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    ST 70 startup



    Posts : 26
    Join date : 2011-09-11

    ST 70 startup

    Post by SnugDynaco on Sun Sep 11, 2011 9:36 pm

    I acquired this previously modified unit, serial 6649027, made in the late 70s. The seller performed three of Adams' recommendations - replacing the molded paper capacitors; the selenium rectifier and the bias filter capacitors. Today, I replaced the can capacitor. Seller advised that all was in good working order at the time of sale, including all of the tubes

    . My question is, how to start this up. Do I need to purchase Variac and perform other tests, lest it goes up like a roman candle? Or do I just bias the unit and let 'er rip?

    I built a Bottlehead Quickie in the meantime, so I've really got two unproven variables to deal with.

    All input (pun intended) is greatly appreciated.


    Posts : 46
    Join date : 2009-10-16
    Age : 60
    Location : Central Florida

    Re: ST 70 startup

    Post by jhoak on Sun Sep 11, 2011 9:58 pm

    A variac is not exactly something that you can just pick up at the local Radio Shack but is a VERY good idea. Especially when bringing up an "untested" piece of equipment.

    Do a Google search on "light bulb variac" for suggestions on how to put together a low budget version that will preserve your hardware if there's a problem.
    Bob Latino

    Posts : 2639
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST 70 startup

    Post by Bob Latino on Mon Sep 12, 2011 7:37 am


    In the absence of a variac try this start-up procedure ...

    1. Plug in all the tubes except the rectifier and see if all the inserted tubes light up and stay on for a minute. This is a test for your power transformer. If all goes well then the amp will stay on without incident.

    Turn the amp OFF and connect the left channel's output terminals to a speaker.

    2. Pull the right channel's output tubes, leave the left channel's output tubes in there and plug in the rectifier tube. Turn the amp on and keep your eye on the rectifier tube and your hand on the power switch. The key time is 10 to 15 seconds after turn on > If you see any sparks (arcing) inside the rectifier tube immediately shut the amp off. If the amp stays on OK without arcing then bias the left channel. If it biases up without incident you are probably OK.

    Turn the amp OFF and connect the right channel's output terminals to a speaker.

    3. Plug in the right channel output tubes (leave the left channel's output tubes in there) and repeat as in # 2 above (eyes on the rectifier and hand on the power switch). If no arcing occurs, then bias up the right channel. When you bias up the right channel the bias on the left channel will drop because all four output tubes share the bias current. Go back and forth between channels until all output tubes have the correct bias.

    4. Play music through the amp and listen for any abnormalities like distortion, noises, excessive hum etc.



    Posts : 26
    Join date : 2011-09-11

    Re: ST 70 startup

    Post by SnugDynaco on Wed Sep 14, 2011 7:03 am

    Thank you. I'll give it a try and report back.


    Posts : 26
    Join date : 2011-09-11

    Re: ST 70 startup

    Post by SnugDynaco on Sat Sep 17, 2011 6:37 pm

    Okay, everything works. A little bit of hum, which cannot be heard when the music is at any appreciable level. No sparking, smoke or death jolts.

    Playing though a pair of Heresy II's, Monster speaker wire, RS interconnects and older NAD CD. Let the upgrading begin!

    Thanks again for the guidance.

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    Re: ST 70 startup

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