New to 7591's - how should they look when running?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mprvise, Sep 4, 2008.

  1. mprvise

    mprvise Member

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    I recently obtained a vintage Ampeg with 7591 power tubes. I LOVE the way this amp sounds, but I fear the tubes may be running hot. It's my understanding that this is a cathode bias amp, so for me that means taking it to a tech to make changes. The amp currently has the original tubes in it. The tubes are glowing blueish when fully warmed up. Either way, I will likely order some JJ's soon and save the original tubes.
     
  2. Blue Strat

    Blue Strat Member

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    Blue glow isn't necessarilly an indication of anything bad, or good.

    http://kcanostubes.com/content/newsletter_details.asp?ArticleID=10

    As with any 40+ year old amp, it requires maintenance. At very least, the cathode bias capacitor should be replaced. When this $1 part fails it will take out your power tubes and possibly your output transformer.

    The amp needs a full cap job if this hasn't been done in the last 20 years.
     
  3. mprvise

    mprvise Member

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    I'll be taking the amp to a tech soon. The amp was stored for 40ish years, and was headed for the dumpster (!!!) when a friend rescued it and called me. Before it went into the closet of no return it was used by a young lady with her accordian for a few years. The amp is all original and has likely never been touched by a tech. Thanks for the reply and link.
     
  4. Trout

    Trout Member

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    I have read a lot of very negative reviews on the JJ 7591's in the audiophile forums. Most of the posts were related to actual tube failures.
    Often in less than 4 hours use.

    You might want to hunt down some NOS, or perhaps get EH or Sovtek.

    Just a thought.
     
  5. brad347

    brad347 Member

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    if the amp was stored for a long time, that's a very good argument to not turn it on at all until you have all the electrolytic capacitors replaced.

    Caps need to be used or they go bad.
     
  6. Doug's Tubes

    Doug's Tubes Member

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    This doesnt come close at all with my experience with the JJ 7591. I've just spent a half hour on Audiogon and Audio Karma and cant find a negative review. Any failures were usually due to the end user failing to set the bias properly, or the amps specs were out of whack and were running the tubes way beyond spec. In fact, I still have some prototypes running in a 1964 Ampeg Reverberocket 4 years later(EH's which are just a repinned 6L6 wouldnt fit in the amp).......

    I've sold hundreds of these with a few failures. The only complaint I've heard from my audiophile customers is that the plate disippation falls short of NOS by a few watts. Otherwise, the tone is nearly equal to NOS.
     
  7. Trout

    Trout Member

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    Try over at DIYaudio.com, it has been discussed over there several times.

    There are other posts as well,
     

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