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    Starting first restore

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    ttownscott

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    Starting first restore

    Post by ttownscott on Sun Aug 12, 2018 3:31 pm

    I started posting in pics, but since I have some questions around resistors, posting here as well.


    Like many I had a Dynaco ST-70 in my youth. I was given one in college and used it all throughout and then left it with one of my roommates when I graduated.

    Now I'm a bit nostalgic and decided I wanted to fit one into my system. Doing a search and wow. These guys are expensive! Looks like free was out of the question, but I found one without tubes for $330 and I actually have a matched set of EL34's for a different amp anyway.

    So after research, I realize there are some first things first, such as replacing the selenium rectifier and upgrading the cap. I ordered the bias kit and a quad cap from Dynakitparts and will check the transformers. Then I plan on striping everything off the chassis and cleaning it up and repainting the transformers. I am optimistic based on the other posts I have seen.

    This is not going to be a VTA70 build. I would like to build one of the VTA120 or M-125 sets after this though. I will just use one of the ebay 6GH8A boards to keep it as stock as possible.

    I would like to get a recommendation of what chassis resisters to use if I replace them and where is a good place to buy them. I am also on the fence about weather it is necessary to use new tube sockets or retension and re-use the existing ones.

    I got these values from the following article.
    https://www.hifizine.com/2010/06/restoring-dynaco-st-70/


    QuantityValueDescriptionSourcePart Number
    210 ohm10 ohm resistor on power socket????????
    215.6 ohmsprecision resistor (for bias set)Dynakitparts.comBRWW-15D6
    25.1 K5.1 K ohm 1 watt resistor Digi-Key PPC5.1KW-1CT-ND
    41000 ohms1000 ohm 1  watt resistor (On EL34 socket)Mouser71-CMF601K0000FHEK
    16.8 K6.8 K ohm 1 watt resistor (filter capacitor) 6800 ohmDigi-KeyPPC6.8KW-1CT-ND
    122 K22 K 1 watt resistor on CapacitorDigi-KeyPPC22KW-1CT-ND
    2475 K475 K ohm 1/2 watt resistor (socket to ground)Digi-KeyCMF475KHFCT-ND

    With out further adieu. Her is what I am starting with.

    The amp with cover:


    Cover off


    The underside



    Look at those original Dynaco caps!


    My first shipment from Dynakitparts
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    dmagazz

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    Re: Starting first restore

    Post by dmagazz on Sun Aug 12, 2018 3:43 pm

    im told(seen in many blogs) the flat domino looking caps last virtually forever. i changed them anyay, but i don't think it was necessary.
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Sun Aug 12, 2018 3:50 pm

    I am going to put a replacement driver board in, but I may do like you did and refresh the stock board so that if I ever get my hands on some 7199 tubes I can try this board out. The replacement board with 6GH8A tubes was about the price of a single 7199 unfortunately.
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    peterh

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    Re: Starting first restore

    Post by peterh on Sun Aug 12, 2018 4:16 pm

    ttownscott wrote:I am going to put a replacement driver board in, but I may do like you did and refresh the stock board so that if I ever get my hands on some 7199 tubes I can try this board out. The replacement board with 6GH8A tubes was about the price of a single 7199 unfortunately.
    There is adaptes on ebay that allows 6U8 tubes to be used instead of 7199
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    WLT

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    criteria time first

    Post by WLT on Sun Aug 12, 2018 4:50 pm

    Your stock amp should clean up and look good with new paint. Good luck no mater what.

    The all important criteria for a rebuild. If you are going to obtain a VTA amp then trying to build out the vintage ST 70 to a high level may not be worthwhile. A simple yet effective rebuild may be enough to postpone the VTA choice for some time.

    I decided to rebuild my vintage Dyna MK IIs by

    1) keeping the amps in stock configuration. This way the amp and the published schematics will always align if need be,
    2) replace all capacitors as they have all exceeded their design life.
    3) optimize the resistor values so they are all within 5% of the published values. Also all important resistors are to be less than 2% from published. Also both channels will be within 2% of each other. I think it makes a big difference.
    4) On My MK IIIs I replaced all resistors with 2 watt, carbon film resistors and used oil caps in the signal path. For the MK IIs I decided to stay with the carbon composition resistors except at a few non critical places. You may want to review capacitor types and replace with the technology you decide is best, or easiest, or cheapest etc. Many opinions will surface on this issue.
    5) replace signal and feedback caps with modern types. Using polypro signal and mica feedback caps is pretty standard. I like the polystyrene for feedback types.

    Back to your questions about sources. Try https://www.justradios.com for all kinds of parts. Good selection and fast shipment. Minimum of $20 only. They do sell resistors in small packs of 5 etc which may be an issue. If you decide to do only the carbon composition types you may have to look a lot more options for sources.
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    solderblob

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    Re: Starting first restore

    Post by solderblob on Sun Aug 12, 2018 5:39 pm

    ttownscott wrote:I am going to put a replacement driver board in, but I may do like you did and refresh the stock board so that if I ever get my hands on some 7199 tubes I can try this board out. The replacement board with 6GH8A tubes was about the price of a single 7199 unfortunately.

    That looks like a pretty nice chassis -- not much rust or staining.  Should make a good project.  Hopefully the transformers are OK.

    Having rebuilt my ST70 twice and my Mk3s twice, here are my $.02 worth:

    Replace the octal tube sockets.  Get them from a reliable source as I've heard there are some available with pin sockets that don't hold up and get loose.

    Do the selenium diode replacement and new quad cap as you're already doing.

    Do the VTA board...  Doing so will replace all caps and resistors (I believe) and provide bias adjustment for each output tube.  Most importantly, it sounds absolutely amazing and is bullet-proof.  I don't believe the cost would be a whole lot more than a 6GH8A board,

    Replace the speaker binding posts, input RCAs, and power cord.

    You can leave the old bias pots in and disconnected so if you ever decide to go back to the old driver board you can.

    Anyway, good luck with the project and hope it turns out great!

    dave
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    dmagazz

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    Re: Starting first restore

    Post by dmagazz on Mon Aug 13, 2018 12:05 am

    yea, like peterh said, i think new caps,a couple $10 tube adaptors,and a couple $10 6u8a tubes, and it'll probably be cheaper than the driver board. like WLT hinted,change what you need to for now. and get it running. im betting the trannys are ok,the rca connectors will be fine,cleant them with 99% isopropyl and maybe slightly tighten / pinch the inner connectors on them. and clean and tighten the sockets,they will be ok if contacts aren't aren't burned.otherwise change them as solderblob said, because if the connections fail you could end up burning out the tube or something. i was given the advice of using pipe cleaners to clean the inners,get some 99% isopropyl alcohol to put on the pipe cleaners,when done use a dental pick, or tiny eyeglass screwdriver to tighten them, just be careful not to go too much, they will bend easy. ohhh. Wink for aesthetics maybe polish the aluminium can cap before you install it. i cleaned my chassi with a old toothbrush and alcohol,and DONT try to clean the transformers with anything if they are tar coated, also alcohol will remove the stamping on the tranny's.(i learned the hard way). also be SURE the bias pots are clean/in good health. test continuity from the sweeper(center contact) to each side turning the pot from min to max each time, watch for dropout(loss of connection) you can clean them with tuner cleaner, or even at least introduce some alcohol and blow them out by the terminals with canned air.then turn them speedily many times from min to max, and check them again.
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    Bob Latino
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    Re: Starting first restore

    Post by Bob Latino on Mon Aug 13, 2018 11:31 am

    ttownscott wrote:
    I got these values from the following article.
    https://www.hifizine.com/2010/06/restoring-dynaco-st-70/

    Note - This article was written by Bill Thomas .. Bill was one of the best resources on the internet for Dynaco tube gear info. Sadly, Bill passed away on 11/3/11 at the age of 72. In early 2012 I made a post about Bill's passing on our forum here along with some links to some of the many articles he posted on the internet over the years .. Link below ..

    Bill Thomas .. A great source of Dynaco tube information ..

    Bob
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Mon Aug 13, 2018 3:18 pm

    Thanks for the great input guys. I spent some time yesterday following along the first two rebuild Dynaco videos pinned in Basket.
    http://dynacotubeaudio.forumotion.com/t2831-rebuilding-your-dynaco-st-70-videos

    Turns out I've got a busy week of evenings then heading out on vacation, so probably be a week before you see me posting any real progress. If I have the transformer validated before I leave I'll order some input sockets, jacks and terminals.

    Looks like justradios.com and dynakitparts will be my go to for a while!
    I also have a vintage console and my grandma's philco radio that I'll be restoring after the Dynaco.

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    Peter W.

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    Re: Starting first restore

    Post by Peter W. on Mon Aug 13, 2018 4:10 pm

    ttownscott wrote:I am going to put a replacement driver board in, but I may do like you did and refresh the stock board so that if I ever get my hands on some 7199 tubes I can try this board out. The replacement board with 6GH8A tubes was about the price of a single 7199 unfortunately.

    If I dig hard enough, I have about 2 dozen 7199s of various vintages and conditions - all unknown (to me). when you get to that need, let me know and I will test up a couple (I have a Hickok 539B, so the test will be reliable). Maybe $8 each + shipping, or so.

    Note that I fully expect 4/5 or more of what I have to be unsellable (not the same as unusable), so I make no promises.
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    PeterCapo

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    Re: Starting first restore

    Post by PeterCapo on Mon Aug 13, 2018 10:46 pm

    7199s are funny. Many if not most test with shorts but they are perfectly fine.
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    dmagazz

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    Re: Starting first restore

    Post by dmagazz on Tue Aug 14, 2018 1:32 am

    your a good egg peter w
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    ttownscott

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    back from vacation

    Post by ttownscott on Mon Aug 27, 2018 7:16 pm

    Ok, back from vacation and had a chance to clean the chassis and paint the transformers last weekend.


    The chassis with parts removed:



    Transformers:



    Cleaned up. There is still some pitting but I can live with it:



    I have some new tube sockets and assorted parts coming in the mail from Dynakitparts. I'll keep posting pics as I progress.
    Cheers
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Sun Sep 09, 2018 2:07 am

    Finally finished her.
    Took a little longer than I thought because of summer vacation.

    Her are the completed photos:
    Top view:


    Bottom view:


    The only thing I am disappointed with is there is some background hum. An equal amount through both speakers. Not sure if I should expect that or not. My Hafler DH-200 was stone silent.

    One thing I wonder if it is the choice of lug on the Quad capacitor that I tied to ground. The diagram shows the lug between 80 and 40, but I used the one between 40 and 30 because it had a hold in it to crimp the wire and was an easier run to the ground lug:

    Quad Cap ground view:
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    PeterCapo

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    Re: Starting first restore

    Post by PeterCapo on Sun Sep 09, 2018 9:06 am

    It should be pretty quiet.  Sorry if I missed it in what you wrote, but are you using a three prong power cord?  If so, you might try using a three prong to two prong adapter to "lift" the ground.  If you do this, I highly recommend one of the following for a measure of safety: http://www.trci.net/products/shock-shield/power-strip

    Also, there are two 10-ohm resistors on the PC board between the node for eyelets 8 and 10 respectively and the ground shell of the respective input jacks.  Try shorting these two 10-ohm resistors and see if it helps.

    I don't think the choice of ground lug on the quad can should matter as long as you have a good solder connection to the adjacent ground lug.
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Sun Sep 09, 2018 2:30 pm

    Thanks Peter.
    I am using just a two prong power strip. I did a stock rebuild with the following exceptions, which I believe are all minor and would not affect sound.

    1. Upgraded Quad Cap from dynakitparts
    2. Diode instead of selenium rectifier
    3. 6GH8A version of driver board.
    4. Upgraded input and output jacks
    5. yellow sheet mods on rectifier tube.

    I'll give the resistor bypass suggestion a try. Thanks!
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    DynakitParts
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    Background Hum

    Post by DynakitParts on Sun Sep 09, 2018 2:50 pm

    Try installing a different 5AR4 rectifier tube.....Kevin
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Sun Sep 09, 2018 4:36 pm

    Thanks Kevin. It is a 60 Cycle hum, not 120 so I think that could indicate the tube. Luckily there is a guy down the street (Literally) who sells tubes on the internet. So I'll have a chat with him about a replacement.

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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Sun Sep 09, 2018 7:44 pm

    So, wasn't the 5AR4. Picked one of those up from the guy down the street at http://worldtubecompany.com

    I'm going to go back through the solder joints and try Peter's suggestion.
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    PeterCapo

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    Re: Starting first restore

    Post by PeterCapo on Sun Sep 09, 2018 8:43 pm

    Just to be clear, the plug on the end of the power cord of your Stereo 70 is two-prong, is that correct?  Is it polarized two prong or non-polarized?  Reason I ask is because some years ago when I owned a pair of Mark IIIs, I flipped the non-polarized plug 180 degrees in the wall socket and got a reduction in hum.
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    Pillo69

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    Re: Starting first restore

    Post by Pillo69 on Mon Sep 10, 2018 6:00 am

    Are the cables of the output transformer (primary) correctly connected?
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Mon Sep 10, 2018 12:07 pm

    Listened to a couple albums through the amp last night. Aside from the background hum it is very pleasant to listen to. I need to solve the hum problem.

    Tonight I'll try bypassing the 10-ohm resistors from eyelets 8 and 10. Is that just a matter of removing and replacing with wire to pin #3 of the bias jack? I think I saw somewhere that running from the eyelet to the ground on the input jack is also an option.

    Pillo69, What might I be looking for regarding OT cables? Output jacks or tube sockets?

    Thanks for the feedback all.
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    PeterCapo

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    Re: Starting first restore

    Post by PeterCapo on Mon Sep 10, 2018 12:16 pm

    The easiest way might be to leave the resistors in place and just wire a jumper directly across each one.  Or, you can accomplish the same thing by connecting the RCA input jack ground shells directly to eyelets 10 and 8 respectively.

    Don't confuse the two 10-ohm resistors connected between the same eyelets and the front panel power take-off sockets.  Those are not the ones you want to short.  What you want is to short the two 10-ohm resistors that are mounted onto the PC board.  Have a look here: http://dynacotubeaudio.forumotion.com/t191-dynaco-st-70-pc-3-driver-board-parts-layout-photo  The two 10-ohm resistors you want to short are between the 82pf capacitors.

    How much hum are we talking about?  If it is a lot, then I am not so sure jumping the resistors will help, but it is still worth a try.

    Another thing to try would be to put shorting RCA plugs into the input jacks and see if it stops the hum, which might then give further direction for troubleshooting.

    By the way, what brand of EL34 are those?
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Tue Sep 11, 2018 1:45 am

    Thanks Peter,

    I was totally thinking you meant the 10 ohm resistors from the bottom of the board to the power port.

    So I just used a pair of alligator clips to do the bypass test and found that my right channel is out. I haven't touched the amp since listening to music last night. I thought it may have been one of the 6GH8A tubes went out, but I swapped them and still the right channel out.

    I tried jumpering the 10 ohm resistors anyway but only noticed possibly a slight decrease in hum if the right channel resistor is shorted and the left not. but I don't think that's anyway conclusive with one channel out now. All the tubes except the right 6GH8A light up and bias is not moving.

    The tubes are
    2 GE 6GH8A
    4 mathched Svetlana EL34
    1 GrooveTube 5AR4

    won't be able to work on it again until Wednesday.

    by the way, this is the board I am using: https://www.ebay.com/itm/NEW-6GH8A-STUFFED-BOARD-Dynaco-ST70-Stereo-70-Tube-Amplifier-PC-3B-PC-3-6GH8A-/372431532449
    Since it is a drop in replacement.

    I had considered changing the caps on the original board, but then I would probably have needed adapters and 6GH8A tubes or expensive 7199 tubes. The amp was tubeless when I bought it, but I already had the quad EL34's
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    PeterCapo

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    Re: Starting first restore

    Post by PeterCapo on Tue Sep 11, 2018 10:50 am

    That sounds about right. That's why I asked how much hum we're talking about.  Jumping the 10-ohm resistors can result in a useful reduction in hum, but it sounds like you've got other things going on.

    For the right side 6GH8A that won't light, try looking at the wires supplying its filament.  There should be a twisted pair of wires going to eyelets 15 & 16 on the PCB.  Trace those wires back and see if they are connected properly.

    Do you know if your EL34s are "good" as well as being reasonably well matched?

    Other than this I am not sure what to tell you.  Sounds like you might have cold solder joints or miswires or something going on.  Don't recall if you have a compete copy of the manual including pictorial wiring diagrams, but I recommend you study it very thoroughly.  Have a look here https://dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf

    And, if you have not already done so, you definitely want to look up safe work practices around dangerous and even lethal high voltages such as are present in your amplifier, including the top side.

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