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    New Project ST70



    Posts : 13
    Join date : 2013-03-05

    New Project ST70

    Post by JackF999 on Tue Mar 05, 2013 2:01 pm

    Hope everyone is doing good !!

    I'm here in Dallas with a ST70 I bought 10 years ago.
    I knew it was in very poor shape, ( as in wires all cut up )
    but because of money, job stress, etc.. I am just now
    getting to assemble it. ( After seeing the VTA board )

    I would appreciate some help in deciding what to replace,
    which options to choose on a VTA basic board, and also
    have some backup when I have a question about assembly.

    I'm so glad I didn't buy other upgrade kits ( like Van Alstine )
    or such, and feel pretty good about buying a VTA board with
    whatever else I need.

    This amp basically only has the original tube sockets, power transformer,
    audio transformers ( the board is original but I don't trust it at all,
    seems some people think putting 100 wonder caps will make an amp 100
    times better ) and some hardware. There is some rusty pitting on the frame
    but the cage looks pretty good. I don't really mind it is not in perfect
    cosmetic shape, I just want good reliable sound.

    I have another ST70 in original shape ( yes still has the selenium diode that
    I know I HAVE to change ) so I do have a physical model to compare to.

    If only someone could enlighten me as to the electronic schematic
    and functioning of a 3 inch solid copper wire, I could just maybe
    get this done.

    I'm going to continue on the next post to separate
    my ramblings with my proposed purchases and questions about
    assembling the new amp.


    Posts : 269
    Join date : 2011-04-04

    Re: New Project ST70

    Post by sailor on Tue Mar 05, 2013 2:27 pm

    I'm not sure I fully understand the condition but I think it is probably similar to one I had a few years ago. The best advise I can give is to sell it on ebay. Without tubes you will probably get about $150 for it. Then buy a full kit. Or buy a kit minus the transformers. But you need to know if your transformers are good before you do that. Buying parts will cost you more than a full kit.


    Posts : 13
    Join date : 2013-03-05

    Plans of purchase and replacements

    Post by JackF999 on Tue Mar 05, 2013 2:46 pm

    ( Could be double post )

    Let me see...

    From tube4hifi..

    I buy a VTA basic board, - $89
    upgraded caps option, - $10
    3 new board tubes, - $39
    probably needs a cap can - $40

    Do most people upgrade to those Russian caps?
    My concern, other than money, is, are they available
    soon, and reliably so recently.

    This amp had no bias pots ???? and I know the VTA board
    has 4 separate, so maybe it would be good to put
    those pentode/triode switches in the holes ?

    Tubes, big honkin output TUBES !!!!!!!!!!!!

    This amp came with 2, I assume to be normal tubes with
    lettering ( 6ca7 el34 ) and greenish PhilipsECG kj
    made in USA. One other tube says ( 6ca7 el34 )PhilipsECG j
    made in USA but the lettering is grey.
    These three all look the same to my eye.

    Now we come to the odd one out ( I orignally thought they were
    all the same ) that is fatter, taller, and the bottom round
    support has small star pokey dingies like the top support plate.
    I just noticed this one also has one ( I give you permission to laugh
    just this once ) little round antenna sticking straight up instead
    of slanting inward like the other one, and gives a vibration when I gently tap it.

    I'd like to use these tubes to get the amp running and then later
    buy those $110 tubes on tube4hifi.

    Okay, I'll let you rest

    Thanks for reading and hopefully responding...

    thanks so much

    Last edited by JackF999 on Tue Mar 05, 2013 3:13 pm; edited 1 time in total (Reason for editing : Correction)


    Posts : 13
    Join date : 2013-03-05

    Sailor me old matey

    Post by JackF999 on Tue Mar 05, 2013 3:09 pm

    Thanks for the candid opinion, Sailor.

    Firstly, I've read about ST70 years ago, but have forgotten much.
    Just how can I verify the condition of the audio transformers,
    hopefully close here in Dallas, with minimal cost ?

    Can I verify the power transformer using a DMM and how
    safely ?

    I'll research on the net for this, but just asking for a quick
    reply to get me started.

    Unfortunately, I've already started complete stripping and have
    put everything but the transformers in a little box.

    I used to see tube testers a couple of years ago in drug stores,
    errrrr,, more than a couple years ago, huh ?
    How to test the tubes here in Dallas ?
    Anybody know of some kind soul who can do this for minimal cost ?

    Sailor, what kit would cost less than rebuilding this amp?
    All the ones I have recently seen ( as in tube4hifi ) are beyond
    my reach or do not make economic sense ( at least in my mind )
    in comparison to rebuilding. I know the transformers would have to test
    good for that comparison.

    Thanks again Sailor..


    Posts : 13
    Join date : 2013-03-05


    Post by JackF999 on Tue Mar 05, 2013 3:29 pm

    Looked at Ebay and found audio transformers going
    for $225 a pair.
    Not besmirching any individual at all ( since I don't know
    about testing audio transformers ) but some say these
    come from a working amp. To my mind a person would have to
    send some signal and hook up a scope to see the true
    characteristics of such a transformer ?

    Newbie ramblings, if you want just ignore........

    I'll research a little more, but if I could verify
    my transformers, that's some savings right there.

    Bob Latino

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

    Re: New Project ST70

    Post by Bob Latino on Tue Mar 05, 2013 5:00 pm


    To verify an ST-70 POWER transformer get out your multitester and set it to AC volts. Remove all the tubes. Turn the amp on. Measure ..

    1. Each of the two RED wires on pins 4 and 6 of the rectifier tube socket to chassis ground > 350 to 380 AC
    2. ACROSS the two WHITE wires on pins 2 and 8 > 5 to 6 volts AC
    3. Where the RED/BLACK wire connects to the Selenium rectifier > about 55 volts AC
    4. ACROSS the ends of the two GREEN wires where they connect to V2 > 6 to 7 volts AC
    5. ACROSS the ends of the two BROWN wires where they connect to V7 > 6 to 7 volts AC
    6. Where the GREEN/YELLOW wire connects to the 7 lug terminals strip to chassis ground > 3 to 4 volts AC
    7. Where the BROWN/YELLOW wire connects to the 7 lug terminals strip to chassis ground > 3 to 4 volts AC

    Verify an OUTPUT transformer - WITH THE AMP OFF when you measure RESISTANCE...

    Primary side - BLUE, BLUE/WHITE, GREEN, GREEN/WHITE, RED - between any combination of these wires you should get between maybe 15 ohms to 150 ohms

    Secondary side - YELLOW, ORANGE, BROWN, BLACK - between the BLACK and any of the other three you should get a very low resistance - maybe 1.5 ohms or less

    Primary to secondary test > You should get NO CONTINUITY or an infinite resistance between any combination of 1 primary and 1 secondary wire.

    NOTE - there could be some output transformer issues that won't show up until high voltage is applied to the primary side when the amp is in operation but usually if the output transformer passes the resistance test - it is OK.



    Posts : 13
    Join date : 2013-03-05

    From a Bob to another Bob

    Post by JackF999 on Wed Mar 06, 2013 5:47 am

    Thank you muchly for your succinct and pertinent

    This is why I'm very seriously considering using
    your VTA upgrade parts for my rebuild.

    You go the extra mile for us DUIyers,,,,, DIYers, yeah that's it.

    I'm going to put this power transformer on a
    wood base, connect the fuse holder, clamp down all
    the wires with 1 inch spacing under a small strip of
    wood and not touch anything except with my Wavetech DMM.

    Does that sound safe ???


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