Some transistors questions .

[seadi123]

Hello everyone I will build some pedals with singe-transistor design so i will need some transistors . So for J-FETs i will buy J201's , to build preamp/buffer/boost . For darlingtons i will use MPSA13 (for fuzz) . Now there seem to be so many alternatives to NPN transistors ; 2n3904 , 2n5088 ,BC107 , BC108 , MPSA18 etc.. . What is the difference between these transistors ? I guess their values .

So , now i should ask , what does the mhz frequency and Hfe mean ?
How does the gain of a transistor affect the tone (mor gain = more clipping ?? ) ? .
I will use these NPNs for overdrive circuits . What if i find a Germanium substitute for these transistors , how will it affect the tone ?

Thanks

PS. i will buy these transistors in bulk (10x) , that's why i'll buy just 1 of a kind

[Seljer]

Just look at a datasheet, there's probably at least dozens things you can differ transistors by.
How much voltage can it withstand before failing? how much current can it withstand before failing? how much gain does it have? up to how high of a frequency does it still work? what kind of package does it come in?

For typical stompbox use you don't have have to worry about the current and voltage ratings too much, the transistor probably won't see more than 9V and at most a couple of 10mA (this of course, isn't the case if you're building and amplifier or using a transistor to switch on a motor or such). Bandwidth isn't that much of an issue because audio is up to 20KHz, you're not building a radio or a fast digital circuit here!

Gain probably has the most effect on sound in actual circuit, but that entirely depends on how the circuit is designed so you have a whole lot of other parameters influencing things. If you look at a transistor datasheet the range specified for gain is usually very wide. Circuit designers typically employ methods that make a circuit stable in regard to these part variations, because having to match or hand select transistors is a nuisance for mass production.

All that said, get yourself a big pile of "general purpose NPN" and a slightly smaller pile of PNP transistors, probably only two types of each (one with higher/lower gain). I don't really remember every using a darlington more than two or three times, if you need one you can always make the arrangement out of two individual transistors.
That'll cover almost everything for bipolars. If you have a specific desire to build odd fuzz pedals then look for various older ones and germaniums as well.

J201s have a very low cutoff voltage making them kind of strange compared to typical JFETs, get some other ones as well! (though a lot of them aren't in production any more in through-hole packages)


Heres what R.G. Keen has to say about this business on geofex http://www.geofex.com/effxfaq/keenslaws.htm

6. Transistors: When in doubt, use a 2N5088. Or a 2N3904 - or a 2N4401 - or a BC549 - or... find cheap, available NPN and PNP devices that you can get easily and use them. In the USA, 2N3904 (NPN) and 2N3906 (PNP) are easily available and can be found almost everywhere. I personally prefer 2N5088 (NPN) and 2N4250 (PNP) because they have slightly better gain and lower noise. 2N4401 (NPN) and 2N4402(PNP) work almost as well. Find good parts you can get, and then figure out why they **won't** work, not how to find magic parts from Timbuktu.

For small signal MOSFETs, the problem is easier. Get 2N7000 or BS170 for N-channel and BS250P for P-channel.

JFETs present a bigger problem. N-channel JFETs vary all over the map, and by an almost intractably large amount. The big problem is that the variation of Vgs and gm values is huge for these parts. This is one place that you may have real problems. However, almost all JFET circuits are set up with trimmer resistors to adjust the operating conditions for the JFET because of the large variation in the JFET. I personally stock 2N5485 and 2N5457 JFETs. BF244C is almost exactly the same as the 2N5485.

On all transistors watch the pinout like crazy . Assume that the device you actually have in your hand is probably different from the device specified in the effect and go look up the pinout for what you actually have. Never assume that a replacement has the same pinout - check it! This one step would probably eliminate half the non-functional first-timer's pedals. The data sheets are available on the internet in almost all cases. Go do the search. Find them.

[seadi123]

Thank you very much I'm still not clear about the gain of the transistors . How does more gain or lower gain affect the tone ?

I found some good Ge transistors on ebay , General Purpose NPN , Hfe min/max = 15/30 http://www.ebay.com/itm/10x-MP37-Russian-USSR-NPN-Germanium-Transistor-Rare-AC176-103NU70-2SD75-/261478604039?pt=LH_DefaultDomain_0&hash=item3ce156dd07

[bluesdevil]

Those Russian GE NPN's are too low in hfe. Just grab some common silicon 2n3904 and 2n5089 (or 2n5088)  NPN's and that will cover most projects. I recommend getting these and J201 jfets from Aron's store on this forum.

[thelonious]

I'm still not clear about the gain of the transistors . How does more gain or lower gain affect the tone ?

Pretty much this:

Gain probably has the most effect on sound in actual circuit, but that entirely depends on how the circuit is designed so you have a whole lot of other parameters influencing things.

In other words, you can talk about how gain affects tone in one circuit, but it's hard to talk about how gain affects tone in all circuits because the transistor will act differently in different circuits. The input impedance of the transistor is roughly the hFE times the emitter resistance, so *in the same circuit* a lower gain transistor might sound warmer because of lost high frequency content. It will also sound a lot quieter. Higher gain transistors will probably sound brighter and louder, and they will push things after them into clipping and/or run out of headroom and sound dirtier. But you can compensate for most of those things by changing other components in the circuit.

[seadi123]

Thank you very much guys