Gain issue with old 2n3904s

Discussion in 'General Electronics Chat' started by Thecomedian, Nov 21, 2013.

  1. Thecomedian

    Thread Starter New Member

    Oct 12, 2013
    I got a lot of Old round can-type copper or "gold" lead mystery op-amps with no identifying marks, and plenty of 2n2222 and 2n3904-6 transistors from a family member, and I was building a few projects, such as

    and had some issues. At first, I thought I'd just found a wrong schematic because the "MUD" control only worked within the last 5-10% of the turn and another schematic shows only a 10k value there. However, I also specifically chose a 2n3904 with β=60-70 in Q3 position.

    I heard that it was possibly a bad transistor, so I tested the rest of the 2n3904's and got multiple values ranging in 60-90, multiples in 100-110, and the rest ranging in 120-180 range. I bought 10 2n3904's from an online parts store, and they all range from 200-250, with most sitting right around 235-245.

    Is it possible that age and heat have damaged the older transistors, or were those values normal for back when Archer still existed? Are the older transistors out of spec for the circuit?
  2. #12


    Nov 30, 2010
    The advertised gain for 2N3904 is 30 to 300. The likely scenario is that Archer got a good price for buying the lowest quality transistors that still qualify within the part number specs.

    It is true that long term heat will degrade transistors, but that is not the likely scenario here.
  3. Meixner


    Sep 26, 2011
    I agree with #12, Archer (Radio Shack) parts were junk.
  4. John P

    AAC Fanatic!

    Oct 14, 2008
    Now hold on a minute, guys. A transistor isn't "better" if it has a high gain. It's an adequate part if it meets its published specs, and if the data sheet says gain between 30 and 300, and it's in that range, then the manufacturer and vendor have done their jobs and you shouldn't complain. If you want a part with higher gain, go look for one. In fact, I'd call it a dangerous design practice to rely on a part with a characteristic that's at one end of its permitted range: the day might come when you order transistors to complete 10000 of your circuits on the production line and you can't find the component that functions the way you want. What then?
  5. #12


    Nov 30, 2010
    Order the right kind of transistors.
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
    As another (now absent) member used to say, a datasheet lists max, min and typical. Since you cannot buy a TYPICAL component, you have to design around what you might actually get.