Reducing output transistors

[gruni]

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

[AndrewT]

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.

[Nigel Goodwin]

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.

[gruni]

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...

[sakis]

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

[gruni]

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

[infinia]

Originally posted by gruni
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...

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.

[sakis]

Originally posted by infinia



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.

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

[infinia]

Originally posted by sakis


the beta issue to my understanding is very secondary

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.

[gruni]

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.