ST-70 acquisition, now what?

Discussion in 'Tube Audio' started by stupidhead, Aug 13, 2017.

  1. stupidhead

    stupidhead aka Ratdogheads Subscriber

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    Hello fellow AKers. I am generally a speaker forum browser but have recently upped my game a bit in the amp dept. Like many here, I was lost in the digital black faced audio world through the 90's and early 2000's, but have been finding my way back to analog. I have acquired a Marantz 2230 totally rehabbed by Pat Font, a 2270 totally rehabbed by Randy Buckner and a 2325 which needs some love before being used anymore. I have also assembled a nice selection of AR speakers which I have been working on myself and have been thoroughly enjoying the whole process.
    A couple years ago I was interested in getting a tube amp and began doing some research. The end result is a Citation II from McShane's tech Don Sachs. It was his personal amp with all the upgrades plus and he was looking to sell it to fund the build of another one.

    glowingredux.JPG

    I am not a tech nor do I have any formal electric/electronic training. I haven't a clue what to do if anything goes south with this and I use it sparingly at the time. Actually using the 2230 as the pre. Very pleased with the resulting sound and have been keeping my eyes out for a proper pre.
    An acquaintance who does estate sales and house clean outs lets me know when he comes across anything he thinks I might be interested in. This past week he texted with news of a Stereo 70 he had come across that I could get for a fair price (under $500). I went over and evaluated as best I could visually, took some quick photos and agreed to pick it up. I will physically pick it up this week but wanted to start the ball rolling on a course of action for it.
    In doing some research I have found Bob Latinos site and kits, which seems to be geared towards building complete kits as opposed to rehabbing old vintage units. He is under 100 miles away from me and certainly in reach for bringing unit to if needed. I am not qualified to make the call as to everything that might make sense to do to this thing but with guidance and encouragement would be likely able to do the work needed myself. Maybe a bit ambitious but I have to start somewhere.

    front.JPG rear.JPG rightside.JPG

    Have I bitten off more than I can chew!

    Geoff
     
  2. 8xlaxx

    8xlaxx Super Member

    Nice find and thank your friend for alerting you to it!

    A couple things come to mind right away when looking at the pics you posted:

    1) No switches, power or mono / stereo. Not a big deal to replace but it would seem that someone has been 'in' the amp.
    2) Red and White sticker on side of chassis might be info on the tech that modified it or possibly an estate sticker?
    3) Some odd ball tubes in the EL34 sockets and others.
    4) Fuse holder looks to have been replaced at some point.

    ...at least the power cord hasn't been cut, so that's a plus! LOL : )

    Be nice if you could pull the chassis and shoot some pics inside / underneath the unit. There will be others on the AK site with more knowledge but I'm going to say that the Selenium Rectifier will need to be swapped out, as well as old caps. Again, not hard to do. The quad cap looks stock, so that might need to go, too. @petercapo and @audioregen (and others) know their 'stuff', so tap them as resources. If you're decent at soldering and have some test equipment then I don't think that you have bitten off more than you can chew.

    Best to you on the project!
     
  3. peterh

    peterh AK Subscriber Subscriber

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    I think i would reduce the price.
    On the plus side they seem unmunged ( not damaged by clueless modders) ,
    on the minus side : the power tubes seem to be a mix of stuff. They should be replaced, the cost
    for 4 decent EL34 is $62 ( http://www.eurotubes.com/store/pc/EL34-c61.htm ). At the same time
    a GZ34 ( rectifier, $16.5 http://www.eurotubes.com/store/pc/Rectifier-Tubes-c4.htm )
    Thus the price should be reduced by at least $78 , let the seller keep the tubes.
    Do however keep the 7199 ( smaller tubes on the board) , they are often in good condition. They are
    also expensive to replace.
    The amp itself could benefit from cleaning, possibly exchange of some electrolytic caps, and
    probably replacing the selenium rectifier inside. This adds up to app $50 This reduced the
    price by an additional $50.
    A bid of $300 is more reasonable.
     
  4. stupidhead

    stupidhead aka Ratdogheads Subscriber

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    Thanx

    Geoff
     
    Last edited: Aug 18, 2017 at 8:06 AM
  5. stupidhead

    stupidhead aka Ratdogheads Subscriber

    Messages:
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    While I appreciate the thought here, a price has been established and agreed upon. I am very comfortable with the purchase price. I have gotten other things from him like a nice set of AR 5s for $80 in very good condition. It all levels off in the end.

    Thanx also for the tube insights, I am so ignorant on these things that guidance is appreciated.

    Geoff
     
  6. gadget73

    gadget73 junk junkie Subscriber

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    Looks like it has two original Mullard EL34 and two Sylvania fat body 6CA7, but one of each per channel. I'd flip that arould so the tubes in each channel match at a minimum.

    Usual route is a physical cleanup first and having a look at whats under the hood vs pics of what it should look like just to see what, if anything, has been changed.
     
  7. 8xlaxx

    8xlaxx Super Member

    I believe that you will find many threads on 'what' to do and 'what others' recommend. Google and research are your friends. Opinions vary all over the place about 'what' to do with an ST-70,...

    One thing that I have found very helpful is to get the schematics and operating manual. The original schematic is large in print and easy to read/follow.

    Also, several purists here recommend to get the ST-70 back to it's original state of operation (stock), then try/swap out parts and kits. I would recommend this option,... If you can, get it to stock operation first.
     
  8. bberkom

    bberkom AK Subscriber Subscriber

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    I rebuilt an ST-70 this year. The driver board was so toasted from heat that I was forced to replace it. I would have liked to rebuild it with the stock circuit first to have a comparison point, but I went ahead with a new VTA board instead since I figured I'd get there eventually. I also replaced the multi section cap. It's a great amp. I'm using EH 6CA7's now but want to try some KT-66's when I can swing it.
     
  9. StuartB

    StuartB AK Subscriber Subscriber

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    I would recommend that you take your time and completely disassemble it. Clean up everything and basically keep the chassis and the transformers. I did that with a unit several years ago. I used Joe Curcio boards to resemble. He has the original assembly manuals and very good instructions to use his boards. He took an awful lot of time with me as this was my first project. I had a blast and I have a unit that will outlast me.

    Search for Curcio Audio. I used the unit to teach myself assembly skills. I am kind of a klutz but by going slow, having good instruction and a supplier that will answer many dense questions I came out in great shape.

    Good luck!
     
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  10. ferninando

    ferninando AK Subscriber Subscriber

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    hope you didnt give more than 250 for it with all thats wrong with it.
    Having said that, it is one of the simplest amps to rreferb/restore.
    if the board is not fried you dont need a new one. Just replace all the Rs and Cs with new .
    Get a quad of new output tubes , EL34 or KT77s. If 7199 are in the drivers are good keep them for now.
    they are nearly unobtanium.
    Check all wireing underneath. A 1N4007 diode replace the selenium rect. All New caps underneath.
    A new can cap. Address the fuse and S/M switch as needed
    Prepare the transformers for black semigloss paint. Polish/derust the chassis.
    You will have something to be proud of in no time and it will sound very very good.
    All this is what I would do. You may want to do it deff. whatever you do, enjoy doing it.
    The amp is worth the effort IMO. I have one.
     
  11. 6DZ7

    6DZ7 Super Member

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    I thought they might be, too, but mine have a square hole in the middle of the plate. Don't know of variations.
     
  12. airtime

    airtime Super Member

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    I had to switch my 7199's in my Sherwood to the 6U8 family. Actually I used a Amperex E80CF. Knowing the 7199 was designed as an audio tube I was very unsure what I was getting myself into switching them (really one good one and one burnt out one) out and doing the socket rewiring.

    Did it and the results were quite good. So if the 7199s go - not to worry. A simple rewire and it's tube rolling time again.
     
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  13. ferninando

    ferninando AK Subscriber Subscriber

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    airtime, good to know. thanks.
    I got some adapters to just in case.
     
  14. superjojo

    superjojo Active Member

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    156
    You have a Don Sachs Citation and your going to waste your time with that crusty ST70?

    Unless its about 50 bucks, I would go back into the listening room and crank that Citation.
     
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  15. airtime

    airtime Super Member

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    I'm told from a VERY reliable source that the adapters can add capacitance and some roll off. If at all possible do the socket rewire. It's pretty simple even on a printed circuit board.
     
  16. Omegaman

    Omegaman Ultra Super Member Subscriber

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  17. Vintagegeek

    Vintagegeek Well-Known Member

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    Agree with gagdet73, he just helped me with my ST-70. I would remove the tubes before powering, if it has an issue it should trip the fuse. Once you know it is safe then clean the chassis, replace the bias circuit, selenium rectifier and retube it. Keep the 7199's and if you sell them you can recoup some of your investment. If the chassis is pitted you can seal it with poly, will prevent further deterioration. Do not sand or use a gritty cleaning agent. This will take whatever plating is left and dull it. You will find many knowledgeable people here that will help you when you have questions. Welcome to the Dynaco world!
     
  18. stupidhead

    stupidhead aka Ratdogheads Subscriber

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    Southern New Hampshire
    Hey now! Thanx for all the input. It actually triggered me to shift a few things around schedule wise and go pick up the unit and start evaluations. As for the cost I paid, which I understood going in was a little higher than many would feel was fair market value, I now own it. A farm fresh unit like this doesn't appear to be that available and as best I can tell this unit has had very little mods if any over the course of time. For my first foray into the world of tube amp restoration this was the model I had my eyes out for and the opportunity arose.
    So I now possess and can move forward. First, yes on 7199 tubes, and I am pleased to hear this is apparently a coup of sorts.

    dirtyboard.JPG

    So a bit of tube extraction and a soft brush, followed by some q tips and mild dish soap makes this look a whole lot better. I even took a bit of metal polish to one surface to see what kind of corrosion was actually involved and not terrible, but certainly more work needed to get it where I want it.

    quickclean.JPG

    The cage cleaned up pretty good as well. After revealing the undercarriage I see no evidence of anything cooked and I am encouraged.

    guts.JPG

    and a close up of the board is the only thing that I have found that seems to maybe identify the actual vintage of the unit. Can anyone give me an idea of potential date of production?

    board.JPG

    So starting to make a parts list I need some feet for this thing yes? Also it would appear as both the s/m switch and power switch were never there and don't know if these pose any challenges in installing proper switches.

    Sorry for the quickie post here but I have to go to work and time is short.
     
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  19. bberkom

    bberkom AK Subscriber Subscriber

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    You can get the replacement switches from a few different places, no big deal. I unhooked the S/M switch on my ST-70 anyways. My power switch caused a bad arcing pop on power off, so I replaced it and added a snubber network, which solved that issue.
     
  20. superjojo

    superjojo Active Member

    Messages:
    156
    Market value and true value are different things. Dont worry if you overpaid. You now have a big project to keep you busy this fall and that can be fun.

    You,ll be spending plenty more money too.... new driver board, components, other parts, tubes etc etc. When your all said and done unfortunetly you,ll have an amp that will be a huge step down from your custom Citation.

    In addition, you will have likely spent more than what the VTA ST70 kit costs. The VTA is a nice upgrade in everyway to the original.... sure you will spend on lots of the same parts to put in your original dynaco but you,ll still have the original transformers and not the beefy ones provided by VTA.

    On the upside, you,ll certainly gain some electronics knowledge too because it will be signficantlly harder to troubleshoot and modify the original amp than simply building the VTA with very clear instructions.

    So again, dont worry about being way underwater on this purchase.... wearing a snorkle is par for the course, i have worn one frequently with audio purchases!
     
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