The general answer to this question, and the several zillion others that all have the form
"I need a part number [insert number here], will a [insert 'nother number here] work OK as a substitute?"
is "look up the datasheets for the two parts and compare the specifications".
If you don't (yet) have the technical background to do the comparison, then the question becomes "I've looked up the datasheets for [insert number here] and a [insert 'nother number here]. They look very similar - will they work work OK as substitutes?" The people who might help you will be very pleased if you give a link to the datasheets so they don't have to go do their own searches to find them.
In many cases, even a cursory reading of the datasheets will tell you if the answer is "no". In the case of the BS170 and BS250, the 170 is an N-channel and the 250 is a P-channel. These are fundamentally different, and so a circuit that works with one will NOT work with the other in all but the most degenerative cases, like using the body diode as a clipping diode. Anything that uses their transistor properties won't work.
jrc386D , well i have a jrc386BD same question .http://semicon.njr.co.jp/njr/hp/productSearchResult.do
New Japan Radio Corporation has the confusing practice of listing their stuff as "NJMxxxx" and labeling it "JRCxxxx". These are listed as the NJM386xx on the web site.
The "B" version is rated for higher power supply voltage and bigger output power.
the jrc , is that the same as a lm ? ( lm386 = jrc386 ? )
That's a good guess, and sometimes, but not always, the letter prefix and suffix indicate that. But you can never trust it fully till you actually look at the datasheet for each maker. In many cases, a second-source manufacturer will say in their datasheet that "this device is pin-for-pin compatible with the YYYYY". Sadly, JRC does not do that. When that happens, you have to compared the actual content of the datasheet for pin designations and device ratings.
But doing that, I see that yes, the JRC386... is pin-for-pin compatible with the LM386. Good guess, but always verify.
i read about lm386 1 , 2 , 3 , 4 being like 1 is the weakest output , the 4 being the strongest .
should i have assumed that the b would represent a 2 in that sequence ?
No. You should have thought "hmmm... it's possible that the B version is a higher power version" but not trusted it until you looked it up. In this case, it was a good guess that it was a higher power version, but I believe that the B version is like the "-4". Note that I have looked up the JRC386BD, but not the LM386-4. There's still some looking left to do.
the uglyface layout i am using calls for a ICM7555 , i have a 555cn cmoss 555 timer , same questions .
Same general answers.