Defective 7591, red plating

Discussion in 'Tube Audio' started by CJVx, Jul 8, 2018.

  1. CJVx

    CJVx AK Subscriber Subscriber

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    Location:
    NJ
    So I was in communication with a seller who sold a pair of NOS Westinghouse 7591's to someone who stated that one was bad and was red plating. I aquired this pair, and sure enough, the one stated as bad, got cherry red after 3 minutes or so in my scott 340b. What causes this when the defect is in the tube and is this tube garbage? Thanks
     

     

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  2. GregBilodeau

    GregBilodeau AK Subscriber Subscriber

    Messages:
    65
    Location:
    Edmonton, AB Canada
    Could be as simple as a bad solder connection on the grid pin. Could also be a broken weld within the tube which renders the tube garbage.

    Gregb
     
  3. Celt

    Celt Peanut Head Staff Member Super Mod

    Messages:
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    Location:
    Paragould, Arkansas
    I once had a defective ketchup bottle which caused red plating all over my fries... :naughty:
     
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  4. gadget73

    gadget73 junk junkie Subscriber

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    Possible the grid spacing or alignment has gotten out of whack somehow. Bad handling perhaps, or just a manufacturing defect.
     
  5. triode17

    triode17 Super Member

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    1,191
    It could well be a signal grid to cathode short. No Vgc = Red Plate.
     
  6. linuxslate

    linuxslate Member

    Messages:
    58
    Location:
    Central Florida, USA
    This is a picture from another thread of one of the 7591XYZ's in one of my amps:

    Note glow on left most tube.

    [​IMG]

    If I move the tubes around, the slight glow follows this particular tube. I'm waiting for a bias meter to come in the mail, but the grid bias voltage (common to all tubes) seems to be set some place between conservative, and very overly conservative.

    Also, the amp works/sounds fine.

    So it must be exactly what gadget73 says.

    Of course, I'll check the current when I get my meter, but short of that showing a (correctable) problem, I'll continue to run it until something worse happens.
     

     

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  7. gadget73

    gadget73 junk junkie Subscriber

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    I actually wish they still made the 7591xyz. Its a perfect sub for a 7355 tube, which one of my amps runs. I'm sort of surprised nobody makes a 7355, it would just be a matter of re-basing a 5881.
     
  8. arts

    arts Super Member

    Messages:
    3,549
    Location:
    Qc, Canada
    The 7591xyz was Sovtek's quick and dirty attempt to cash in on the 7591 market without actually going to the effort of designing or just reverse engineering a genuine 7591.While the 5881/6L6WGC ''kinda-sorta'' worked in the application,biasing and drive requirments are substantially different,as is overall performance.

    They were really a poorly thought out idea,as you still needed a tech to either re-bias the amp (many of which couldn't supply enough negative voltage without modification,if at all) or install the now-included ''Bias Kit'' in cathode bias models.

    I was never a fan of this tube; the initial offering made no mention whatsoever of the substantially differing bias requirements,which caused many problems for owners who were not technically savvy.And in addition,you were now dependant on an ''oddball'' tube.
     
  9. linuxslate

    linuxslate Member

    Messages:
    58
    Location:
    Central Florida, USA
    Whoever made/rebuilt my amp put a bias pot in it, so if the Russians interfere in my listening, my understanding is that I have lots of basically drop-in (drop-in and re-bias) options.
     

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