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Thread: Fender Twin-Amp Pro Tube question, replacing 6L6 with Vintage 5881's

  1. #1
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    Fender Twin-Amp Pro Tube question, replacing 6L6 with Vintage 5881's

    Hello, I would like an expert opinion on what level I should adjust my bias when replacing my 4 6L6 tubes with vintage tung-sol 5881's. I realize they are 23 watts vs 30 and wondering if the bias should be set at less than the 60ma that is listed in the manual. Any advise is welcome. Thanks.

  2. #2
    Old Timer J M Fahey's Avatar
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    Hi Foossh.
    I would respect the manual suggestions.
    I fear biasing them colder will, well, ... sound colder; don't think you want that.
    Where did you get a set of (presumably matched) Tung Sol 5881's?
    They must have cost a fortune.

  3. #3
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    My experience with a buddy's Pro Tube Twin is that 60mV was too cool, increasing to 80mV was a noticeable improvement, with the stock tubes (30 watt GT branded 6L6GC, Sovteks I think).
    60mV is less than 14 watts plate dissipation, as the B+ is a very reasonable and stiff 450V.
    I would try your TS 5881 at 50mV and 60mV, just don't go much hotter than that.
    But there's a lot of global negative feedback on that amp, which tends to negate the character of the tubes. And it's a super loud amp to push into overdrive, even with the naff stock speakers. So such rare expensive tubes wouldn't be used to best effect, IMO - save them for a tweed type, ideally a 5F6A Bassman / clone. Pete.

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    Thanks for the feedback, I believe I will try the bias at both 50mv and 60mv and see which sounds better, its at 60mv now but wanted to protect the lesser powered tubes a little bit. The other potential problem is I have two pairs of matched tubes but not necessarily does each pair match the other. I got them from an old radio/tv repairman in the midwest, there are 2 NOS Raytheon branded Tung Sol, and 2 Tung Sol branded, they look identical except for the printing on the tubes. I do have an old Sencore tester and they all 4 look pretty close to 90 on the emissions test but I don't know how reliable that is for matching. I also scored about 15 nos 12ax7's, various brands: GE, Amperex, RCA, and Syvania for 10$ each so I am also switching them between different positions with the most powerful ones in the V1 and V2 spots so far to get the best tone. I know its not the perfect situation but I am kind of a geek trying out these 1950's/60's tubes. The guy is looking through a couple more boxes he has stashed away to see if he can find me some 6L6 GC's but until then I will use what I got. They already sound much better than the sovtek 5881 WXT's that came in the amp.
    And your right this amp is loud, I use the 1/4 power switch in the house and switch back n forth between the clean and dirty channels and try out different pedals on the clean channel and compare to the stock distortion. These 5881's would have sounded good in my 1966 blackface bassman I recently traded off.

  5. #5
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    'I have two pairs of matched tubes but not necessarily does each pair match the other'
    As long as one from each pair is either side of the OT, then you should be good.
    Try the Sovteks at a higher current before you write them off.
    And try the amp through some different speakers - maybe they've changed, but the ones I tried weren't nice.
    Tube rolling is fun, but there's way more scope for improvement from the speakers. Pete.

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