I need to replace one of the four 2SB554 transistors in an Adcom GFA-555II power amplifier.
Each transistor is connected through a 0.22-ohm resistor for current sharing and bias stability.
Do I need to replace all four with balanced transistors or can I replace just the one defective transistor?
Thanks for all your help.
Each transistor is connected through a 0.22-ohm resistor for current sharing and bias stability.
Do I need to replace all four with balanced transistors or can I replace just the one defective transistor?
Thanks for all your help.
Should ensure they are the replacement has the same gain class - otherwise let the resistors do their job? The datasheet I found suggests there are two gain classes, O and R
Never replace just one single transistor in a parallel group. Unless you still have some from that original batch. They really need to be from the same production run. There’s also the fact that if one died (due to overstress?) the rest may be overstressed as well.
Where are you planning to get 2SB554’s these days?
Where are you planning to get 2SB554’s these days?
I was planning on getting it from eBay. What do you think? They are difficult to find, especially ones that have a gain rating.
Get them from e-bay and you are guaranteed getting fakes, which will probably destroy the amplifier. Toshiba does NOT make them anymore (havent since 1990), and since they are so desirable any real ones will have been scooped up long ago. Second sources like ISC may or may not be any good - and most certainly will be different from the originals. Some of the stuff they make is ok, others are known to be garbage. I have never tried their D424/B554. Unless you have a source of these parts that’s been sitting around since the 80’s and you know where they came from, I would steer clear of them entirely. If you buy NTE285’s, they will be ISC’s B554, re-marked. If you want to try that experiment, do so with caution. Others will warn you to stay away from NTE replacements entirely - and I can’t say I disagree.
The most suitable replacement available these days is the ON MJ21193 for O grade, and MJ21195 for R grade. It is advisable to replace the other side (2SD424) with the MJ21194/6, but it won’t be fatal if you don’t. Any difference will likely be imperceptible, although it could measure a bit higher THD. The MJ15024 will work, but is not as good a match as the others. Again, ON Semi parts procured from E-bay are just as much a no-no for the same reasons.
The most suitable replacement available these days is the ON MJ21193 for O grade, and MJ21195 for R grade. It is advisable to replace the other side (2SD424) with the MJ21194/6, but it won’t be fatal if you don’t. Any difference will likely be imperceptible, although it could measure a bit higher THD. The MJ15024 will work, but is not as good a match as the others. Again, ON Semi parts procured from E-bay are just as much a no-no for the same reasons.