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    SP9 Octal version hum

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    garageguy55

    Posts : 4
    Join date : 2010-01-04

    SP9 Octal version hum

    Post by garageguy55 on Fri Mar 08, 2013 8:07 am

    A while back I built an octal version SP9 pre amp . It went together without any problems and sounds much better than the Pas 3 it replaced. The problem is noises on start up which go away and are replaced by hum. When measuring the pin out voltages, G1 of all four 6SL7 tubes reads about 50 volts instead of 129, all other readings are within limits. Any ideas?

    tubes4hifi
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    Join date : 2008-11-30

    Re: SP9 Octal version hum

    Post by tubes4hifi on Fri Mar 15, 2013 2:09 pm

    which version of the board do you have (v1, v2, v3)? Do you have all the B+ jumpers on the board?
    what are your P1 and C1 readings on those tubes with low G1 voltages?

    garageguy55

    Posts : 4
    Join date : 2010-01-04

    Re: SP9 Octal version hum

    Post by garageguy55 on Fri Mar 15, 2013 2:56 pm

    How can I identify which version it is? All the B+ jumpers are in place. All 4 tubes have low G1 voltages - P1 is 266 on all 4, and C1 is from 123 to 141.

    tubes4hifi
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    Re: SP9 Octal version hum

    Post by tubes4hifi on Fri Mar 15, 2013 4:50 pm

    unless you have version 1, all boards are marked in the top right corner with the version number.
    It should also be noted on the documentation you received with the circuit board.
    Your P1 and C1 voltages are correct, so I don't know how your G1 voltages could be off by so much,
    they should be within 1-3 volts of C1 (about 129vdc to ground).
    So double check the values of the associated resistors, R6,7,8 and R13,14,17

    garageguy55

    Posts : 4
    Join date : 2010-01-04

    Re: SP9 Octal version hum

    Post by garageguy55 on Fri Mar 15, 2013 6:02 pm

    It must be version 1 as there is no marking.R6 & R13 are 470K, R7 & R17 are 220 ohms, and R8 &R14 are 1K.

    tubes4hifi
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    Re: SP9 Octal version hum

    Post by tubes4hifi on Fri Mar 15, 2013 8:02 pm

    that's all correct. So other than strange G1 voltages, does it sound OK?
    I know you mentioned hum, but that is likely a ground loop or wiring problem.

    tubes4hifi
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    Re: SP9 Octal version hum

    Post by tubes4hifi on Fri Mar 15, 2013 8:06 pm

    how much voltage do you have on the cathode of the rectifier tube? Should be around 300vdc

    garageguy55

    Posts : 4
    Join date : 2010-01-04

    Re: SP9 Octal version hum

    Post by garageguy55 on Fri Mar 15, 2013 8:47 pm

    rectifier tube cathode voltage is 311. It sounds good but the hum is very annoying and the noise on start up is strange. What are the differences between version 1,2 and 3?

    tubes4hifi
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    Re: SP9 Octal version hum

    Post by tubes4hifi on Fri Mar 15, 2013 9:42 pm

    minor differences in layout. I don't know where your hum is coming from.

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