[SOLVED] in finding an equivalent transistor, what are the important parameters?

[maark6000]

I'm trouble-shooting a pre-amp from a 70's Fender Rhodes electric piano. Pretty simple device... but I realized that I should poke around to see what equivalent transistors I might need. With rare exception, the designer selected 2N3392s for his go-to transistor. Amplification, buffering, tone controls... all 2N3392s. The top rail is 25 V, single sided. The CBO's and CEO's are actually a bit wimpy at 25 V for this device, but the continuous Ic current is an impressive 500 mA. Hfe is between 150 / 300 (min / max). Since this is an audio device, albeit NOT hifi by any means... what are the important parameters for me to hit in choosing a replacement? Do I find one with a similar Hfe? Do I worry about finding something with an Ic current max of around 500 mA or better? Do I worry about low noise?

So far, in staying in the TO-92 family, the transistor I do have lying around is the 2N2222. 1A Ic current and similar Hfe.

What do you think, is that a decent choice to replace the 2N3392? Below is the schematic:

 

[Warpspeed]

Just had a thumb through my ancient transistor data book.

The 2N2222 would probably work, although its not a perfect match, and it would mean crossing over its legs. Not my choice !

A much close equivalent electrically and also in a TO-92 plastic pack with the correct ECB connections would be a 2N2925. This should still be a fairly common easy to find device.

Just had quick a look at e-bay, and they are there.
https://www.ebay.com/sch/i.html?_od...RC0.A0.H0.X2N2925+.TRS1&_nkw=2N2925+&_sacat=0

If you are willing to go to a TO-18 metal can, a BC107 would also be correct electrically, although the pin out of this is triangular.

The book of words describe all three devices as low noise, high gain, general purpose amplifier, from 1uA to 100mA collector current.

 

[Audioguru]

The original 2N3392 was called "general purpose", not low noise. The circuit has low currents in the transistors so 500mA max does not matter. The transistors are reasonably biased so high hFE does not matter.
Many Japanese transistors like the 2SC945 have the same plastic case and pinning.
if you buy an old 2N2925 then buy a real one from a real electronic parts distributor like Digikey, not a fake one from ebay.

 

[maark6000]

Thank you! Tony, out of curiosity, what is this ancient transistor data book of which you speak?

 

[Warpspeed]

This National Transistor book was published in 1974, and at that time these devices were current items, so the data available was fairly comprehensive.

 

[maark6000]

So cool! I just bought one on e-bay for $12.

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can I ask another question regarding the 2N3392? Below is a picture from Mouser's website... apparently one can still buy the 2N3392, although I'm not sure it's the same transistor. Notice the @ Ic (mA) column, why is it 2.0 mA? Is that the max current the transistor can handle? That's a long way off from 500 mA.

 

[Warpspeed]

Notice the @ Ic (mA) column, why is it 2.0 mA? Is that the max current the transistor can handle?

If you look it says "Hfe @ 4.5 vots @ 2 mA
Its just specifying the typical kind of operating conditions under which the stated current gain has been measured.

The rated maximums are also quoted as 25v and 100 mA.

 

[maark6000]

ah, okay, seeing that now. derp! hmm, i suppose if they are still available, i should just get the same part. Also, according to mouser's datasheet, it looks like the 2N2924 is a closer match as far as hfe goes. I'll just get some of each, if one doesn't work i'll try the other. thanks so much everyone...

 

[Warpspeed]

Its pretty common for these types of devices to be mass produced by the millions.
Once they are spit out of the transistor making machine, they are usually automatically individually tested for Hfe gain, and sometimes other parameters.

If you look, the 2N2931, 2N2932, and 2N3933 are all identical, except for gain.
Its very likely they are absolutely identical devices, individually stamped with the appropriate part number after being sorted into batches after production testing.

In many well designed circuits the transistor individual parameters are not that critical. Almost anything will work satisfactorily.

If you are doing a serious restoration, its always nice to be able to source the original genuine parts, but it will almost certainly function perfectly well with a great many of the available alternatives.