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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    Newbie Troubleshooting: Blown 47Ohm R on PC3

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    garyt808


    Posts : 3
    Join date : 2017-08-19

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    Post by garyt808 Sat Aug 19, 2017 7:50 am

    Hi Everyone,

    Sorry in advance for my newbie question.  I'm not very well versed in tube amp design but am trying to learn.

    I have a completely stock ST70 with 7199 tubes and original parts (except for the selenium rectifier).  The 47 Ohm resistor connected to pad 9 blew on the PC3 while I was listening to it.  It looks like the 620 ohm and 47 ohm resistor is a voltage divider to reference the EL34 Screen Grid, so I think this is the screen grid resistor?

    In any case, it seems like one of my tubes probably ran away (I really don't know, the amp was running for about 12 hours straight at low volume, made a crackling sound in one channel and I ran to turn it off), and caused the 47 ohm resistor to blow.  

    First of all, is what I said above even true?

    In addition to replacing the 47 ohm resistor, should I replace my EL34's?  Could the 7199 also have caused this to blow?  Since the EL34 screen grid is connected to the output transformer, do I need to worry about this too?

    Again, thank you for your patience and for teaching me!

    Gary
    peterh
    peterh


    Posts : 1823
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Newbie Troubleshooting:  Blown 47Ohm R on PC3 Empty Re: Newbie Troubleshooting: Blown 47Ohm R on PC3

    Post by peterh Sat Aug 19, 2017 8:16 am

    garyt808 wrote:Hi Everyone,

    Sorry in advance for my newbie question.  I'm not very well versed in tube amp design but am trying to learn.

    I have a completely stock ST70 with 7199 tubes and original parts (except for the selenium rectifier).  The 47 Ohm resistor connected to pad 9 blew on the PC3 while I was listening to it.  It looks like the 620 ohm and 47 ohm resistor is a voltage divider to reference the EL34 Screen Grid, so I think this is the screen grid resistor?

    In any case, it seems like one of my tubes probably ran away (I really don't know, the amp was running for about 12 hours straight at low volume, made a crackling sound in one channel and I ran to turn it off), and caused the 47 ohm resistor to blow.  

    First of all, is what I said above even true?

    In addition to replacing the 47 ohm resistor, should I replace my EL34's?  Could the 7199 also have caused this to blow?  Since the EL34 screen grid is connected to the output transformer, do I need to worry about this too?

    Again, thank you for your patience and for teaching me!

    Gary

    This was an oddball problem !
    To burn the 47ohm resistor you need some power. It seems unlikely that this power comes
    via 7199/620ohm. The probable way is through the 390pF cap between the screen and the
    47 ohm resistor. Install a replacement cap, 390pF and at least 500V rating.
    Your tubes is probably not damaged, change 47ohm 390pF and power on.( do check the
    other resistors too)
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    Newbie Troubleshooting:  Blown 47Ohm R on PC3 Empty Re: Newbie Troubleshooting: Blown 47Ohm R on PC3

    Post by Guest Sat Aug 19, 2017 9:32 pm

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    Last edited by PeterCapo on Thu Dec 03, 2020 11:38 pm; edited 1 time in total
    peterh
    peterh


    Posts : 1823
    Join date : 2012-12-25
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    Post by peterh Sun Aug 20, 2017 2:35 am

    PeterCapo wrote:Make that 1000 VDC for the 390pf cap, as the original was.  Have a read here https://www.audioasylum.com/forums/dynaco/messages/12/6416.html

    Mouser Part #: 598-CDV16FF391JO3F  http://www.mouser.com/ProductDetail/Cornell-Dubilier-CDE/CDV16FF391JO3F/?qs=sGAEpiMZZMtLiKaZgV7flU44AUKbBD%2fX3kgUEfF6fKM%3d

    Thanks for this. A was unaware of the voltage.
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    garyt808


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    Post by garyt808 Sun Aug 20, 2017 6:13 am

    Thank you for your help! I'm glad my tubes and OPT's, are probably ok...
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    wildiowa


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    Join date : 2012-03-19

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    Post by wildiowa Sun Aug 20, 2017 8:20 am

    Might be a waste of money, but I always kinda thought when there were fireworks it was best to change out the tubes involved just in case they were the cause or there is an intermittent problem that kicked off the runaway. Not all of them, just on the side that blew up. Maybe that is just a knee jerk reaction but I just kinda start from scratch just to be safe.
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    Post by Guest Sun Aug 20, 2017 1:56 pm

    .


    Last edited by PeterCapo on Thu Dec 03, 2020 11:38 pm; edited 1 time in total
    peterh
    peterh


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    Post by peterh Sun Aug 20, 2017 3:00 pm

    wildiowa wrote:Might be a waste of money, but I always kinda thought when there were fireworks it was best to change out the tubes involved just in case they were the cause or there is an intermittent problem that kicked off the runaway. Not all of them, just on the side that blew up. Maybe that is just a knee jerk reaction but I just kinda start from scratch just to be safe.

    The 47 ohm resistor is in the 7199 cathode, but the 7199 cannot supply that kind of
    current. Thus i don't think the 7199 is unharmed.
    The EL34 is not supplying the path that burned the 47ohm, the path is from the
    B+ via output transformer ( screen path) . The EL34 should not been affected by this.

    There is however some logic in stocking up spare tubes, a quad EL34 and a GZ34
    is what the doctor recommends. Store them until needed.
    The 7199 should probably not be replaced, they are made of unobtanium, instead
    similar tubes ( 6u8 is one ) + socket adapters ( that takes care of different pin layout) is needed some
    time in the future. Don't do this now, you might choose to upgrade to a VTA board
    when that day comes, they use 3 ECC82 instead.
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    garyt808


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    Post by garyt808 Sun Aug 20, 2017 9:26 pm

    After a closer examination I can see a hair line crack in the 390pf domino capacitor.

    This may sound a bit strange, but I have 2 ST70's. One completely stock (the amp in question) and a modern reproduction ST70 kit that I built.

    My plan was to keep the stock ST70 stock, and modify the reproduction ST70. That way I can keep a baseline for the mods. However, since I don't have time to mod right now I gave the reproduction to my dad to enjoy. Very Happy

    Keeping with the theme of keeping this ST70 stock, is there any recommended source for NOS domino capacitors?

    I might have to bite the bullet and use modern capacitors in place of the dominos.





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