tips on selecting replacement transistors when the original type is unknown

[Ptron]

I'm wondering if anyone has any tips for choosing replacement transistors when you don't know what the originals were.

In my particular case I'm looking for driver transistor for a small amp built into a Wurlitzer electric piano. It's a quasi-complimentary configuration so I would replace the other driver as well. The original says TMPS A05 which could be a house # as I can find no cross reference match. Here's what I know:

- It's silicon
- It's NPN
- It's TO92
- It's pinout is EBC (although that's easily dealt with by twisting the legs around)

I was thinking I would just use somthing like %150 of the rail voltage as a guidline for min. voltage rating. (although I think the rail is pretty low and not many transistors wouldn't fit this guidline) Then I would use V(rail)/R(collector) for a min. current rating. From there I guess the choice is pretty arbitrary? Does this seem sound?

In the past I had a similar situation but it was a preamp transistor. I just threw a 2N3904 it worked fine.

[Rick Erickson]

The original says TMPS A05

That would likely be an MPSA05, A very common NPN small signal transistor. An MPSA06 can be substituted. You can check the specs and chose another transistor, but to my knowlege, the MPS series is still readily available.

RE

[Amp Kat]

Use MPSA-05 High Voltage, general purpose amp. TO-92. CB voltage 300v, CE voltage 300, 0.5 amps max collector current and EB voltage 8v. Sometimes it's a manufacturors code that's pretty hard to tell .

[Rick Erickson]

Use MPSA-05 High Voltage, general purpose amp. TO-92. CB voltage 300v, CE voltage 300, 0.5 amps max collector current and EB voltage 8v. Sometimes it's a manufacturors code that's pretty hard to tell .

The MPSA05 is a 60V device, which should be plenty for a Wurlitzer SS piano amp driver stage. It sounds like you are quoting the specs of an MPSA42, which will handle 300V.

RE

[Amp Kat]

The MPSA05 is a 60V device, which should be plenty for a Wurlitzer SS piano amp driver stage. It sounds like you are quoting the specs of an MPSA42, which will handle 300V.

RE

Damn Rick, that's what I get for trusting the NTE / ECG cross refernce book
It crosses the mpsa05 to a 287 but it also crosses several others so they just use the higher rated part which is the mpsa42 for all of them. thanks,

[Rick Erickson]

I see that on ebay a lot. A seller will list a generic part # and then quote the NTE cross reference part's specs, like there's any correlation between the two.

[Ptron]

Thanks folks. I've seen the MPSA prefix before but I didn't put it together. I was fooled by the way it's written on the transistor: TMPS is one "word" and AO5 is another.

I am curious though what to do when you really don't know, like when the original isn't there and there's no schematic available. One can make a reasonable guess as to weather it's silicon or not by looking at the other transistors and if it's still not obvious, measuring their Vbe drop. Rail voltage can be found and measured. Pinout and polarity could probably be figured out by the place/configuration it's used in but I suppose there are a few that would stump me. But what about from there?

[Enzo]

Transistors are incredibly flexible. In a tube circuit you cannot just slap a 12AU7 in there when you run out of 12AX7s. But in a transistor circuit, ANY small signal transistor will work in most of our circuits as long as voltage and current ratings are considered.

Unless it is real old or an effect pedal, it won't often be germanium. I don't recall the last time I needed a Ge xstr. I think I sent a bag of them to KB several years ago and that was the last i touched a germanium. You can figure NPN or PNP from the application. In a small effect, you may have to decide if it is a bipolar or a JFET by looking at the application, as well.

First match the package. TO92 is the small signal xstr package. TO220 and TO3 are common power packages. You want something that will fit. For TO92s, also consider continent of origin. American types like 2N3906, MPSA06, etc are pinned out EBC left to right looking at the flat side. Asian TO92s are ECB. There are SOOOO many types available in both pedigrees that it makes little sense to look for a sub from the other side of the ocean unless it is a right now emergency repair. By the way, I don't mean where they are made, I mean where the number originated.

Now consider the voltage. In the circuit, a xstr may have only a volt or two across it. But in almost any circuit it is possible for the part to see the entire rail voltage at some point. Other parts fail or just large signal peaks. SO always select a voltage rating higher than the supply rail. In little signal circuits, it isd usually a single rail, but in a p-p output stage there are both pos and neg rails, and they add together. +/-50v rails means potentially 100v across a part. So 140v or better for them.

Now look at current. This is more important in power xstrs, but even so pay attention. A small xstr mught be driving some LEDS or relay coils, and that might need more current that just amplifying a signal. Small xstrs might have a current rating of even 800ma, while another has a rating of only 50ma. They will all work, but if the application is a input stage of a pedal, then a heavy duty MPSA06 might not be the best choice. Like using a 300 watt speaker on your 12 watt amp. It works, but... On the other hand it is not a critical thing, I worry about enough a lot more than I worry about too much.

Gain - hfe - you might see something in the gain rating like 50-400. WHat that means ot that the gain of the part is in large part dependent on the circuit it finds itself in. I don't worry about it so much with signal xstrs. I like to keep it in the ballpark.

SOme transistors are Darlington xstrs. These xstrs are like two xstrs in series in one package. It gives extra high gain - like 1000 instead of 50. SOme power amps use darlington output transistors since they sort of act as their own drivers, saving a part or two. You cannot use a regular xstr in place of a darlington xstr, and vice versa.

Power. I do have to consider the power dissi;pation rating when talking power xstrs, but it is not usually an issue with the small signal types.

Package/voltage/current is my main mantra, so I don't worry about the...(wait for it)...Oms. (sorry)

I stock a pretty large selection, moer than I probably need to to get by, so I often have the exact part. Designers often use common parts because they are already in the OEM inventory, or they are familiar and easy to source. Just like a tube designer uses 6L6s and 12AX7s instead of something similar but rare. I have favorites, like the 2SC1815, 2N4401, whatever. I keep some low noise types for where it matters, and some high voltage types like those MPSA43/MPSA93.

The MCM catalog has a page with a Transistor Selector Guide. It lists a bunch of very common types in each package, many with their complements listed. It is a place to start.

[Ptron]

Hey thanks Enzo. Another post of yours I've added to my saved collection.

[Steve Conner]

Enzo is dead right about the transistors. Most of them are more or less interchangeable. I've always seemed to get by with the following:

NPN:
2N3904 (basic small signal transistor)
2N5551 (higher voltage)
MPSA42 (even higher voltage)
MJE340 (high voltage, more power)
MJ15024 (for when you really want to go up to 11)

and their PNP complements

[Paulo Palmieri]

Hello everybody,

Iīm new here and I would like to participate to trade technical information.
Iīm a repair tech in Brazil and I do maintenace in vintage electric pianos, synths and guitar amps.

Iīm restoring a wurlitzer 200A and almost all of the transistors was replaced by similar and I couldnīt find the original replacements. 2 of those are BC547 and BC338! I believe that they are not correct for this case...

The problem that occurs is basically a distortion in the sound.

I just have the original part number but itīs not a common number, itīs a manufacturer part number and I didnīt find anything on the web.

Iīm needing the following cross reference parts for the Wurlitzer transistors with respective manufacturerīs number:

*137159-1 (NPN) TO-92
*203719 (PNP) TO-92


Any help would be much appreciated!

Thanks!

Paulo Palmieri
www.vintagesynth.com.br