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    ST-120 Tune Up

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    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    ST-120 Tune Up

    Post by arledgsc on Sun May 15, 2016 2:39 pm

    Today I wired up the power transformer +5V rectifier winding as a bucker to mainly reduce plate voltage.  I wanted to eliminate the WZ68 and use straight SS rectifiers (WS1) while keeping the plate voltage at or below +500V.   I figured to get about a 4% reduction and that holds true.  With WZ68 I had 491V plate voltage and 497V with the bucker circuit wired in and WS1.

    Disclaimer:  You may lose line voltage isolation by introducing it in the transformer secondary.  I wired Neutral to the +5V winding and then the mains to keep the potential near ground in the transformer secondary.   Also have the chassis grounded and the Furman power conditioner has ground fault monitoring.  The +5V winding can easily handle the current so I am not too worried.  Your setup may be different though.

    When I had the chassis open I also noticed my Dueland preamp-to-phase inverter coupling caps were leaking all over the bottom of the chassis.  That really pisses me off especially for what they cost.  I had some Mundorf SIO 0.15uF caps and substituted those.  Wow!  I was missing a lot of low level detail with the leaky caps.  And now have more bass as well.  I think I am going to like the Mundorf SIOs.  So today has been productive.


    Maintarget

    Posts : 209
    Join date : 2013-02-10

    Re: ST-120 Tune Up

    Post by Maintarget on Sun May 15, 2016 4:30 pm

    I have the ST-120 with 1200 hours and the Russian PIO caps no complaints here, My SP-14 has the Mondorf SIO amazing sound had wondered about swapping out the Russian caps for the Mondorfs in the ST-120 keep us updated as they break in
    Curious how did the Dueland's perform initially, after how many hours of break in? and how many hours before the melt down occurred?
    I'm guessing it gets a little warm under the hood.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-120 Tune Up

    Post by arledgsc on Sun May 15, 2016 5:50 pm

    Maintarget wrote:I have the ST-120 with 1200 hours and the Russian PIO caps no complaints here, My SP-14 has the Mondorf SIO amazing sound had wondered about swapping out the Russian caps for the Mondorfs in the ST-120 keep us updated as they break in
    Curious how did the Dueland's perform initially, after how many hours of break in? and how many hours before the melt down occurred?
    I'm guessing it gets a little warm under the hood.

    The Dueland caps sounded great when installed 2 yrs ago and wasn't much of run in period. Warm with a lot of detail. I use the ST-120 every day and at least 1,000 hrs. per year so about 2k hrs total on the Dueland's.  I guess they just deteriorated and my hearing has been stuffering for while so probably did not notice a slow decline.  The ears are fix but the amp sounded bright and little raspy in the vocals.  The insides of the ST-120 must get warmer than I imagine.  But a cap shouldn't leak in a tube amp so glad to find something else.  A cooler pre-amp or speakers are probably best applications for the Dueland CAST types.

    The Mundorfs silver-in-oil (SIO) are settling in and clean sounding in comparison.  I believe the SIOs have a lot of detail and Mundorf's gold types are warmer sounding.  So far so good and no real complaints.  But I am liking the detail for a change.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-120 Tune Up

    Post by arledgsc on Mon May 16, 2016 4:49 pm

    Just a note that I looked up the specs for the Mundorf silver-oil caps and they are rated up to 70 deg C.  I mounted them inside the ST-120 chassis but, after some thought, will relocated them outside on top of the driver board. 

    Interesting the Mundorf silver-gold-oil caps are rated up to 85 deg C.

    After about 8 hours the Mundorf SIOs are sounding great.  Lots of bass now, dense mid-range harmonics, and much detail.   They are (or seem to be) jacked up on the high end a bit and hence (perhaps) the extra detail.   But they are smooth, wide, and deep.  I see why they are described as liquid.  My not be for everyone if you shy away from high frequencies.

    sKiZo

    Posts : 1309
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST-120 Tune Up

    Post by sKiZo on Mon May 16, 2016 6:10 pm

    ... can always roll in some 5963's ... those tend to be a bit darker than most ...

    I have just the opposite "problem" here. Currently running a 6sn7 center with Westinghouse 5963's in the sides, and that combo is a bit too dark for my tastes. Fortunately, I can get around that by running a tube that would normally be too bright for me in the Maverick TubeMagic that I use as a buffer. I'd say that setup tickles perfect. It's all about what comes out the spout, right?

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-120 Tune Up

    Post by arledgsc on Mon May 16, 2016 10:03 pm

    I don't really like to do too many changes before deciding how they affect sound quality. But very pleased with these ST-120 changes. Assume the SS rectifier has something to do with tighter, better bass. The highs are abundant but clean and plenty of sparkle and pop but in a relaxed way. Not grating or ear burning at all. I had a glare issue before the rework and no longer. Plus I am breaking in new tubes all around but those are coming around and not fully broken in (month). The Mundorf change is a thumbs up though. May be other caps that sound better but liking what I hear. Compared to last week the amp stepped up a notch or two.


    corndog71

    Posts : 451
    Join date : 2013-03-19
    Location : It can get windy here

    Re: ST-120 Tune Up

    Post by corndog71 on Mon May 16, 2016 11:00 pm

    This isn't the first time I've read about duelunds falling apart over time.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-120 Tune Up

    Post by arledgsc on Tue May 17, 2016 5:40 pm

    corndog71 wrote:This isn't the first time I've read about duelunds falling apart over time.  

    I read that also. It appears many oil and wax impregnated caps don't really like to be subjected to higher than average temperatures.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-120 Tune Up

    Post by arledgsc on Wed Aug 10, 2016 12:19 pm

    Three months have passed since wiring the ST-120 transformer rectifier heater winding as a bucker with the goal of using straight solid state rectification while keeping plate voltage below 500V. I could not be more pleased as bass is tighter and better imaging/ soundstage placements.

    The Mundorf silver-in oil coupling caps have settled in. The Duelands were warmer sounding while the Mundorfs are natural and allow the sound to come on through.

    I have been waffling back and forth between the Shuguang Natural Sound KT-88s and Tung Sol KT-120s. I like the KT-120s tonally but after a long run-in period Shuguang's have settled in and have much better clarity on the high end. Plenty of sparkle but refined. Bass is decent as well as more low level resolution. I must confess that I had to change the ST-120 negative feedback caps from stock 220pF to 260pF to help tame the highs a tiny bit. This change works with KT-120s as well with my system and ears. And the amp runs much cooler with KT-88s is a huge plus as well.

    I have gone through four DACs and two sets of speakers in the past 4 years. The ST-120 is the only component that remains in the system. Required a few tweaks to keep up but delivers the goods daily. Remarkable for a $1,000 amp.

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