EDIT (even before the first reply) to say I realize this can be a pretty broad topic... just looking for a basic starting place to begin learning from.
What do I need to look for besides:
-Polarity
-Vce max
-Ic Max
Terms I've recently come across, that i wasn't aware of their relevance in design spec:
-frequency Ft (high freq osc can reduce gain to near zero)
-epitaxial vs homotaxial (from what I've recently learned epi will require emitter resistors to compensate for Rbe whereas homotax not always)
I'm sure there's more to consider that I'm missing. What should I be thinking about when I make a substitution?
What do I need to look for besides:
-Polarity
-Vce max
-Ic Max
Terms I've recently come across, that i wasn't aware of their relevance in design spec:
-frequency Ft (high freq osc can reduce gain to near zero)
-epitaxial vs homotaxial (from what I've recently learned epi will require emitter resistors to compensate for Rbe whereas homotax not always)
I'm sure there's more to consider that I'm missing. What should I be thinking about when I make a substitution?
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I would also check:
- Hfe because you would like this to be similar otherwise your drivers might not be able to drive them
- capacitance, similar to above
- size, package, dissipation
- Hfe because you would like this to be similar otherwise your drivers might not be able to drive them
- capacitance, similar to above
- size, package, dissipation
Very unlikely you will get any hometaxial (not homo BTW) transistors unless they are genuine RCA NOS or pulls. Safe to assume all transistors will be of the epi type and need emitter resistors. Even with old hometaxials I still recommend and used emitter resistors. Useful for short circuit protection circuits and temperature stability.
It’s not just about ratings. You have to be concerned with fT ( relative speed) and how beta varies with collector current. The driver transistor needs to be faster than the output transistor, or bad things always happen. Both need to be *fast enough* for the application. That depends on how much open loop gain the amp has and how it is compensated. There is no one size fits all. The flatter the beta versus current, the more linear everything is. The higher the distortion performance you expect, the better this has to be. In all cases beta linearity is much more critical in the DRIVER than it is in the output. Get it there first (and it’s easier). High performance Japanese drivers used with old school epi-base Motorola
outputs can result in very high performance if attention to detail is paid.
The whole hometaxial vs. epitaxial thing has to do with ruggedness. Hometaxials were the best, handing their full power up to at least half, and in many cases, all of their breakdown voltage capability. Power handing in ALL other types falls off power transistors falls off at increased voltage. Generally, the faster they are, the worse it gets - but there is no hard and fast rule. You have to consult the device data sheet, or use types known to be “better” than their spec.
You cannot buy hometaxial power transistors anymore. No one makes them. Best you can do is old stock. There are many types of epitaxials made now, and triple diffused types, ring emitters, and hybrids with different features. You can get ballasted multiple emitter types using epi-base or triple diffused princesses. Most high performance audio outputs are one of those two types. Many get similar ruggedness to the old hometaxial types. Many far exceed their spec. Again, you have to look at the data sheets and take on as much tribal knowledge as you can get your hands on.
outputs can result in very high performance if attention to detail is paid.
The whole hometaxial vs. epitaxial thing has to do with ruggedness. Hometaxials were the best, handing their full power up to at least half, and in many cases, all of their breakdown voltage capability. Power handing in ALL other types falls off power transistors falls off at increased voltage. Generally, the faster they are, the worse it gets - but there is no hard and fast rule. You have to consult the device data sheet, or use types known to be “better” than their spec.
You cannot buy hometaxial power transistors anymore. No one makes them. Best you can do is old stock. There are many types of epitaxials made now, and triple diffused types, ring emitters, and hybrids with different features. You can get ballasted multiple emitter types using epi-base or triple diffused princesses. Most high performance audio outputs are one of those two types. Many get similar ruggedness to the old hometaxial types. Many far exceed their spec. Again, you have to look at the data sheets and take on as much tribal knowledge as you can get your hands on.
All great answers and thank you! I found myself in a situation with an old power section (70s bass amp) and gained an education on a few things I was completely unaware of.
I like the idea of adding emitter resistors for safety's sake even on homotaxial transistors. I remember hearing a tech say he liked to add 1ohm (maybe 1w can't remember) cathode resistors on power tubes to sort of act like a fuse and protect the tubes in case of a fault.
I like the idea of adding emitter resistors for safety's sake even on homotaxial transistors. I remember hearing a tech say he liked to add 1ohm (maybe 1w can't remember) cathode resistors on power tubes to sort of act like a fuse and protect the tubes in case of a fault.