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    Final wiring procedures on ST-120

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    musky37

    Posts : 4
    Join date : 2012-02-17

    Final wiring procedures on ST-120

    Post by musky37 on Wed Feb 22, 2012 4:52 pm

    First post here--just about to finish wiring the ST-120. I've been careful and have taken it slow (excellent instructions Bob!). Just wondering if there are things I can do to the internal wiring or checking of soldering to help ensure sonic success. I know Bob has suggested not running wires adjacent (parallel) to the twisted wires. Is it recommended to bundle wires together with twisties? I assume I should put the bottom on the amp before powering it up for the first time and performing the first warm start. Thanks for your responses.

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: Final wiring procedures on ST-120

    Post by mantha3 on Wed Feb 22, 2012 9:15 pm

    Take a photo of the underside and post it here. Great idea for the final steps... You are at a good stage to make tweeks if advised by Bob and Roy.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Final wiring procedures on ST-120

    Post by Bob Latino on Thu Feb 23, 2012 8:36 am

    musky37 wrote:First post here--just about to finish wiring the ST-120. I've been careful and have taken it slow (excellent instructions Bob!). Just wondering if there are things I can do to the internal wiring or checking of soldering to help ensure sonic success. I know Bob has suggested not running wires adjacent (parallel) to the twisted wires. Is it recommended to bundle wires together with twisties? I assume I should put the bottom on the amp before powering it up for the first time and performing the first warm start. Thanks for your responses.

    The general approach when wiring amps is to TWIST all wires carrying AC current and to "try" not to run too many sets of wires parallel to each other for long distances. I don't use "twisties" myself although the wiring will look neater if you do use them.

    You don't have to place the bottom cover on the amp when you start it up for the first time. After the amp has been run a little and you know that it is STABLE, shut it off, and then put on the bottom cover.

    Bob

    musky37

    Posts : 4
    Join date : 2012-02-17

    Re: Final wiring procedures on ST-120

    Post by musky37 on Tue Feb 28, 2012 11:21 am

    OK--I'll try to get a shot of my wiring job here now. I've had a bit of a problem on initial power up. I checked out the time delay board per the instructions, voltages measured ok. I then installed all tubes except the right side power tubes and successfully biased the left side. I then installed the right side power tubes and powered up. About five seconds after the time delay board clicked, there was a momentary arc across the rectifier tube so I shut her down. I checked my wiring and repeated the procedure to same effect. I then called Bob (nice that he lives a couple of towns over from me!) who instructed me to bias each side independently and play music through each side independently (with only two power tubes installed at a time) and this worked fine. He suggested I may have a bad power tube so I drove over and grabbed some known-good tubes, tried them out but got the same arc on power up when all four power tubes were installed. I've got a new rectifier tube coming the mail today from tubedepot, will try that and hopefully it will not arc. Will report back soon. Here's the shot:

    [img][/img]

    musky37

    Posts : 4
    Join date : 2012-02-17

    Re: Final wiring procedures on ST-120

    Post by musky37 on Wed Feb 29, 2012 10:01 am

    Well--it appears the electron gods were smiling on me. The new sovtek rectifier tube arrived via overnight delivery which cost more than the tube did but who cares? When I performed the initial power up procedure per the instructions again from the beginning, and put in the right side two power tubes and powered up, I was just waiting for the arc and pop and when it didn't happen I thanked my lucky stars. I then was able to bias four tubes successfully for the first time in my lifetime. Can't wait now to put it into my system which is tri-amped with homemade speakers--the ST-120 will be driving the midrange--have SS amps for subs and tweeters.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Final wiring procedures on ST-120

    Post by Bob Latino on Wed Feb 29, 2012 11:02 am

    Hi Carl,

    Glad that you are up and running ... The 5AR4/GZ34 rectifier on Dynaco ST-70, Mark III, Mark IV and the VTA ST-120 is the tube most likely to "bite the dust". Those rectifier tubes have 360+ volts on 4 pins and the long term high voltage abuse takes its toll on these tubes. The real problem is that these GZ34 tubes made today are not as "robust" (read that robust as > not as well made) as those tubes made in the 1950's and 1960's in the USA and Great Britain. This is the reason why people hoard OR drive the price up on NOS or even USED older Mullard or USA made GZ34's. If you can get your hands on one without remortgaging your home to come up with the funds, buy one.

    Another way to get around this is to use an older USA made 5U4 rectifier tube in one of the above listed amps. A 5U4 rectifier tube will work in place of a GZ34 BUT with a slight loss of power on your amp as a 5U4 has a higher voltage drop than a GZ34. You may not notice the loss of power, however, unless you push the amp really hard. You will probably gain rectifier tube life.

    Bob

    costerdock

    Posts : 33
    Join date : 2011-07-27

    Re: Final wiring procedures on ST-120

    Post by costerdock on Thu Mar 01, 2012 10:57 pm

    My sovteks rectifiers didn't last long at all - few hours - I've been happy with the SS webers. Someday I'll get some old mullards.

    musky37

    Posts : 4
    Join date : 2012-02-17

    Re: Final wiring procedures on ST-120

    Post by musky37 on Fri Mar 16, 2012 9:08 am

    So far so good--in fact, great!! My ST-120 has been up and running on average 4 hours/day, sometimes 12 with no problems. It's powering my midranges in a tri-amp configuration feeding homemade speakers and sounding fantastic. Here's a shot of the amp under the blue LED lights of my home-built stereo cabinet (solid maple). I was concerned about putting the amp in the cabinet and closing the doors even though it is well ventilated in the back, but I found as long as I leave the doors ajar, heat does not build up very much at all. I'll also attach a photo of the cabinet...

    [img][/img]

    [img][/img]

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