I just acquired an amplifier with two bad output transistors. They are no longer available. In fact, none of the suitable substitutes are available either. But I found one that seems to be pretty close. However, they differ substantially in something called the "current gain bandwidth product" which is 4Mhz on the replacement unit and 30Mhz on the original part. Everything else is pretty good. So what is this spec? And what does it mean, exactly?
That is the “speed” specification. It’s the frequency at which the AC current gain extrapolates to 1. Let me guess - you are replacing an old (obsolete) TO-3?
Yep, you got it! I cannot believe how few transistors there are available in the TO-3 package. NOTHING is available. My amp (a Sansui AU-919) is useless without some work around.
One dude from Croatia took some suitable TO-3P packaged units and soldered on some round pins to the emitters and bases and a screw lug onto the collector and inserted them into the through holes, and screwed the collector pin down too. That looks monster difficult. I’m thinking of just mounting them to the other side of the heat sink and forget the pass through. May have to drill a few holes tho. I’m in the contemplative stage. Pretty bummed tho.
One dude from Croatia took some suitable TO-3P packaged units and soldered on some round pins to the emitters and bases and a screw lug onto the collector and inserted them into the through holes, and screwed the collector pin down too. That looks monster difficult. I’m thinking of just mounting them to the other side of the heat sink and forget the pass through. May have to drill a few holes tho. I’m in the contemplative stage. Pretty bummed tho.
I am very new to solid state, and I have another question. The metal case of a TO-3 is the collector, isn’t it? Don’t the screws that tie down the metal care to the heat sink ground the collector? I know the mica wafer under the transistor is supposed to insulate it from the hear sink, but how to you prevent the screws from grounding the collector?
And also, how do you connect anything to the collector? Nothing is ever soldered to the metal case of a TO-3. Or to the screws that secure it. So what am I confused about?
Mounting screws will NOT ground the collector of either TO-3P L or N package. There is an annular ring of plastic surrounding the hole. The real TO-3’s require the extra step of putting a plastic bushing around the screw where it goes through the heat sink. In that respect, the plastic packages are easier. A piece of #18 stranded wire can be used to connect the center leg to the OPPOSITE mounting hole of a flatpack. Heat shrink should be used to insulate the connection. It is sometimes preferable to re-drill the heat sink entirely, to properly mount flatpacks. In any case, you want good solid mechanical construction, that can stand up to normal handling of the equipment. That is NOT always possible. You do not want a rats nest with flying leads waiting to be broken off. No amount of SOA will protect you there.
Using MJ21193/4 will almost always WORK to replace high speed TO-3 outputs. Whether or not you return to original performance depends on the circuit design. The MJs are sustained-beta devices, just not any faster the old epi-base parts and hFE is toward the lower end of the spectrum for sustained-beta types. Some circuits will work just as well with those (ie, no significant change in distortion spectrum) others won’t. It also depends somewhat on the original designer’s choice of other parts like the drivers and their bias currents. You may or may not even have the equipment to verify this.
Using MJ21193/4 will almost always WORK to replace high speed TO-3 outputs. Whether or not you return to original performance depends on the circuit design. The MJs are sustained-beta devices, just not any faster the old epi-base parts and hFE is toward the lower end of the spectrum for sustained-beta types. Some circuits will work just as well with those (ie, no significant change in distortion spectrum) others won’t. It also depends somewhat on the original designer’s choice of other parts like the drivers and their bias currents. You may or may not even have the equipment to verify this.