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Thread: Cathode Bias Red Plating Discussion

  1. #36
    Supporting Member eschertron's Avatar
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    I've had iffy (read: better than nothing) results tracing a board by sticking a high-intensity diode on a wand under the board. It may reveal just enough shadowy information to suss out some of the unknown connections. If the board's not fiberglass/semi-opaque then this won't work so well.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
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    don't forget the joker g1's Avatar
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    Quote Originally Posted by eschertron View Post
    I've had iffy (read: better than nothing) results tracing a board by sticking a high-intensity diode on a wand under the board.
    I was wondering what kind of diode divining rod you were building when it struck me you probably meant the light emitting variety.

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    The voices in my head are idiots!

  3. #38
    Lifetime Member Enzo's Avatar
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    I had the same reaction... Some sort of sensing diode??? Then, oh... a light. I use a 100 watt bulb. I have a clamp on work light.

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    Education is what you're left with after you have forgotten what you have learned.

  4. #39
    Supporting Member eschertron's Avatar
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    Yes, typing past my bedtime leads to errors of omission. I'm thinking with the PCB still installed in the chassis, there might not be room to get a work light under the board.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    Distortion is a form of signal degradation. It is pretty easy to define what a good "clean" signal is, but trying to achieve just the right kind of "broken" can be pretty maddening. - glebert

  5. #40
    Old Timer J M Fahey's Avatar
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    I was chiming in to say something and I see you are near, but not exactly there.

    I think that "somebody" who must be an avid "Forum reader" or a Graduate at "You Tube University" has added the "Ruby Mod" or whatever they call that dreaded invention of biasing tubes with a Zener diode.

    FWIW it might even come from Factory itself.

    In any case, either Factory or user added Zener diode shorted, Service "Tech" thought "oh! ... a diode!!! I have a few of those!!!" and replaced it with 1 x 1N4007 ... or 2 in series, same thing ... and oriented so they can pass current ("why would the original one(s) be oriented the wrong way"?) [headscratch] ... the perfect Storm.

    Diodes get forward biased under operation , drop only 1 or 2V, and give power tubes all the current they want, so they happily redplate *and* overload +B down so much that it about halves and in a short time will also burn the PT.

    Of course Multimeter, set to a resistance reading scale, does NOT show any diode(s) there.
    Out of curiosity, I would measure cathode to ground, on the diode scale.

    An alternative theory (hey!! I saw it at Alex Jones Channel so it must be true ) could be that you have there a properly oriented (replacement) Zener, but way lower Zener voltage, so tubes are always underbiased (voltage wise) and over biased (current wise). Pick one.

    I wonder why the mains fuse does not blow, although I would not be surprised at finding it "reinforced".

    So I suggest first of all remove those damn cathode diodes and plase sensible value cathode resistors.

    Amp "should" work as normal.

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    Juan Manuel Fahey

  6. #41
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Mr_bibbles View Post
    Cathode resistor voltage drop (Measuring at both sides of resistor with tubes in) 1.985VDC

    There are two Resistors wired in parallel, Possibly a modification, possibly intentional. There is no short from cathode to ground, but the tubes are still red-plating even with both resistors removed from the circuit.
    If it flies like a diode, swims like a diode and quacks like a diode, then itīs a diode. (slightly adapted to current situation )

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    I was chiming in to say something and I see you are near, but not exactly there.

    I think that "somebody" who must be an avid "Forum reader" or a Graduate at "You Tube University" has added the "Ruby Mod" or whatever they call that dreaded invention of biasing tubes with a Zener diode.

    FWIW it might even come from Factory itself.

    In any case, either Factory or user added Zener diode shorted, Service "Tech" thought "oh! ... a diode!!! I have a few of those!!!" and replaced it with 1 x 1N4007 ... or 2 in series, same thing ... and oriented so they can pass current ("why would the original one(s) be oriented the wrong way"?) [headscratch] ... the perfect Storm.

    Diodes get forward biased under operation , drop only 1 or 2V, and give power tubes all the current they want, so they happily redplate *and* overload +B down so much that it about halves and in a short time will also burn the PT.

    Of course Multimeter, set to a resistance reading scale, does NOT show any diode(s) there.
    Out of curiosity, I would measure cathode to ground, on the diode scale.

    An alternative theory (hey!! I saw it at Alex Jones Channel so it must be true ) could be that you have there a properly oriented (replacement) Zener, but way lower Zener voltage, so tubes are always underbiased (voltage wise) and over biased (current wise). Pick one.

    I wonder why the mains fuse does not blow, although I would not be surprised at finding it "reinforced".

    So I suggest first of all remove those damn cathode diodes and plase sensible value cathode resistors.

    Amp "should" work as normal.
    Fahey! Thank you- This seems like the answer I was looking for. I have never heard of that diode implementation before but this would all make sense as to why it was working with the diode removed.

    But I'm having a terrible day. One of those days where everything you touch turns to shit.

    When I (re-)removed the diode and powered it up I magic smoked the 1k (grid?) resistors in parallel. I have no idea how, It could have been the tubes, left a snipped lead in, left it diconnected for testing and forgot to re-connect... I repaired the damage (and managed to shock the shit out of myself in the process... like I said, terrible day...) and fired it up on the limiter, worked fine, added different tubes, worked fine, not pulling too much current. BUT now there is no signal / hum / buzz. Traced a signal through to V1, noticed there is NO PLATE VOLTAGE on any preamp tubes, but there is 350 on the plates of the power section. Open dropping resistor? any other suggestions? I noticed there is only 2 diodes near the primaries for a rectifier... they measure OL / 0.000 on the diode tester, but if this IS the rectifier and it WAS shorted, wouldn't the DC be missing from the output section?

    On the plus side, the red plating is gone.

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  8. #43
    don't forget the joker g1's Avatar
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    This should be unrelated. They would not have a separate rectifier for the preamp supply. Like you say, the power tubes are still getting plate voltage, so there must be a dropper resistor or fuse open somewhere.

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  9. #44
    Old Timer J M Fahey's Avatar
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    Agree.
    Only you didnīt notice because bthe redplating tubes problem is way more dangerous and you (and probably an earlier Tech) focused on it only.
    Hey, if at the same time a dog is biting your ankle and a mosquito is biting your ear ... what would be the focus of your attention?

    I wouldnīt even be surprised that an earlier Tech disconnected the preamp trying to find what was pulling his +V down

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    Juan Manuel Fahey

  10. #45
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    I had these lurking in my folder of random schematics. Hope they help elucidate the problem.

    Click image for larger version. 

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  11. #46
    Supporting Member eschertron's Avatar
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    I don't see D3 and D4 on the drawing. At least you have confirmed the two x 500R cathode resistors are stock

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    If it still won't get loud enough, it's probably broken. - Steve Conner
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    Distortion is a form of signal degradation. It is pretty easy to define what a good "clean" signal is, but trying to achieve just the right kind of "broken" can be pretty maddening. - glebert

  12. #47
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    Quote Originally Posted by zozobra View Post
    i had these lurking in my folder of random schematics. Hope they help elucidate the problem.

    Click image for larger version. 

Name:	rev_jr_pro_PRE.png 
Views:	31 
Size:	49.6 KB 
ID:	49318
    Click image for larger version. 

Name:	rev_jr_pro_PA.png 
Views:	36 
Size:	47.9 KB 
ID:	49319
    Click image for larger version. 

Name:	rev_jr_pro_PSU1.png 
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Size:	24.7 KB 
ID:	49320
    Click image for larger version. 

Name:	rev_jr_pro_PSU2.png 
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ID:	49321
    zozobra! I could fucking kiss you! <3<3<3!

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  13. #48
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    Quote Originally Posted by J M Fahey View Post
    Agree.
    Only you didnīt notice because bthe redplating tubes problem is way more dangerous and you (and probably an earlier Tech) focused on it only.
    Hey, if at the same time a dog is biting your ankle and a mosquito is biting your ear ... what would be the focus of your attention?

    I wouldnīt even be surprised that an earlier Tech disconnected the preamp trying to find what was pulling his +V down
    Actually the amplifier was passing signal before, while it was red plating. Pre-amp out was OK.... I think I smoked a dropper to the preamp? Now that we have a schematic I can probably manage the rest on my own. I'll be back if I hit any brick walls, but there should be a national "Music Electronics Forum Appreciation Day". You old heads are saints for spending your time here and helping out doofuses like myself.

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  14. #49
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    No problem. Hope they're useful.

    The krank's are interesting designs to dissect, at least in how they copied something and ended up with a semi-successful product. At their very core they are basically tweaks of the Sovtek Mig-100H with a dual recto-ish poweramp with no NFB and vox presence. This makes a lot of sense as the guy who ran krank used to import them and flip them on with a few modifications. The Mig circuit isn't one that has been copied a great deal as most people probably think they're cheap Russian crap but I rather like them. They aren't terribly subtle though!

    The Mig-100H in itself is pretty interesting as all the Mig-100's apparently use the same PCB and they really kludged a conversion from a 3+CF topology of the Mig-100 to the 4 stage plate driven stack of the 100H. Notably the easiest way to achieve this was by referencing the 3rd to 4th stage divider to the cathode of the 3rd stage rather than ground. This has sparked a fair few arguments about feedback and bias shifting and what tonal effects this may have. I was certainly guilty of this until someone pointed out it was just kludge to get it to work on an existing PCB and that the designer probably didn't give a shit about either of those things; they just wanted a 4 stage high gain ripper! The kranks reference to the 4th stage cathode.

    Some people find some parts of the krank design a bit questionable as the shift/sweep and presence controls do have HV on them. Both could have been avoided by either changing their placement or adding a cap without any real tonal penalty.

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