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    All bugs worked out, Mark-3's sounding great and running cool!

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    GlacierJohn

    Posts : 38
    Join date : 2014-04-17

    All bugs worked out, Mark-3's sounding great and running cool!

    Post by GlacierJohn on Thu Apr 24, 2014 10:12 am

    Well it was a bit of a process, but not bad really. What I think happened was a bad solder joint on the power supply ground during my re-build led to that runaway bias. After a couple days trouble shooting I discovered the bad joint and fixed it, what I didn't know is the runaway bias damaged the rectifier tube. The tube apparently was a bit intermittent, coming up normally for about four minutes, then dropping off so that it takes full pot to get about 1.4 volts. I think that day that I played the amps for 30-40 minutes, the rectifier kicked in a was running full bias for 10-15 minutes, that's why the amp got so hot.

    Yesterday I put an old, still good Sylvania GZ34 in the amp and it biased perfectly and held steady for 30 minutes. Then I hooked both amps up to my system and played music, wow is all I can say. These are really nice amps, lots of good clean power and now they are running nice and cool. Based on some internet advise about setting the bias a little lower due to today's higher line voltage (mine is 123 volts), I set my bias at 1.50 rather than 1.56. Anyway, thank you for your patience and good ideas while I struggled with this over the last week.

    John

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: All bugs worked out, Mark-3's sounding great and running cool!

    Post by GP49 on Thu Apr 24, 2014 10:56 am

    Glad to hear your amp is making music! That Sylvania GZ34 might just last as long as you need it to, by the way. I know of one that has been in a Stereo 70 used semi-regularly over the past forty years!

    As for setting the bias, though the difference between a reading of 1.5 volts and 1.56 volts is minuscule, there really is no need to do that due to higher line voltage, contrary to some of the advice out there. No matter what the line voltage is, a reading of 1.56 volts indicates the identical idle current through the tube regardless of line voltage (Ohm's Law is just that...a law, and immutable). But with today's tubes, which are less durable and rugged than the genuine Mullard EL34s of old, it is a good idea to set the bias lower anyway, for reasons of tube longevity.

    ENJOY YOUR Mk IIIs!!!

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: All bugs worked out, Mark-3's sounding great and running cool!

    Post by sKiZo on Thu Apr 24, 2014 3:10 pm

    Another happy ending!

    Bad solder joint ... hmmmm ... where have I heard that before?  Suspect 

    Worth repeating, a real good magnifier and non-metallic prod are your handiest tools when tracking down a problem in a point to point build.

    The older tubes are just a lot stronger due to the rare (and some would say dangerous) metals used in their construction. Wonder if you have to call in a hazmat squad if any of the magic smoke gets out? I'm trying to build up extra quantities of radon and black mold just in case ... I hear that's supposed to counteract it.  Wink 




    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: All bugs worked out, Mark-3's sounding great and running cool!

    Post by j beede on Thu Apr 24, 2014 10:26 pm

    FYI: I rescued a pair of MkIII from their outdoor storage under a picnic table. They were rebuilt "just for fun". I swapped them in for a Threshold power amp to do a listening test. That was about three years ago and the MkIII are still in place "being tested"  Smile 

    "

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: All bugs worked out, Mark-3's sounding great and running cool!

    Post by sKiZo on Fri Apr 25, 2014 2:12 pm

    Can I have them for testing when you're done testing them?

    No big rush ...  clown 

    GlacierJohn

    Posts : 38
    Join date : 2014-04-17

    Re: All bugs worked out, Mark-3's sounding great and running cool!

    Post by GlacierJohn on Fri Apr 25, 2014 6:43 pm

    Looks like they came out pretty clean after being stored outdoors. Very nice looking amps...

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: All bugs worked out, Mark-3's sounding great and running cool!

    Post by tubes4hifi on Fri Apr 25, 2014 9:28 pm

    even in the $5000 plus region of SS amps, most of them sound like crap compared to a well executed tube design even with 1/4 the power and 1/4th the price.
    SS SUCKS !!!!!!!!!!! just my own opinion of course . . . . likely shared by 99% of the people on this forum!!!

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: All bugs worked out, Mark-3's sounding great and running cool!

    Post by j beede on Sat Apr 26, 2014 4:23 pm

    FYI: The quad cap "cans" are just for show. I built boards using series pairs of modern electrolytics that fit under the chassis with no mods. The full restoration took about 40 hours--not including trips to the hardware store and local electronics component shop!


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