Do Black Plate 6L6 Tubes Cause Red Plating in Mac Amps?

Discussion in 'McIntosh Audio' started by AA6U, Apr 9, 2018.

  1. AA6U

    AA6U Member

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    I'm hoping someone can get me the correct information regarding whether black plate 6L6GCs by any manufacturer (but especially RCA) can cause red plating in Mac amps (no other cause other than the tube having a black plate)? The friend who told me this said that there is no explanation for it, but, because of the red plate danger, I should always avoid using black plate 6L6 tubes in Mac amps.

    I know that McIntosh-labeled tubes were mostly black plate, so this puzzles me. I also searched the internet and could not find anything on this unexplained phenomenon.

    Has anyone else heard of this (or experienced it) or it just some unsubstantiated rumor/urban legend floating around?
     

     

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  2. chuckworkb

    chuckworkb AK Subscriber Subscriber

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    Why not reach out to McIntosh Service and ask them to comment. You can call or send an email.
     
  3. whoaru99

    whoaru99 Epic Member

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    Sounds like some sort of urban legend/nonsense to me.
     
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  4. BillWojo

    BillWojo AK Subscriber Subscriber

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    I have a special set of machine's that will dispose of your RCA 6L6 black plates. PM me for my address. Machine is listed in my Sig below.

    BillWojo
     
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  5. AA6U

    AA6U Member

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    I posed the question to McIntosh. I'll post their answer when I receive it.
     
  6. AA6U

    AA6U Member

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    Here is what McIntosh said:

    "McIntosh amps will correctly perform with 6L6GC tubes. The earlier 6L6 tubes do not have the voltage rating to work in these amps. Red platting is not an issue using 6L6GC tubes as long as the amp is in proper operational condition."

    Perhaps whoever started the red plate rumor was using non-GC tubes.
     
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  7. tuberadios69

    tuberadios69 AK Subscriber Subscriber

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    Standard 6L6 output tubes do not have a glass envelope, it is metal. The 6L6G has a glass envelope as does the 6L6GC. The 6L6 tube envelope is metal so no way can you see the plates or even tell (until it's too late) if red plating occurred internally. As McIntosh states, 6L6 tubes (or 6L6G) were not rated for higher plate voltages like the 6L6GC and are not recommend for use in an MC30 or MC240. The MC30 did come with 1614 output tubes which had a metal envelope but they were rated for higher plate voltages than standard 6L6 metal tubes. I think your friend was referring to RCA 6L6GC glass tubes which did have black plates internally. I have never heard that they were more susceptible to red plating than any other tubes. In fact, they are very highly regarded and NOS samples go for big bucks today. Plus you don't have to use matched output tubes in the old MC series of tube amps. They were designed to compensate for mismatched output tubes due to the output transformer design, I believe. However, use unmatched output tubes in a Dynaco ST70 and you probably will see red plating as one tube robs the other of current and goes into thermal runaway. That I have seen happen myself.
     
  8. AA6U

    AA6U Member

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    I think you are spot on. Perhaps the rumor began with someone having a red plate experience with non-GC black plate tubes and then assumed that all black plate tubes would red plate. My friend specifically said that RCA black plates were bad, but I have some McIntosh-branded black plate tubes that were made by RCA. I wouldn't think that McIntosh would sell tubes that would unexplainably red plate; that would be a PR nightmare in the audiophile world.
     
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  9. quartersaw

    quartersaw Well-Known Member

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    I volunteer to assume the risk of using RCA black plate 6L6GC's that anyone may want to dispose of... :)
     
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  10. jlovda

    jlovda Things I loved from the 60's and 70's Subscriber

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    In two sentences or less would someone be kind enough to describe red plating?
     
  11. Pio1980

    Pio1980 AK Member Subscriber

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    Usually caused by incorrect or loss of control grid (G1) bias, the outer part of the innards of an output tube glows an abnormal dull to bright red. If allowed to continue, will result in the destruction of the tube and possibly associated components.
     
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  12. Pio1980

    Pio1980 AK Member Subscriber

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    Additionally, underrated or defective multi-grid output tubes will usually fail spectacularly with internal fireworks from a screen grid (G2) arc-over, effectively destroying the tube.
     
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  13. Rockon1

    Rockon1 Well-Known Member

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    Its when the plates of a tube get so hot they actually glow red. It has to be way over the max dissipation to do so.

    This pic is on the mild side and may be caused by being biased too hot. They can get much brighter-0especially if theres an issue with the bias supply. Bob

    redglow.jpg
     
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  14. Pio1980

    Pio1980 AK Member Subscriber

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    In some examples, an output tube may spontaneously red-plate from a runaway bias cancelling cycling, caused by cathode material being sputtered onto the control grid by being overstressed. In this condition, the tube is defective and must be replaced.
     
  15. whoaru99

    whoaru99 Epic Member

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    Exactly what it sounds like.

    Tubes running past their capability enough so the structure actually starts to glow somewhere from a dull red to beyond.

    Fairly mild case on rightmost tube -
    images (1).jpeg

    Chernobyl in 3...2...
    8eos_zps20008052.jpg
     
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  16. jlovda

    jlovda Things I loved from the 60's and 70's Subscriber

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    Thank you all for the tutorial! it wasn't clear to me whether the red was a deposit on the plate or the glowing of hot metal. :thumbsup:
     
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  17. ron-c

    ron-c AK Subscriber Subscriber

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    6L6, 6L6G, 6L6GA, and 6L6GB are all glass bottle early versions that will not work (for very long!) in a Mc amp as they have a Max plate watts of 19, Max plate volts of 360 and Max screen volts of 270 @ 2.5 amps.
    6L6GB is Max plate watts of 30, Max plate Volts of 500, Max screen volts 450 @ 5 amps.

    Ron-C
     
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  18. 62caddy

    62caddy Trust but verify Subscriber

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    Either that, or something else was wrong with the amplifier to begin with.
     
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  19. motorstereo

    motorstereo AK Subscriber Subscriber

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    Or I wonder if they can be installed one pin off like a kt88 without the centering pin? There's red plating, green plating and magic smoke all at once when that happens:yikes:
     
  20. 62caddy

    62caddy Trust but verify Subscriber

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    ...or if tube pins not making contact at all points in the connector sleeves. Yes that can do it too.

    Good point on the locating pin.
     
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