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    TAD 6550A-STR in ST-120

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    MarcVBelgium

    Posts : 81
    Join date : 2013-12-06
    Age : 62
    Location : Antwerp, BELGIUM

    TAD 6550A-STR in ST-120

    Post by MarcVBelgium on Mon Feb 08, 2016 9:10 am

    Hello to all.

    Recently I noticed a strange situation with the TAD 6550A-STR tubes on my ST-120. In november, while playing some music,
    I heard a distorsion in the left channel....Checking, I found the front tube had zero bias. Non-adjustable, so I swapped
    with a new tube (I Always buy an extra pair of tubes, all "matched")... Problem solved. Conclusion : bad tube.
    Important : no pops, clicks, highlighting, fireworks,....... nothing of the kind. Just from 0.55 to 0 in 1 second.

    Yesterday I noticed a slight 50Hz hum in the right channel......... same thing, this time rear right tube with no bias, not adjustable.....
    Swapped with new (last one...)tube and.... problem solved. Again, no pops, clicks or fireworks of any kind...

    The amp uses a GZ34 (1960's old Mullard), center 12BH7 and 2 12AU7...

    What I'm wondering about : is the amp blowing tubes and should I check certain things, or..... are TAD 6550's rather weak tubes ?

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: TAD 6550A-STR in ST-120

    Post by GP49 on Mon Feb 08, 2016 1:07 pm

    Nothing current is as good as the old, legacy tubes were.* You might try to reflow the solder and add some more to the plate and cathode pins of your dead 6550s.  If one or the other goes open there will be no current flow, whether as idling cathode current ("bias") or otherwise, and the unbalanced output, no longer push-pull, will lose its hum-nulling properties.  If the grid pin had gone open, the tube would have run away and red-plated.

    Those tubes are useless now so there is nothing to lose.

    * which reminds me, I just last week found three pair of NOS Mullard EL34s and a 5AR4 in a box in a bedroom closet, where a relative put them while "helping" me move in 1993!

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: TAD 6550A-STR in ST-120

    Post by deepee99 on Tue Feb 09, 2016 2:53 pm

    GP49 wrote:

    * which reminds me, I just last week found three pair of NOS Mullard EL34s and a 5AR4 in a box in a bedroom closet, where a relative put them while "helping" me move in 1993!
    Nice score. Like finding a Franklin in your coat pocket from a winter ago.
    Not sure about the NOS Tung-Sol 6550s (I just sold all 17 of mine): I find the Gold Lions KT-88s very robust and good-sounding. I've had one go short on me and (again, my opinion) they hold their vacuum much better than the KT-120s. Think the real action in the driver and pre-amp slots where NOS tubes are a must.

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