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    Need help/suggestions on this ST70.

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    poconoman

    Posts : 106
    Join date : 2013-01-17

    Need help/suggestions on this ST70.

    Post by poconoman on Fri Oct 17, 2014 12:46 pm

    What I think this is, is a V2 ST70. It has been modded, that it doesn't have bias control, two cans and no rectifier. Is it fixed bias? Bring it back to tube rec? Also, I'd like to change the two 50MFD caps. What's the best option? ANY mods/changes is welcome.

    I'm not an 'expert' tech by no means. I can solder and I know about safety. But with some guidance and some step by step, I can do it.

    Thank you

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    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: Need help/suggestions on this ST70.

    Post by MontanaWay on Fri Oct 17, 2014 12:56 pm

    well it does have the two BIAS potentiometers fitted, so I am assuming it has BIAS adjustment, biasing two tubes per channel.
    It is using what looks like two diodes as the rectifier on a tag strip.
    And quite an older version driver board.
    If this was my ST70, I would upgrade as follows:
    New multi cap, 80/40/40/20, get rid of both caps now in use. Check those rectifier diodes are 1N4007, if not, replace them with those.
    New VTA driver board, this will then also give individual BIAS control for each tube, much better than the current setup. Will need some mods, but pretty straight forward.

    poconoman

    Posts : 106
    Join date : 2013-01-17

    Re: Need help/suggestions on this ST70.

    Post by poconoman on Fri Oct 17, 2014 1:10 pm

    MontanaWay wrote:well it does have the two BIAS potentiometers fitted, so I am assuming it has BIAS, biasing two tubes per channel.
    It is using what looks like two diodes as the rectifier on a tag strip.
    And quite an older version driver board.
    If this was my ST70, I would upgrade as follows:
    New multi cap, 80/40/40/20, get rid of both caps now in use. Check those rectifier diodes are 1N4007, if not, replace them with those.
    New VTA driver board, this will then also give individual BIAS control for each tube, much better than the current setup. Will need some mods, but pretty straight forward.

    Thanks MW!

    Would I need to bring back the tube rectifier or the diodes are fine? Just change them?

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: Need help/suggestions on this ST70.

    Post by MontanaWay on Fri Oct 17, 2014 1:15 pm

    poconoman wrote:
    MontanaWay wrote:well it does have the two BIAS potentiometers fitted, so I am assuming it has BIAS, biasing two tubes per channel.
    It is using what looks like two diodes as the rectifier on a tag strip.
    And quite an older version driver board.
    If this was my ST70, I would upgrade as follows:
    New multi cap, 80/40/40/20, get rid of both caps now in use. Check those rectifier diodes are 1N4007, if not, replace them with those.
    New VTA driver board, this will then also give individual BIAS control for each tube, much better than the current setup. Will need some mods, but pretty straight forward.

    Thanks MW!

    Would I need to bring back the tube rectifier?

    not really a must have I guess, 1N4007's, rated at 1000V will be more than enough to handle this amp. If you want the look of a tube rectifier, then sure.
    Also, since it is using the original itty bitty Dynaco power transformer, stick to EL34's, you'd really be pushing that transformer if you want to use say KT88's!
    The VTA driver board will make a WORLD of difference in the sound this little guy with give you, at around $100 or so, it is REALLY worth the upgrade! With it too, you can do quite a bit of tube rolling as well, trying out all the 12AU7 variations.
    Have fun!

    GP49

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

    Re: Need help/suggestions on this ST70.

    Post by GP49 on Fri Oct 17, 2014 11:12 pm

    It appears to have a bandpass filter on the input, to prevent extreme high and low frequencies from getting into the amplifer. Possibly it was copied from the Panor Dynaco Stereo 70 II. Panor copied it without permission from Sound Valves (originally Sound Values) which "reissued" the Stereo 70 after buying the parts stock when Dynaco shut down. Sound Valves copied it without permission from Frank Van Alstine's "Super 70" modification.

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: Need help/suggestions on this ST70.

    Post by MontanaWay on Fri Oct 17, 2014 11:17 pm

    GP49 wrote:It appears to have a bandpass filter on the input, to prevent extreme high and low frequencies from getting into the amplifer. Possibly it was copied from the Panor Dynaco Stereo 70 II.  Panor copied it without permission from Sound Valves (originally Sound Values) which "reissued" the Stereo 70 after buying the parts stock when Dynaco shut down.  Sound Valves copied it without permission from Frank Van Alstine's "Super 70" modification.

    these would more than likely not be required for the VTA driver board upgrade.

    mazeeff

    Posts : 114
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    Age : 61
    Location : Grand Rapids, Michigan, USA

    Re: Need help/suggestions on this ST70.

    Post by mazeeff on Sat Oct 18, 2014 5:42 am

    I may be mistaken, but it looks like the 3A fuse is missing, and has been jumpered. The on/off switch looks a little funky as well! If you plan to work on this, I would add a fuse and proper on/off switch prior to turning it on. That fuse is there for safety reasons!

    Mike

    MontanaWay

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    Re: Need help/suggestions on this ST70.

    Post by MontanaWay on Sat Oct 18, 2014 10:10 am

    mazeeff wrote:I may be mistaken, but it looks like the 3A fuse is missing, and has been jumpered. The on/off switch looks a little funky as well! If you plan to work on this, I would add a fuse and proper on/off switch prior to turning it on. That fuse is there for safety reasons!

    Mike

    well spotted!...yes, absolutely, re install a fuse and better switch assembly! Why anyone would want to eliminate a fuse, especially for a tube power amp....... affraid

    mazeeff

    Posts : 114
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    Age : 61
    Location : Grand Rapids, Michigan, USA

    Re: Need help/suggestions on this ST70.

    Post by mazeeff on Sat Oct 18, 2014 10:57 am

    Also, when the SS (Solid State) rectification was installed, the white wires from the transformer were simply cut, and left open. Not good! From the looks of things, whoever did the prior work, simply used parts from the junkbox. Not always a bad thing, but anytime, I see electrical tape, I start to worry. The Gold cap can looks like a roman candle to me!!! Might be a good candidate for a strip and rebuild. Mine was the same way. Everything in it, except the transformers, chassis, and bias pots was replaced.

    Mike

    peterh

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    Location : gothenburg, sweden

    Re: Need help/suggestions on this ST70.

    Post by peterh on Sat Oct 18, 2014 1:22 pm

    mazeeff wrote:Also, when the SS (Solid State) rectification was installed, the white wires from the transformer were simply cut, and left open. Not good! From the looks of things, whoever did the prior work, simply used parts from the junkbox. Not always a bad thing, but anytime, I see electrical tape, I start to worry. The Gold cap can looks like a roman candle to me!!! Might be a good candidate for a strip and rebuild. Mine was the same way. Everything in it, except the transformers, chassis, and bias pots was replaced.

    Mike

    The 5V windings for the rectifier may be of great value ;
    As they are well insulated one could use them to adjust mains voltage; put them
    in series with the primary and you will add or subtract 5V depending on polarity.
    As i understand a lot of people in US has problems with mains above 120V, thus by
    using the 5V one could subtract 5 V .


    zx

    Posts : 194
    Join date : 2011-08-05

    Re: Need help/suggestions on this ST70.

    Post by zx on Sat Oct 18, 2014 2:27 pm

    Thanks for input on the 5 V winding....but the 120-26 Ac come from the wall..put the Dynaco MK3s at 510-20 DCV...way high for good sound ....Like ARC i feel 460-80 is the sweet spot for most ..50-60 watt tube amps B+.......Varaic works best....an saves tubes!

    Just my finding.................




    Thanks for the site Bob............

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: Need help/suggestions on this ST70.

    Post by MontanaWay on Sat Oct 18, 2014 2:35 pm

    going back to my first reply, there are basically two ways to get this ST70 up to speed.
    Do some changes, ie multi cap, fuse, switch and maybe VTA driver pcb, OR, as has already been suggested, do a complete 'ground up' restoration, strip the chassis, clean and polish, or paint, the chassis, you should be able to re use the tube sockets, clean and re paint the transformers, maybe even consider getting the higher rated power transformer, re wire the lot, either re use or replace the speaker terminals, VTA driver pcb, new multi cap, maybe even go to tube or Weber rectifier. By doing this total restore with the up graded power transformer, now you can also start using KT77, 88 etc..

    poconoman

    Posts : 106
    Join date : 2013-01-17

    Re: Need help/suggestions on this ST70.

    Post by poconoman on Mon Oct 20, 2014 10:07 am

    Great suggestions everyone!

    As an option, would anyone recommend a good amp tech in the NYC, NNJ and NEPA area?

    Thanks!

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Need help/suggestions on this ST70.

    Post by Bob Latino on Mon Oct 20, 2014 5:47 pm

    poconoman wrote:Great suggestions everyone!

    As an option, would anyone recommend a good amp tech in the NYC, NNJ and NEPA area?

    Thanks!

    Yes - Get in touch with Sal Brisindi in Freehold, NJ ... Sal is the best Dynaco tech that I know of in the NY, NJ area. You can Email Sal directly at salb203 (at) aol (dot) com ..

    Bob

    poconoman

    Posts : 106
    Join date : 2013-01-17

    Re: Need help/suggestions on this ST70.

    Post by poconoman on Tue Oct 21, 2014 10:28 am

    Bob Latino wrote:
    poconoman wrote:Great suggestions everyone!

    As an option, would anyone recommend a good amp tech in the NYC, NNJ and NEPA area?

    Thanks!

    Yes - Get in touch with Sal Brisindi in Freehold, NJ ... Sal is the best Dynaco tech that I know of in the NY, NJ area. You can Email Sal directly at salb203 (at) aol (dot) com ..

    Bob

    Thank you Bob!

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