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    Newbie Troubleshooting: Blown 47Ohm R on PC3

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    garyt808

    Posts : 3
    Join date : 2017-08-19

    Newbie Troubleshooting: Blown 47Ohm R on PC3

    Post by garyt808 on 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
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    peterh

    Posts : 837
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Newbie Troubleshooting: Blown 47Ohm R on PC3

    Post by peterh on 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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    PeterCapo

    Posts : 426
    Join date : 2008-12-05

    Re: Newbie Troubleshooting: Blown 47Ohm R on PC3

    Post by PeterCapo on Sat Aug 19, 2017 9:32 pm

    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
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    peterh

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

    Re: Newbie Troubleshooting: Blown 47Ohm R on PC3

    Post by peterh on 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.

    garyt808

    Posts : 3
    Join date : 2017-08-19

    Re: Newbie Troubleshooting: Blown 47Ohm R on PC3

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

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

    wildiowa

    Posts : 167
    Join date : 2012-03-19

    Re: Newbie Troubleshooting: Blown 47Ohm R on PC3

    Post by wildiowa on 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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    PeterCapo

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    Re: Newbie Troubleshooting: Blown 47Ohm R on PC3

    Post by PeterCapo on Sun Aug 20, 2017 1:56 pm

    Reasonable to at least check the tubes.
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    peterh

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    Re: Newbie Troubleshooting: Blown 47Ohm R on PC3

    Post by peterh on 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.

    garyt808

    Posts : 3
    Join date : 2017-08-19

    Re: Newbie Troubleshooting: Blown 47Ohm R on PC3

    Post by garyt808 on 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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    PeterCapo

    Posts : 426
    Join date : 2008-12-05

    Re: Newbie Troubleshooting: Blown 47Ohm R on PC3

    Post by PeterCapo on Sun Aug 20, 2017 10:12 pm

    Unless you find a source for NOS, the current production choices for 390pf @1000VDC are quite limited.  The CDE part I posted a link to (in Post n°3) is about it.  It is a mica cap, and I *think* the original domino was also mica (possibly ceramic, but I think it was mica).  Fortunately, this CDE part is a very good choice, in any case.

    As far as using your original, stock Stereo 70 as a baseline against which to compare another, modded Stereo 70, it's not really a valid baseline for what the original Stereo 70 was capable of unless it has all the same advantages in fresh parts quality like the modded versions enjoy.

    In other words, assuming you have not fully restored it, what you have now is a baseline for what a well-used, decades-old Stereo 70 sounds like.  But, to know what the original Dyanco Stereo 70 can sound like at its best, you’d need to refresh most or all of the parts 1:1, and perhaps also build-in certain enhancements to level the playing field with the modded versions, all the while sticking to the original Dynaco circuit design.

    Of course, it's fine to keep it as original as possible, with as many original parts as practical, but it likely will not represent the full sonic potential of the original Dynaco circuit.

    http://www.audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf

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