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    Help with Identification - ST70



    Posts : 5
    Join date : 2014-09-23

    Help with Identification - ST70

    Post by fastbulbous on Tue Sep 23, 2014 12:24 pm

    Hi There and thanks for allowing me in...

    I am newly registered to the site, seeking help on identifying the series (approx date) and the different wiring modifications as shown in the pictures below.

    I am being offered this as a project (at a very reasonable price) but would welcome a guide as to how it looks to your expert eyes?

    It looks as if the tube rectification has been modified along with quite a few other mods which, I am not really up to speed on.

    I would welcome any and all suggestions as to if this is a worthwhile project to embark on?

    Currently this fires up but has sound only on one channel.

    Look forward to your responses and thanks in advance


    Re: Help with Identification - ST70

    Post by Guest on Tue Sep 23, 2014 1:00 pm

    well I think it would be a worth while project. But, if it was me, I would pretty much gut it and start again.
    The wiring is pretty messy and I would probably also switch to the VTA driver board.
    The diode rectifier should be OK, but you'll need to check all the high voltages.
    As to one channel not working, it could be bad solder joint, bad tube/s, wire/s come off.
    By doing a restoration of sorts and updating the driver board, you should end up with a first class tube power amp!
    Also, by changing to the VTA driver pcb, you'll end up with individual BIAS adjustment for each power tube, a big improvement over your current setup, not to mention a MUCH better sounding amp!


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    Join date : 2014-09-23

    Re: Help with Identification - ST70

    Post by fastbulbous on Tue Sep 23, 2014 9:58 pm

    Many thanks for your reply.

    I will try it as it is, but realistically, I agree, I have to rebuild it some how and upgrade it also. That was my intention in considering it as a rebuild project.

    I would be interested to know if the transformers chassis, cover, original wiring etc. are Series 1..?

    The seller keeps changing tack - originally saying it was a series II, but from my research it looks older and looks like a Series I with the tube rectification modification and on/off switches and mono switch removed along with changes to the speaker output terminals.

    Any pointers on the model type would be greatly appreciated?

    In the final analysis I suppose its all moot if I am rebuilding it but provenance and all that.

    I already agreed to buy it so we are off an running now.

    Thanks in advance (again).


    Re: Help with Identification - ST70

    Post by Guest on Tue Sep 23, 2014 10:07 pm

    are ALL the tubes glowing when switched on, that is, are all their filaments working?
    I would still check B+ to make sure that at least is good.
    If all the above is good, then perhaps swap some tubes over, that is, take a tube from the working channel and swap it over to the non working, do make sure though that ALL tubes are in at all times!
    I could not really tell you if it is the really early model or not, the driver board for sure is not the first generation, difficult to say from the photo but from what I can see is that the transformer is not original series either, perhaps other members can chime in here.


    Posts : 1309
    Join date : 2008-11-30

    Re: Help with Identification - ST70

    Post by tubes4hifi on Sat Sep 27, 2014 2:06 am

    it's definitely an older series 1 Dyna ST70 with lots of mods. Personally I would put the tube rectifier back in.
    It's probably a 6GH8 type driver board which sounds similar to the original, but as MontanaWay has suggested,
    the VTA driver is a HUGE improvement in sound quality and allows individual bias adjustment of each output tube.
    You can download the original ST70 manual on my website


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    Join date : 2014-09-23

    Re: Help with Identification - ST70

    Post by fastbulbous on Sat Sep 27, 2014 5:06 am

    Many thanks for the information and suggestions.

    Much appreciated Very Happy

    Captain Coconut

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    Age : 64
    Location : Great White North

    Re: Help with Identification - ST70

    Post by Captain Coconut on Sat Sep 27, 2014 9:16 am

    fastbulbous huh? Shouldn't it be fast n' bulbous? Captain Beefheart, right? I know all the Captains! Laughing


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    Join date : 2014-09-23

    Re: Help with Identification - ST70

    Post by fastbulbous on Sat Sep 27, 2014 10:36 am

    A squid eating dough in a polyethylene bag is fast and bulbous. Got me?
    That's right, The Mascara Snake. Fast and bulbous! Also, a tin teardrop!
    Bulbous, also tapered.

    mark four

    Posts : 33
    Join date : 2011-08-13
    Age : 61
    Location : philadelphia , home of dynaco

    stereo 70

    Post by mark four on Sun Sep 28, 2014 12:32 pm

    i have the next best thing .a pair of mark fours and i love the hell out of them . great sound . good luck with your dynaco   ."  the earth is matter , the stars are matter , but it doesn't matter " happy listening


    Posts : 163
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    Re: Help with Identification - ST70

    Post by stewdan on Sun Sep 28, 2014 3:04 pm

    Hi Fastbulbous -- Welcome to the Forum.

    You were asking were to start:

    1) There is a table and a schematic at the back-end of the Dynaco ST70 Manual (via the link from Roy) that shows representative voltages at various points along the circuit of a stock ST70.

    2) I looked at your underside wiring picture but it is not very enlargeable, before it gets too fuzzy to read. So first can you upload a higher resolution picture, so that we can blow it up to  see details of the wiring??

    3) The things that your wiring picture called out to me are: (a) GZ34 Tube replaced by diodes, (b) there is some sort of jack on the front panel (to the left of the RCA Inputs) that replaced the Stereo/Mono Switch (possibly turning the amp into a mono only version????), (c) On the bottom right of the picture there are some additional capacitors, but I cant tell how they are wired into the circuit (don't know if they replace all or part or add additional capacitance to Quad Cap sections (Silver Tube) that is on the top of the chassis, (d) On the back panel someone replaced the two original speaker connectors with something better (two-wire as opposed to four-wire) but can't tell what speaker impedance is supported (4 ohm or 8 ohm or 16 ohm), you would need to check the colors of the transformer wires that attach to tell what ohmage is supported, (e) On back side, it looks like the ON-OFF Switch is missing (or replaced??). There may be additional mods, but that is all I can see now with the fuzzy picture.

    4) If you don't already have one, I would suggest getting a 5 or 10 Amp Variac and use it at reduced voltage (instead of 120v output, use say 40v output, so that you would have 1/3 of the schematic or table listed voltage on the wiring side of the chassis when you check voltages).

    Hope this helps, if you have questions, ask them.


    Posts : 5
    Join date : 2014-09-23

    Re: Help with Identification - ST70

    Post by fastbulbous on Sun Sep 28, 2014 10:51 pm


    Many thanks for your comments. I will take hold of the amp at the end of this week so will be able to start to have a closer look at it.

    One other thing I think I may have detected is that it appears the tubes on the driver board are plugged in using adapters (the large white plastic bases at the base of each tube).

    I am keen to put it back to something like original but taking opportunity to benefit from any sonic improvements available from new kits and parts.

    Not in any hurry though  - just try to learn and enjoy the experience.

    I am sure I will be back with more Q's later.

    Thanks everyone, will update as we move forward.

    edit: Looked again and I think you are right Tube4hifi the driver board is for 6GHA Tubes

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    Re: Help with Identification - ST70

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