Hello!
I have a question about the output transistors in my Optonica.
It is equipped with 2SB558 / 2SD428.
Would there be any problems putting in MJ15024G / MJ15025 instead?
They are a lot beefier, but I have quite a few gathering dust.
I have a question about the output transistors in my Optonica.
It is equipped with 2SB558 / 2SD428.
Would there be any problems putting in MJ15024G / MJ15025 instead?
They are a lot beefier, but I have quite a few gathering dust.
In most cases you can put 15024/5 in place of any unobtainium TO-3 audio output. Even the super fast ones (which these are not). The only potential issue with extreme overkill is being able to adjust the bias low enough. Vbe is going to run lower on these. If it is hard-wired, the bias might run too high. If it’s adjustable you might run out of range. This is fixable if you know what you’re doing.
I even use them to replace impossible to source devices like Sanken LAPTs. When it’s a choice between doing nothing and binning it, using the 15024, or jacking in a flatpack, the customer will usually choose putting in something that fits. With the older Toshiba, Sanyo, Hitachi etc. parts I don’t even think twice - just put ‘em in and do what’s needed to fix the bias. I have other choices on hand that would be closer match (less overkill) but only because of collecting for 30 years. Now a 15024 is as cheap as any other alternative.
I even use them to replace impossible to source devices like Sanken LAPTs. When it’s a choice between doing nothing and binning it, using the 15024, or jacking in a flatpack, the customer will usually choose putting in something that fits. With the older Toshiba, Sanyo, Hitachi etc. parts I don’t even think twice - just put ‘em in and do what’s needed to fix the bias. I have other choices on hand that would be closer match (less overkill) but only because of collecting for 30 years. Now a 15024 is as cheap as any other alternative.
Thank you!
I noticed they were both pretty slow devices. Only thing I could find on google was issues about oscillation if faster devices was used.
I will give it a go!
I noticed they were both pretty slow devices. Only thing I could find on google was issues about oscillation if faster devices was used.
I will give it a go!
Yes, oscillation is definitely something to watch out for - it doesn't take long for an oscillating power amp to fry its Zobel network resistor. And if its borderline you won't know till it goes wrong, so you need to drive it hard with a 'scope and various different loads to look for the slightest sign of oscillation to gain confidence - this is a tweeter-destroying issue, and normal protection circuits are always going to catch this.
If a zobel network is drawing current, a dim bulb limiter will light up. And if the zobel resistor is getting warm you know to dig into it. The speed difference is negligible - both single digit MHz. I seriously doubt oscillation would be an issue.
Thank you for replying!
Should I use a dummy load and drive the amp hard when looking for oscillation?
Should I use a dummy load and drive the amp hard when looking for oscillation?