Hello,
I hope this has not been covered previously, but I'm anxious to learn. Can anyone suggest a reference text or a web site that provides a comprehensive guide to selecting substitute bipolar and FET transistors? I currently have a copy of Towers' "International Transistor Selector" circa 1984. I also have an NTE replacement directory. But I believe this is outdated and limited in the information provided. If I knew what would be a suitable candidate for a replacement then I look at the manufacturer's data sheet for more complete information.
Secondly, is there a good tutorial about how to evaluate the data and determine if there will be any improvement in using it as a replacement? I've read about certain transistors being "faster", how can this be determined from looking at a data sheet?
Thirdly, can someone make a recommendation on a moderately priced test instrument to evaluate and match transistors? What should a person look for?
I certainly appreciate any information and guidance.
I hope this has not been covered previously, but I'm anxious to learn. Can anyone suggest a reference text or a web site that provides a comprehensive guide to selecting substitute bipolar and FET transistors? I currently have a copy of Towers' "International Transistor Selector" circa 1984. I also have an NTE replacement directory. But I believe this is outdated and limited in the information provided. If I knew what would be a suitable candidate for a replacement then I look at the manufacturer's data sheet for more complete information.
Secondly, is there a good tutorial about how to evaluate the data and determine if there will be any improvement in using it as a replacement? I've read about certain transistors being "faster", how can this be determined from looking at a data sheet?
Thirdly, can someone make a recommendation on a moderately priced test instrument to evaluate and match transistors? What should a person look for?
I certainly appreciate any information and guidance.
Hi,
In repairing stuff you often want equivalents if you have to use them
to be worse in terms of speed if anything, to maintain stability.
You don't want your repair to self destruct.
There so many transistor parameters, the importance of each varying
with how the transistor is used, generic "better" replacements don't exist.
Comes down to device selection in the first place, why that device ?
How should it carefully be chosen, and was it in the first place ?
What are the important parameters in that circuit position ?
You would need to know your design stuff.....
rgds, sreten.
Ft is the basic speed parameter, but junction capacitances can matter.
In repairing stuff you often want equivalents if you have to use them
to be worse in terms of speed if anything, to maintain stability.
You don't want your repair to self destruct.
There so many transistor parameters, the importance of each varying
with how the transistor is used, generic "better" replacements don't exist.
Comes down to device selection in the first place, why that device ?
How should it carefully be chosen, and was it in the first place ?
What are the important parameters in that circuit position ?
You would need to know your design stuff.....
rgds, sreten.
Ft is the basic speed parameter, but junction capacitances can matter.
I'll second everything sreten said, and advise you stay away from NTE generics, never had good results with them. 🙁
Mike
Mike
Moderately priced is a relative term - but - the Sencore Super Cricket tester is highly regarded and sought after. If you get one of these you can be pretty sure that you can get your $$$ back upon resale.
Sencore Super Cricket TF46 Transistor and FET tester | eBay
Attachments
In general, look for a replacement device that's physically the same (package and pinouts). Then for a breakdown voltage that's equal or better than the old device, and it should also be more than any possible voltage that can appear across the device under worst case conditions (some other part failed). Next, the speed should be similar. A bit slower may be safer, but not always. A bit faster may cause instability, but so can a slower device. Next, look at the gain (Hfe). You want to be in the same range, though you can often get away with surprisingly more or less. If it's a FET, make sure the gate capacitance isn't a lot different and that the circuit can drive it.
When I replace a part, the ideal thing is if there's another working one of the same type. I'll put that on the curve tracer and try to find something with similar characteristics, having hopefully already covered the stuff above.
Finally, there is an unpublished list (known only to the initiated) of devices that have traditionally given good performance in audio circuits. You can get an idea of these by looking at a lot of schematics and see what pops up frequently in certain circuit locations.
When I replace a part, the ideal thing is if there's another working one of the same type. I'll put that on the curve tracer and try to find something with similar characteristics, having hopefully already covered the stuff above.
Finally, there is an unpublished list (known only to the initiated) of devices that have traditionally given good performance in audio circuits. You can get an idea of these by looking at a lot of schematics and see what pops up frequently in certain circuit locations.
I've had good luck with NTE replacements, also the GE & RCA predecessor lines, but they cost too much now that Newark & mouser take debit cards. (used to have to buy locally unless you were a business and could be invoiced). I install bipolar transistors with faster Ft sometimes, but I put a capacitor across the B-E junction to slow it down- usually a 22 pf. To prevent unnecessary oscillation. Look at the famous "TIP mod" for the dynakit Stereo 120. replacing the old 5 digit RCA transistors with 2n3055 or TIP3055 caused all kinds of oscillation posibilities. In the power supply protector of that amp, I had to bleed off gain with a B-E resistor on the TO3 transistor, because the newer higher gain parts were regulating at 1.5 amps Vcc out instead of 6.5 amps. Looking at your device to figure out what happened helps assess the results of your substitutions.
No advice on Fets, only device I have with them is switcher power supplies.
I'm pretty bold, I repaired the ST120 the first time in 1985 with no schematic, no manual, no scope, and so much stuff was fried I couldn't tell if the power supply transistors were NPN or PNP or what the resistor values were. Tried some things, got it working, but didn't find out until 2 years ago that 6.5A was the P.S. limit, tested and found my ST120 was putting out 4 watts/channel. No wonder the St70 tube amp with bad tubes was louder.
Having a curve tracer or transistor tester is nice, but a dead amp with good power transformer is a lot cheaper. My B&K 2120 scope was $40, less than 2 probes at $50 each. Scope is very handy, especially when oscillation can happen.
No advice on Fets, only device I have with them is switcher power supplies.
I'm pretty bold, I repaired the ST120 the first time in 1985 with no schematic, no manual, no scope, and so much stuff was fried I couldn't tell if the power supply transistors were NPN or PNP or what the resistor values were. Tried some things, got it working, but didn't find out until 2 years ago that 6.5A was the P.S. limit, tested and found my ST120 was putting out 4 watts/channel. No wonder the St70 tube amp with bad tubes was louder.
Having a curve tracer or transistor tester is nice, but a dead amp with good power transformer is a lot cheaper. My B&K 2120 scope was $40, less than 2 probes at $50 each. Scope is very handy, especially when oscillation can happen.
Last edited:
Though you can still find the occasional Tek 575 at hamfests, they're a boat anchor and tube based. The more modern versions are still quite expensive, even though many are several decades old. I'd suggest searching the net for schematics, as they aren't hard to build as a scope accessory. National Semiconductor published an FET curve tracer for the scope many years ago, and that's not hard to build, but it only does small signal FETs. Member Gootee was selling a very nice curve tracer kit, but I don't think he offers it anymore- maybe PM him.
Thanks for the tip, Conrad. I seem to remember an article in Audio Amateur, 1993/1994 I believe about building a curve tracer. The Tek 575 is often on eBay, but I'm running out of room. I have a very clever Peak Atlas DCA semiconductor analyser that gives pin ID, device type, Hfe, Ic, Vbe, Ib etc. All in a "palm sized" package.
- Status
- Not open for further replies.