BC212 Transistor Question (Marshall amplifier repair)

Discussion in 'General Electronics Chat' started by sweatyk, Aug 14, 2013.

  1. sweatyk

    Thread Starter Member

    Aug 13, 2013
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    First of all hello to everyone and this is my 1st post here. I feel really dumb here and way out of my league with the knowledge that most of you guys have. But I have been staring at data sheets till I'm crossed eyed and have just one question. I'm replacing a BC212 transistor and of course they are obsolete. I did some scouting on the internet and found a recommendation of using a BC556 as a substitute. I have an order with Mouser Electronics and this is the last item I need. What they have in stock there is a BC556ABU. Is there much difference and will it work in place of the BC212? Any comments would be appreciated and thanks in advance.
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    The BC 556 might work, both are pnp and the 556 has a higher voltage and power rating but it depends upon the application.

    Both transistors come in a variety of pinouts depending upon end letter so you need to ensure a match.

    The BC 212 has a current rating of 200mA and a frequency Ft of 200 MHz, whereas the 556 has a currrent rating of only 100mA and and Ft of 150 MHz so you need to check the application here.
     
  3. sweatyk

    Thread Starter Member

    Aug 13, 2013
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    Thanks Studiot!
    I am also considering just biting the bullet and go to ebay to get the exact ones so I know it will be right. Unfortunately obsolete items come at a premium.
     
  4. Ramussons

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    May 3, 2013
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    #12 likes this.
  5. studiot

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    As RAS has said you have many possible replacements, the question is why did the original fail?
    Was it overcurrent, voltage or power?

    Have you identified the pinouts and where are you in the world?
    I have some BC212 and 212L stock.
     
  6. sweatyk

    Thread Starter Member

    Aug 13, 2013
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    Ras thanks for that link. I will save it for future reference. This is part of the output circuit on a solid state guitar amp (Marshall Valvestate) that I messed up on. It worked prior to this with the exception of the reverb. I was checking the output voltage on the IC that controls it when my hand twitched and shorted the output transistors. Given the age of the amp and the usage it has had I have decided to replace the transistors and diodes that were affected in this circuit. Most of which is available at Mouser except the BC212 and also the BC182 that I need. They did not read short but as a precaution I feel that changing them would be better so that I don't have to go back in it at a later date.

    Studiot I live in the US on the east coast in sunny Florida. If you had a couple of the BC212's and the BC182's I would like to see how much you would charge plus the shipping.

    Another question is looking at the data sheet for transistors what does the Hfe mean? When looking for a replacement or equivalent how important is that number?
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    You are welcome to a couple of each for the postage (they were European/UK types). They will go in a small envelope.

    But you need to determine pinout since they were produced in several configurations.

    The common ones are shown below. The 212 and 182 were complementary.

    The current gain Hfe (also called beta, β) is not likely to be relevant (ie adequate) as they were a good modern transistor.

    I can tell you the measured value of any I send if you wish.

    PM me.
     
  8. sweatyk

    Thread Starter Member

    Aug 13, 2013
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    On your drawing of the 212....isnt that backwards? From that view I thought it was EBC?

    Does this forum have a "PM". I didn't see one unless I'm not looking too hard.
     
    Last edited by a moderator: Aug 15, 2013
  9. studiot

    AAC Fanatic!

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    The pinouts are the conventional way looking at the transistor from the wire end ie up under it as it is mounted on the board.

    To send a PM, left click a user's name and choose send a private message.
     
  10. sweatyk

    Thread Starter Member

    Aug 13, 2013
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    The "PM" feature is not in the list of options when I left click. Not sure why.

    Just hit me up at <SNIP>
     
    Last edited by a moderator: Aug 15, 2013
  11. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The PM feature is not available for new members.
    You will get the feature once you have reached the 10 posts count.

    Bertus
     
  12. sweatyk

    Thread Starter Member

    Aug 13, 2013
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    Thanks Bertus!!!
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Here I have a zip-file with the data of a lot of transistors:
    transistor_tabel.xls.zip

    Unpacked you will find a excel sheet that you can search.

    Bertus
     
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  14. sweatyk

    Thread Starter Member

    Aug 13, 2013
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    Thanks for the link to that spreadsheet. Did you create that?
     
  15. sweatyk

    Thread Starter Member

    Aug 13, 2013
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    Studiot as soon as I can get to 10 posts I will PM you with the info. I had neglected to fully read the terms and agreement when registering here and didn't see that.

    Thanks Bertus for editing my post with my email. That was foolish of me.
     
  16. studiot

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    You should have an email to reply to.
     
  17. sweatyk

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    Aug 13, 2013
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    Being new to this sight I wasn't sure..........well there's my 10th post!!!!!!!
     
  18. studiot

    AAC Fanatic!

    Nov 9, 2007
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    OK, I have found a few BC212A which are the same pinout as the BC 212. So that is OK

    Needless to say I have dozens of the alternative BC212L type.

    The BC182 are more problematic as I have none for that pinout. They are all BC182L.

    Now I have plenty of both types of BC214C and BC183C which are part of the same series, but better transistors. In particular they are higher gain, lower noise types.
    The only thing is that they have a lower theorectical guaranteed breakdown voltage of 30 volts instead of 45.
    My BC212 measures 96 volts at breakdown.
    I have measured some and the actual breakdown voltage is 55-60 for the BC214 and 75 volts for the BC183.

    Again I have plenty of the L variety.

    Looking at the Marshall it has ±12 volt rails and ±45 volt rails, if it is the model I think.
    So the other transistors may be fine as they will not see the full rail.

    What do you think and what circuit information do you have?
     
  19. sweatyk

    Thread Starter Member

    Aug 13, 2013
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    Studiot
    It is a model 8080. The transistors are TR11 and TR12. I am attaching the schematic. Being you have way more knowledge with this I will trust your judgement. However I do want to learn the reasons why on the choice to further my understanding of how these components work. Let me know what you think. Thanks again!
     
  20. studiot

    AAC Fanatic!

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    First let me say that I have never worked on this particular model, so anyone with personal knowledge is welcome to chip in (pun intended) here.

    That is what makes AAC special - The high level of cooperation between members.

    Now did you say you are refurbishing this amp?

    The transistors in question are not actually necessary for the operation it should work perfectly without them.

    They form (part of) the protection circuit so if you are refurbishing you can remove them to get the main parts working properly. so long as you don't overdrive things.
    They work by monitoring the output darlington emitter currents across the emitter resistors, R118 and R96. They only turn on when there is sufficient current through these resistors to provide base bias and drive. They work by then shunting some of the darlington base drive signal to earth in overdrive conditions.

    Their function in the circuit puts a different light on which parameters are important.

    They will never see a greater voltage than the BE pedestal voltage if their respective darlington, TR8 or TR10 which will be somewhere between 1.5 and 3 volts I guess.
    So voltage rating is not an issue.
    Current rating may be another matter and why Marshall went for a 200mA transistor rather than the more usual 100mA small signal job, so we should stick with that.
    Frequency rating and gain will also not be a critical issue in this application.
    However the resistor pairs R117/R116 and R115/R97 will have been chosen in relation to the Vbe characteristics of the BC212 and BC182 so this characteristic is important in any substitution.

    How is the rest of the refurb going?

    That only leaves the issue of pinout to resolve.

    So please check if the centre terminal of TR12 is connected to the junction of R117 and R116 and not D5 or earth, and similarly the centre wire of TR11 to the junction of R115 and R97.

    Edit a rethink on the voltage ratings

    The output followers will have a voltage gain of about 1 so the earlier voltage amp stages of the power amp will need to swing the bases of the darlingtons, depending upon the load

    For 8 ohms the peak required is about 40 volts for 100 watts and 35 for 80watts
    For 4 ohms the peak is 28 and 25 volts
    For 2 ohms it is 20 volts and 18 volts
     
    Last edited: Aug 17, 2013
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