Home Forums GuitarPCB Build Support muff'n question

This topic contains 11 replies, has 3 voices, and was last updated by  Billy 4 weeks ago.

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  • #6689

    brako
    Participant

    hello,

    like i’m french it’s not totaly clear…

    i’m finiching to build a civil war. like i put BC549C transistor, so what have i to put in R25? nothing?

    thank u !

    #6692

    Billy
    Moderator

    In the build notes it advises if you use a high gain transistor like the BC549C you should use a resistance of 33K as a starting point so I’d socket R25 and try different values

    The BC549C can go from 100 to 800 hFE according to the datasheet

    The ones I have all test using a DCA55 from 420 to 560 hFE

    #6695

    brako
    Participant

    in the build notes it’s notice for the 2n5133 or 2n133 transistor but anything for the bc549c…

    the ram, triangle, foxy and mayo used  2n5133 or 2n133 but all other used the bc549c.

     

    #6696

    brako
    Participant

    when u say high gain transistor, so this is the part of the effect who influence which gain? input gain? disto gain? output gain?

     

    #6699

    Billy
    Moderator

    It says in the build document

    R25-As noted at the bottom of the parts list, the 2N5133 was lower gain, in fact, the specs state the minimum gain as 60hFE, whereas the 2N5088 has a minimum gain of 300hFE. If you use a 2N5133, R25 should be a jumper. If you use a 2N5088 or other higher gain transistor, you will need to insert a resistor in R25 to keep the pedal from squealing when all knobs are turned completely up. 33K is a good value to start with. A stock Mayo calls for the2N5133 and a 2k7 value in R25, but this same guidance applies if you change transistors

    A BC549C is a higher gain transistor, the gain in this case refers to the hFE or β (beta) value of the transistor

    http://www.learningaboutelectronics.com/Articles/What-is-hfe-of-a-transistor

     

     

    #6704

    brako
    Participant

    ok thank u very much!

    i understand a little more 😉

    so i think that i have to use a roto4 on R25 to test different value..

    1 : jumper

    2 : 33k

    3 : 2K7

    4 : ?

    is it a good idea?

     

     

    #6705

    Billy
    Moderator

    Use SIL single inline sockets

    Just cut the number you need (2) and solder them into the pads the idea is to stop the transistor oscillating (squealing) I’d start with 33K as advised in the build document place the resistor in the sockets turn all the controls to maximum and see if it squeals if it still squeals try a higher value if it doesn’t you can try lower values if you want

    as an example I’ve used them here to test different transistors in a PUFF build what I do is rather than remove them once I decide what component I’m using I simply solder one component leg into a socket to hold it in place

     

    #6707

    brako
    Participant

    ok ok!

    and what product a “squealing” in the sound?

     

    #6708

    Billy
    Moderator

    Q4 the original was a low gain 2N5133 which didn’t require resistance at R25 having a low minimum hFE of around 60

    Because you are using a higher hFE BC549C you will need some resistance at R25 to stop the transistor being amplified too much causing it to oscillate and make a squealing sound when all the controls are at maximum

    The hFE value is the factor by which the base current is amplified so for example if your base current is 1mA and you are using a 2N5133 with a hFE of 100 your current at the collector (output) would be 1 x 100 or 100mA

    If you use a higher gain BC549C with an hFE of 470 for example then it would be 470mA over 4 times as much and this may cause oscillation causing it to squeal so you add resistance to stop that happening

    Put in SIL sockets and experiment for learning, start with a jumper and see if it does indeed squeal then add resistance 10K at a time until it stops

     

     

    #6715

    brako
    Participant

    i’ll test with the roto in order to have different control on R25, i think it’ll be an interesant parametre.

    thank u prof!

    #6732

    Matt
    Participant

    Correct me if I’m wrong, but if you’re trying to find a good R value, couldn’t you wire in a pot and test it until it squeals and then back it off and measure it? Or am I overthinking this?

    #6739

    Billy
    Moderator

    Yes you can wire a pot as a variable resistor and  measure it as you say Matt

    Either sockets or a pot would do the job, I’d use sockets myself because you wouldn’t have to desolder them and start at 33K as suggested

    I think Brako likes the idea of a varying squeal looking at what he says!

    I personally wouldn’t use a roto tone you’re unlikely to get much of a varying squeal and it’s a bit of a waste if for some reason you want it I’d suggest a DPDT with resistance on one throw and none on the other

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