistors have something to do with this ....
i would better look at the "mechano" ....many times i lost chinese amps cause one of the transitors wasnt firmly attached ....also design issues like too long pcb with too thin traces makes biger resistance from the center located transitors that are much better "wired "
the beta issue to my understanding is very secondary
sakis said:
emitter degeneration helps certainly, but not knowing the specifics of the design it's just conjecture.
In case of large mismatch and dynamic overloads the local feedback time constant and SOA it cant react fast enough.
infinia: there is no output except for the offset so i cant use it. the drivers arent shorted so perhaps they really work but in that case somewhere else in the signal way there must be something broken...
sakis: you are right, there are 0R22 resistors on the emitters so there should be a bit load balancing.
sakis: you are right, there are 0R22 resistors on the emitters so there should be a bit load balancing.
gruni said:
ahh 0,22 is on the lower side
0,33 minimum and 0,47 is better for sharing purposes.
the lower value of the emitter ballast along with lower bias current makes output device matching more important.
OK more trouble shooting ahead.
edit> when you get it running spend some time setting the bias current ie check the specs for DC voltage across 0,22.
If you don't have specs I would make sure it's 0.3 Vdc or so. Let it sit after initial adj. and recheck it periodically. (must be stable with temp) when it goes lower then you have less sharing.
If you don't have specs I would make sure it's 0.3 Vdc or so. Let it sit after initial adj. and recheck it periodically. (must be stable with temp) when it goes lower then you have less sharing.
That can't be right for a ClassAB amplifier.infinia said:
0.025V = 25mV across the emitter resistor is much closer for an EF output stage.
If it's a CFP output stage then <=10mVre will be the norm.
your aspect
remains the same as you treat i hifi to high end amplifier ...this one isnt
matching is critical but secondary
remains the same as you treat i hifi to high end amplifier ...this one isnt
matching is critical but secondary
you remind me
of a boy that used to work for me edjucated guy ....from good school in computers .... his experties in software, networks and so on were realy amazing .... but on the other hand for every other problem his first word is format .....
meaning to be able to troubleshoot a chinese amplifier you need to follow the spirit .....you cannot think of any chinese amplifier as a leach amp ...it may be based on the principal but its alot changed from that to meet the low quality specs of the components firstly ....then there is more things ....
the botom line is that design rules and principals as often disscused in the forum do not apply to these devices ....
i cant say if is good or bad ...its simply another way for sure cheaper ....
check Andrew T posts ..... you will see that a 60 +60 volt rail amplifier give or take will require at least 10 transistors per board to produce 250w ....is this what the book say ????? is this what the math say ???? one way or another many comercial designers not especially chinese share another opinion
regards sakis
back to the post .....
---- check voltage all arround
---- if the amp features an op amp in the input there is a chance that the network that supply the op amp with 18+18 volts is gone
place the brand of your device .... there is a possibility to find a schematic ....
things will be much easier after that
( small coment .... some of these amplifiers work in class G can you define what class is your amp ???? you can easilly do that by checking the psu ..... )
of a boy that used to work for me edjucated guy ....from good school in computers .... his experties in software, networks and so on were realy amazing .... but on the other hand for every other problem his first word is format .....
meaning to be able to troubleshoot a chinese amplifier you need to follow the spirit .....you cannot think of any chinese amplifier as a leach amp ...it may be based on the principal but its alot changed from that to meet the low quality specs of the components firstly ....then there is more things ....
the botom line is that design rules and principals as often disscused in the forum do not apply to these devices ....
i cant say if is good or bad ...its simply another way for sure cheaper ....
check Andrew T posts ..... you will see that a 60 +60 volt rail amplifier give or take will require at least 10 transistors per board to produce 250w ....is this what the book say ????? is this what the math say ???? one way or another many comercial designers not especially chinese share another opinion
regards sakis
back to the post .....
---- check voltage all arround
---- if the amp features an op amp in the input there is a chance that the network that supply the op amp with 18+18 volts is gone
place the brand of your device .... there is a possibility to find a schematic ....
things will be much easier after that
( small coment .... some of these amplifiers work in class G can you define what class is your amp ???? you can easilly do that by checking the psu ..... )
Re: you remind me
Perhaps you think I assumed too much in your eyes but maybe not, but I think you got me in the wrong peg sakis. I have maybe alot more experience and education than you know. Pro amp with 6 devices on a rail w/low voltage... tell alot don'tcha think. Even with a switched rail voltage the song remains the same.
But Andrew is very conservative in his foundation, and for me that can be good but in the marketplace who knows.
Perhaps you think I assumed too much in your eyes but maybe not, but I think you got me in the wrong peg sakis. I have maybe alot more experience and education than you know. Pro amp with 6 devices on a rail w/low voltage... tell alot don'tcha think. Even with a switched rail voltage the song remains the same.
But Andrew is very conservative in his foundation, and for me that can be good but in the marketplace who knows.
Re: you remind me
its a behringer bx3000 bass head. well, it of course is the cheapest one around, one might say its not profitable repairing it but i dont have the money to buy a new one.
the 15-0-15 supply is ok, it also feeds the preamp stage that works perfectly. i dont really understand the output stage as the transformer only has a 88v rail with no potential to the 18-0-18 rail and no center tap...
sakis said:
its a behringer bx3000 bass head. well, it of course is the cheapest one around, one might say its not profitable repairing it but i dont have the money to buy a new one.
the 15-0-15 supply is ok, it also feeds the preamp stage that works perfectly. i dont really understand the output stage as the transformer only has a 88v rail with no potential to the 18-0-18 rail and no center tap...
post ....
post pictures of it
meanwhile i may take a look for a schematic
chinese regards sakis he he
post pictures of it
meanwhile i may take a look for a schematic
chinese regards sakis he he
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