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    Is this tube red plating?

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    jfine

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    Is this tube red plating?

    Post by jfine on Thu Jul 13, 2017 9:56 pm

    This is a 5AR4 chinese (shuguang) tube, about 100 hours on it.

    M125 monblocks.

    First 2 pics are the tube in question, the other 2 pics are from the other monoblock.

    There is some red on the left upper of the tube, just noticed it.


    What happens if I let this tube fail, will it take out other tubes, caps, etc.,?









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    corndog71

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    Re: Is this tube red plating?

    Post by corndog71 on Thu Jul 13, 2017 10:23 pm

    No.  

    This is.

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    jfine

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    Re: Is this tube red plating?

    Post by jfine on Fri Jul 14, 2017 12:29 am

    Yes seen things like that, just wondering if possible mine is an early stage.
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    Peter W.

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    Re: Is this tube red plating?

    Post by Peter W. on Fri Jul 14, 2017 8:14 am

    Typically, when a rectifier tube 'red-plates' it is on the verge of spectacular failure. The filament will sag against the cathode and then, more-or-less *SLAG*. Clearly, the first two are acting unhappily and look to be on the edge.

    What happens if you switch the pairs between the amps? Does the problem follow the tubes or remain with the amp? That would be the first test I would do, and requires no instruments to do so. Let us know.

    Jim McShane

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    Re: Is this tube red plating?

    Post by Jim McShane on Fri Jul 14, 2017 11:22 am

    Whether red on the plate is a sign of imminent meltdown is not a "cut and dried" determination. A small area of red plating is generally not a precursor to meltdown - in fact it is/was often considered normal and was mentioned in owner's manuals, etc. Some tube data sheets also mention that a small amount of red is normal and acceptable as well.

    But large areas of red are certainly detrimental - it's an indication of overheating which is not good. Even if catastrophic failure does not occur it certainly is not beneficial for long tube life.
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    Peter W.

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    Re: Is this tube red plating?

    Post by Peter W. on Fri Jul 14, 2017 11:31 am

    Jim McShane wrote:Whether red on the plate is a sign of imminent meltdown is not a "cut and dried" determination. A small area of red plating is generally not a precursor to meltdown - in fact it is/was often considered normal and was mentioned in owner's manuals, etc. Some tube data sheets also mention that a small amount of red is normal and acceptable as well.

    But large areas of red are certainly detrimental - it's an indication of overheating which is not good. Even if catastrophic failure does not occur it certainly is not beneficial for long tube life.

    True.

    But I would still be curious as to whether the issue followed the tubes or is related to the individual amp. If the latter, further tests might be useful. If the former, the age, brand and service-life of the one pair as compared to the other might also be useful to compare.
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    jfine

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    Re: Is this tube red plating?

    Post by jfine on Fri Jul 14, 2017 11:52 am

    Switched the 5AR4's between the 2 M125's, the same tube displayed the red, so it followed the tube.

    Let's suppose it is beginning to fail, if I let this tube fail, will it take out other tubes, caps, etc.,? If it just fails on its own and doesn't damage anything else, I would just run it til it dies. If there's impending danger, then I probably should not turn the amp on again until it's replaced...
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    peterh

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    Re: Is this tube red plating?

    Post by peterh on Fri Jul 14, 2017 12:49 pm

    jfine wrote:Switched the 5AR4's between the 2 M125's, the same tube displayed the red, so it followed the tube.

    Let's suppose it is beginning to fail, if I let this tube fail, will it take out other tubes, caps, etc.,? If it just fails on its own and doesn't damage anything else, I would just run it til it dies. If there's impending danger, then I probably should not turn the amp on again until it's replaced...

    Order a new. Or two. During delivery delay use the amp as usual ( but not unattended), the fuse might not protect from a failing 5ar4 so keep a close watch.
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    jfine

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    Re: Is this tube red plating?

    Post by jfine on Fri Jul 14, 2017 1:16 pm

    peterh wrote:
    Order a new. Or two. During delivery delay use the amp as usual ( but not unattended), the fuse might not protect from a failing 5ar4 so keep a close watch.

    So ideally, in the M125 anyway, the fuse *should* catch it, but is it an unknown set of possibilities of what might happen if it were to fail?
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    Peter W.

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    Re: Is this tube red plating?

    Post by Peter W. on Fri Jul 14, 2017 1:30 pm

    jfine wrote:
    peterh wrote:
    Order a new. Or two. During delivery delay use the amp as usual ( but not unattended), the fuse might not protect from a failing 5ar4 so keep a close watch.

    So ideally, in the M125 anyway, the fuse *should* catch it, but is it an unknown set of possibilities of what might happen if it were to fail?

    I have ranted in the past on fuses. I believe that the VTA M125 comes with a 5A fuse. That is, at 120 VAC, 600 watts. I also believe that it is supplied with a conventional spiral-wound "Slo-Blo" fuse. Such a fuse will operate at 135% of rating for 60 minutes. At 200% of rating for two (2) Full Minutes. It takes a fraction of a few seconds to destroy a power-transformer in a short situation.

    So, the short answer without being snarky is that most fuses are designed to protect real-estate, not the equipment they are in. Draw your own conclusions.
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    peterh

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    Re: Is this tube red plating?

    Post by peterh on Fri Jul 14, 2017 1:39 pm

    jfine wrote:
    peterh wrote:
    Order a new. Or two. During delivery delay use the amp as usual ( but not unattended), the fuse might not protect from a failing 5ar4 so keep a close watch.

    So ideally, in the M125 anyway, the fuse *should* catch it, but is it an unknown set of possibilities of what might happen if it were to fail?
    The single fuse is installed at the mains inlet, and is large enough to cope with
    worst case startup currents but it might not blow if the 5ar4 shorts.

    An aftermarket fuse in the B+ chain could be 250mA and does not have to
    copy with turn-on surges as the amp has a valve rectifier.
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    deepee99

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    Re: Is this tube red plating?

    Post by deepee99 on Fri Jul 14, 2017 2:41 pm

    jfine,
    On the advice of a poster here a year or two ago, I installed some fairly spendy 3-amp slo-blows in my M-125s and never had one fail at start-up surge. And there are different kinds of slo-blow fuses, too. The 5-amp automotive variety we are supplied with do on the M-125 kits, on rare occasion, have the ability to vaporize internally and become perfect conductors when they blow. John Fluke worked this out years ago and that's why there's a pair of fuses in even the cheapest of his meters.
    Remember the adage that all those spendy downstream tubes, chokes and transformers exist to protect the 50-cent fuse Smile c.f. Peter W's post here.
    In early tube-rolling days I blew lots of stuff up while the M-125's line fuse continued to pass current and even seemed to smile wickedly back when I pulled it.
    Some perfectly healthy tube rectifiers kinda sorta look like they're red-plating because you're looking mostly at the filament, but from the pix it looks like you've got a problem, Houston. That tube ain't going to heal itself; get it out of there before it hurts something. The 5-amp slo-blow fuse won't save you.
    While I have sung praises of their signal and output tubes, cheap Chinese rectifiers are an expensive accident waiting to happen.
    Four routes to go here, all of which I have tried in M-125s:
    New-issue Tung-Sol 5AR4s from Jim McShane for about $40 a pop. You can prolly get them cheaper elsewhere but at least Jim torture-tests his. Never had a problem with them;
    The grand-daddy Mullard GZ 33, 34 or 37s, which will cost you your first-born but with proper care will live forever and look really cool;
    The Weber Copper-Cap WZ68 hybrids, discussed here: http://dynacotubeaudio.forumotion.com/t2743-weber-coppercap-versus-tube-rectifier
    Or just say, "screw it" and go to a solid-state power supply. This will insult the purists amongst this lot, but my tubes, ears, iron and speakers don't seem to mind.
    PeterW, I think you started a thread on the vagaries of cheap v. expensive fuses. Gotta link?
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    LeGrace

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    Re: Is this tube red plating?

    Post by LeGrace on Fri Jul 14, 2017 2:51 pm

    Well after 6 weeks of glorious trouble free operation today another red plate. (ref Mullard GZ37 rectifier) As with last event fuse did not blow. Mine was not a maybe, at least half the plate area glowing red. I was reading a book and when I looked up I noticed it.

    But unlike last time no hyperventilating. I simply shut off amps and let the tubes cool. After about 15 min turn amps back on and first thing is check the bias. Down to 0.5 volts on left pair, there's the issue! Adjust back up to 1 v and amp is happy again. No angry red glow anymore. Why it shifted this much is an entirely different question. But explains the red plate.

    So IME suggestion, double check your bias.
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    jfine

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    Re: Is this tube red plating?

    Post by jfine on Fri Jul 14, 2017 2:55 pm

    deepee99 wrote:jfine,
    Some perfectly healthy tube rectifiers kinda sorta look like they're red-plating because you're looking mostly at the filament, but from the pix it looks like you've got a problem, Houston. That tube ain't going to heal itself; get it out of there before it hurts something.

    So give me the blow by blow scenarios what *could* happen, take out a transformer, catch fire, take out all my power tubes, fry caps/resistors, ?

    Call me a masochist but half of me wants to see what happens, watch the tube fail and the amp just shuts down, that would be a learning experience...but start costing me $$$ then I'll pass...

    Checked bias and it's dead on, in fact, it only changed since I had these amps, when I added the yellow paper diode mods...
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    LeGrace

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    Re: Is this tube red plating?

    Post by LeGrace on Fri Jul 14, 2017 3:02 pm

    The diode mod seems to be a good insurance policy. In that I've done it and had two rectifiers go nuclear with no knock on effects. I'm starting to get in tune with the rhythm now, why I immediately suspected a bias issue. Starting to gain an appreciation for auto biasing circuits LOL>
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    deepee99

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    Re: Is this tube red plating?

    Post by deepee99 on Fri Jul 14, 2017 3:26 pm

    Hmmm,
    You guys may be asking questions above my pay-grade...
    Any time you change rectifier tubes, even of the same brand and type, you've got to adjust and keep a hawk's-eye on bias for the first dozen hours at least. Start low and work your way up.
    If you're going to be a purist and use tube rectifiers, more power to ya.

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    peterh

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    Re: Is this tube red plating?

    Post by peterh on Fri Jul 14, 2017 3:43 pm

    jfine wrote:
    deepee99 wrote:jfine,
    Some perfectly healthy tube rectifiers kinda sorta look like they're red-plating because you're looking mostly at the filament, but from the pix it looks like you've got a problem, Houston. That tube ain't going to heal itself; get it out of there before it hurts something.

    So give me the blow by blow scenarios what *could* happen, take out a transformer, catch fire, take out all my power tubes, fry caps/resistors, ?

    Call me a masochist but half of me wants to see what happens, watch the tube fail and the amp just shuts down, that would be a learning experience...but start costing me $$$ then I'll pass...

    Checked bias and it's dead on, in fact, it only changed since I had these amps, when I added the yellow paper diode mods...
    If you already done the "yellow sheet mod" you don't have to worry. A shorted 5ar4
    won't harm your transformer.
    What you have looks like a 5ar4 that have reduced emission on part of one cathode,
    thus all current will flow on part of the cathode and anode. That's the part
    that is a weak glow on your picture. Change the tube and be happy.
    Btw, a cheap JJ gz34 will do just fine for a fraction of the cost.
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    Tubes4ever

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    Re: Is this tube red plating?

    Post by Tubes4ever on Fri Jul 14, 2017 7:03 pm

    jfine,

    If the tube shorts internally the Slo Blow fuse will blow within 1/2 second.  You won't damage the transformer or any other components.  I speak from experience.  The Slo Blows blow rather quickly with the massive increase in transformer primary current due to the short in the high voltage winding.  This assumes you haven't done the diode mod which is prerectifying your high voltage.  If you have done the diode mod, a shorted side in the 5AR4 will simply turn the tube into a Weber copper cap minus the internal dropping resistor.  At any rate, any glow on the 5AR4 plate is not normal.  Get another one.


    Last edited by Tubes4ever on Fri Jul 14, 2017 7:58 pm; edited 1 time in total
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    j beede

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    Re: Is this tube red plating?

    Post by j beede on Fri Jul 14, 2017 7:06 pm

    FYI: I have been using Sovtek 5AR4 from Guitar Center for 5+ years in multiple pairs of MkIII and am very happy with them. I pay from $13 to $20 each depending on what promotions GC is running at the moment.

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