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Transistor Question

Discussion in 'General Electronics Discussion' started by gomez, Feb 27, 2014.

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  1. gomez

    gomez

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    Feb 27, 2014
    Greetings!

    I'm in the midst of trying to restore an old Rock-ola 454 juke from the early 1970's. When I got to the power amp section, two of the output transistors were missing. The transistors are 2N3055c, which seem to be very hard to find.

    I don't understand enough about transistors to determine if there is a suitable substitute. As I mentioned, the original is a 2N3055c. There are other variations that are still available, including the 2N3055 and 2N3055G. They seem similar. However, there are slight differences in their specs. Given the application, the differences may or may not be relevant. If there are any transistor gurus out there who can help, I truly would appreciate it.

    Thanks!
     
  2. duke37

    duke37

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    771
    Jan 9, 2011
    The 2N3055 transistors seem to have a wide spread of gain. The 2N3055C could have a bit higher gain than the others.
    The TIP3055 may have a little more gain but is in a plastic package.
    A pair of cheap 3055s could be used to find if there are other faults and may be good enough.

    Since two have failed, they may be in a push-pull amplifier and they ideally they should be matched.
     
  3. gomez

    gomez

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    0
    Feb 27, 2014
    Thanks for the information. When you referred to push-pull amplifier, are you referring to type of design where one matched set is NPN and the other PNP?. A friend of mine had mentioned something to the effect that some amplifiers are designed where one set of transistors operate on the top of a sine wave and the other set operate on the bottom part. I'm a bit of a newbie and still trying to absorb some of the basic principles about how transistors work. One thing I do know from looking at the schematic; both channels are powered by two matching NPN 2n3055 transistors (no letter designation on the schematic). The actual unit, itself, has a matching pair of 2N3055c transistors for one channel and the other channel circuit board has only an empty space where, I assume, another matched pair once resided.
    I cross referenced the 2 (C & G) and only spotted 2 differences:
    Collector Base Voltage: C is 80V, G is 100V
    Collector Emitter Voltage: C is 60V, G is 70V
    Would these differences be anything to be concerned about?
    Pin position appears to be same.
    Thanks!
     
  4. BobK

    BobK

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    1,688
    Jan 5, 2010
    If those are the only differences, you can safely substitute the G for the C.

    Bob
     
  5. gomez

    gomez

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    Feb 27, 2014
    Thanks Bob!
     
  6. duke37

    duke37

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    Jan 9, 2011
    There are several ways of making a push-pull amplifier. I doubt if a 1960 amp would use a PNP transistor.
    To use two NPN transistors, they can be configured in series (totem pole) or drive a transformer.

    Matching is done by measuring the transistor characteristics.
    The transistors you have are similar, you have no information to say they are matched.

    Just for interest, show us the schematic of the output stage.
     
  7. OLIVE2222

    OLIVE2222

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    Oct 2, 2011
  8. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    My word, that is complicated.

    The HT is 64V plus variation so I would go for the G version. Maybe a higher voltage transistor could be found. There is feedback so you should be able to get away without close matching.

    Before fitting the transistors, check the voltage at the emitter of Q6, it should be about 32.5V and should be able to show an AC signal. Check C41, 1500µF, for leakage.
     
  9. gomez

    gomez

    4
    0
    Feb 27, 2014
    Output stage schematic

    Hey Duke,

    I hope these (3 attachments) come out OK. The schematic section for this unit folds like a map and measures 3' x 5' when completely unfolded. I did my best to isolate just the section that was needed. However, best I could do was in 3 scans.

    I certainly appreciate all of the help (from all contributors)....and learning more than I knew before I posted the question.

    After 3 unsuccessful attempts to get a decent image with file size restriction, I resorted to a file drop site. URLs below.

    Page 1 of 3:
    http://www.filedropper.com/rock-olaampsectschpg1of3_1
    Page 2 of 3:
    http://www.filedropper.com/rock-olaampsectschpg2of3_1
    Page 3 of 3:
    http://www.filedropper.com/rock-olaampsectschpg3of3_1

    Thanks, again
     
  10. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
  11. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    Filedropper seem to want me to download a program. I am not doing that.

    Olivier's suggestion should be considered.

    Many years ago I made a Lindsley-Hood amplifier. This used 2N3442 transistors in the output. These are rated 140V.

    I do not think the transistor type is critical in this application but it needs to be strong enough.

    The 2N3055 is in a TO3 case, this is common so it should be easy to find a transistor with this fitment. The transistor must be fitted to a heat sink, use thermal grease to make a good thermal contact.. You will need minimum thickness and maximum area.
     
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