Transistor question

[dlazzarini]

I have a hard time finding certain transistors. Still trying to learn what equivalents are out there. What could be substituted for a bc550c? I can find bc550cta, cbu, a1 etc. will these all work?

 

[Nostradoomus]

Don’t worry about anything after BC550C, they’re all different manufacturing codes for the same series of transistor.

 

[dlazzarini]

Thanks I kind of thought so.

 

[dlazzarini]

Don’t worry about anything after BC550C, they’re all different manufacturing codes for the same series of transistor.

I have a few bc548c’s. Would these be comparable or is there a noticeable difference

 

[Chuck D. Bones]

BC550C are low-noise, high gain NPN silicon transistors in a TO-92 package. 2N5088, 2N5089 & 2N5210 are all good replacements. The BC550C is rated for 45V, 2N5210 is rated for 50V, the others are rated for 35V. Should make no difference in 99% of the pedals we would build. One other thing to keep in mind when substituting transistors: sometime the pinouts are different. Such is the case here. The pinout for 2N5088, 2N5089, 2N5210 and most other TO-92 transistors is shown below. The BC550 has C & E swapped. If you sub a 2N transistor for the BC550, you have to put it in "backwards."

 

[dlazzarini]

Now I’m confused. So the bc448c doesn’t fall in the same category as the 550c? This needs to be put in backwards

 

[Chuck D. Bones]

I think you meant BC548C. It has the same pinout as BC550C. BC548C is noisier than BC550C. Depending on where it is used, you may notice a difference. BC548C is rated for 30V, compared to 45V for BC550C, so I would not use it in the Triangulum, Isoceles or Integral Preamp pedals. Datasheet is attached.

 

[dlazzarini]

Thanks Chuck. I’m just trying to get a better understanding of the parts I’m using and so far transistors seem to be the most confusing to me.

 

[Chuck D. Bones]

Understandable. There are quite a few different transistor types and many variations on how they are used in a circuit. Then there's the datasheet, crammed with arcane information, most of which is of little interest to a pedal designer. Short of getting a degree in Electrical Engineering, you might search the 'net for lecture notes from introductory college courses on transistor circuit design. That's where I find some of the best design and analysis information. You might have to wade thru a little math though.

If that doesn't work for you, I'm happy to answer questions.

 

[Nostradoomus]

I wish I would have went to school for EE but I didn’t really become interested in it til my late 20s and now there’s no way in hell I can afford that haha. The autodidactic route is working out just fine though.

 

[Chuck D. Bones]

You didn't miss much, most of the practical electronics I learned was outside of school.

 

[Nostradoomus]

Yeah I’d just like a little grasp on theory but much like guitar that stuff went out the window for feel and hands on approaches haha

 

[dlazzarini]

Thanks for all of the input and a push in the right direction. Yeah the data sheets are where I was hitting a road block. Couldn’t find anything that was helpful at all. I’ll see what else I can find following your lead. Thanks again

 

[Chuck D. Bones]

These are the most useful (to us) pieces of info in a bipolar transistor datasheet:

Absolute Maximum Ratings - don't exceed any of these, not even for a millisecond. If you stay under 75% on all of them, your transistors will outlive you.

ICBO - collector cutoff current, AKA leakage current. A big deal with germanium transistors because most of them are pretty damned leaky. Pedal designers exploit this phenomenon. Some pedals, like Tone Bender, require a certain amount of leakage to work properly. With the small-signal silicon transistors we use in guitar pedals, ICBO is so low that it might as well be zero.

HFE & Hfe - current gain is important with germanium transistors because their gain can be pretty low. Usually doesn't matter much with silicon transistors because the ones we use have gains well in excess of 100, even at the low currents in guitar pedals. Fuzz-Face circuits are sensitive to HFE and ICBO, which is why a lot of them have bias trimmers and/or require transistors that are hand selected.

NF - noise figure tells us how much noise the transistor adds. Lower is better. Only the low-noise transistors advertise their noise figure. The others are too embarrassed.

Packaging information - pay attention to the pinout.

All that other stuff in the datasheet can be ignored for guitar pedals. This includes power dissipation and thermal characteristics because in 90% of pedals, the transistor dissipation is a less than 10mW.

 

[dlazzarini]

Thanks Chuck. This is very helpful

 

[giovanni]

You didn't miss much, most of the practical electronics I learned was outside of school.

+1 to that. I learned theory in college but I never learned how to read a data sheet, how to solder, how to test/debug, etc. until I started building pedals.

 

[Chuck D. Bones]

OJT: On the Job Training

It's the kind of learnin' that sticks with ya.