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    Power Transformer Options - ST-70

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    Kentley

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    Location : Worcester, MA

    Re: Power Transformer Options - ST-70

    Post by Kentley on Wed Jun 15, 2016 8:46 pm

    How old are your output tubes? My intuition is screaming "new set of output tubes". Yes, bad ones can cause the fuse to blow and also cause all manner of alien sound to accompany their death. A hard-earned lesson I learned the hard way {!!} - always check the easily switched-out stuff first. And always have a spare set of these easily switched-out parts. You'll need 'em sooner or later anyway.

    GP49

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    Re: Power Transformer Options - ST-70

    Post by GP49 on Wed Jun 15, 2016 11:50 pm

    Troubleshooting should always follow a logical, step-by-step progression, ideally one that isolates the operation to one part of the circuit first, then going on to the other parts in succession.

    Remove ALL the tubes.  Install a 3A fuse (lay in a supply of them...doesn't have to be slow-blow).  Plug in the tubeless amplifier to about 100VAC (using your VariAC to reduce the line voltage) and see if the fuse blows.  If it does, suspect the transformer or the wiring to it.

    If it doesn't, install the rectifier tube.  If the fuse now blows, try a substitute rectifier tube.  If it still blows with a known good tube, suspect the filter capacitors (in a stock Stereo 70, the quad cap, if you use separate capacitors, suspect those).

    If it does not blow with a known good rectifier tube, then install the small signal tubes.  If it doesn't blow, install the output tubes...one channel at a time.
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    Aljaheejus

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    Join date : 2010-01-03

    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Thu Jun 16, 2016 12:55 am

    GP49,
    Thanks for the methodology and suggested steps to carry it out!

    Did you see the resistance readings I posted on the transformer and case, once removed? Nothing there to cause the fuse to blow, right?

    GP49

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    Re: Power Transformer Options - ST-70

    Post by GP49 on Thu Jun 16, 2016 1:21 am

    Nothing in your measurements is obviously sticking out at me...the obvious fault would have been an unusually low resistance to the metal outer shell into which you installed the toroid.

    The following may seem odd, but:

    Check the polarity of the smoothing capacitors in the bias voltage circuit. Make sure they are POSITIVE-SIDE-GROUNDED. Sometimes, someone will instinctively wire them up negative-ground because virtually always, smoothing capacitors are wired that way in the usual positive-voltage power supplies. But the bias voltage circuit in amplifiers is NEGATIVE VOLTAGE. It's a dumb mistake that is more likely made by someone with experience in electronics than by a novice!

    This fault will probably draw excess current and destroy the capacitors and perhaps the bias rectifier, too...and if that has happened it will still blow the fuse with all tubes out.
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    Bob Latino
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    Re: Power Transformer Options - ST-70

    Post by Bob Latino on Fri Sep 02, 2016 4:52 pm



    Someone Emailed me about the Dynaco ST-70 torroidal power transformers as sold on Ebay so I though I would put my thoughts on this thread ... Link below ..

    Dynaco torroidal replacement power transformer on Ebay

    Yes - at about $75 USD + shipping they cost less than a Dynakitparts replacement PA-060 power transformer BUT > You will need the mount kit + $6 USD and you should use a shield because without bell covers and the raw windings showing, there is more of a possibility of that PT radiating hum/noise into the amp's circuitry.

    Also once mounted, the amp will have a different look to it. Because of the different appearance of the amp, it will probably bring less money should you decide to sell the amp in the future. People buying on Ebay like "unmolested" Dynaco tube gear. You would not be the first person who "upgraded" a piece of Dynaco tube gear and found out later it was worth less than you originally paid for the piece BEFORE you modified the item.

    Just my opinion but > If you do need a replacement PA-060 power transformer, you should bite the bullet and get a replacement PT from Dynakitparts. Yes - it will cost more but you will retain the original look of the amp and it will be worth more than an amp upgraded with a torroidal PT.

    Bob
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    Aljaheejus

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    Join date : 2010-01-03

    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Fri Sep 02, 2016 5:33 pm

    Kentley wrote:How old are your output tubes? My intuition is screaming "new set of output tubes". Yes, bad ones can cause the fuse to blow and also cause all manner of alien sound to accompany their death. A hard-earned lesson I learned the hard way {!!} - always check the easily switched-out stuff first. And always have a spare set of these easily switched-out parts. You'll need 'em sooner or later anyway.

    Thanks! I have to get back to this project!! I will try another set of output tubes - that's the four EL 34s, right?

    Also, could the bias settings that I made have caused the fuse to blow if I replaced the tubes in different locations from the location they were in when I balanced the bias? I was compensating a lot to balance the tubes. Then, may have replaced them in opposite channels, or in different sockets of each channel... (Each channel on the DIYtube board has an adjustment for the two tubes on that channel, as I understand it.)

    koojinchen

    Posts : 2
    Join date : 2017-06-20

    Replacing ST-70 PA060 mains transformer with Sowter 9079

    Post by koojinchen on Tue Jun 20, 2017 10:38 am

    i have recently been given a Dynaco ST-70 tube amp to switch out and replace the original 110V transformer to one that capable of handling 240V, i have purchased the "SOWTER 9079 MAINS TRANSFORMER" as the new transformer, but because i have very little knowledge in tube amplifiers and not to mention reading schematics, it would be great if someone coule help me out here. Smile i also would have sent it to a repair store if it were possible but there isnt one in the country i live in.

    the main problem is that i don't know what color lead goes to which # pin of the components because the color codes of the new transformer are different than that of the original one. ive attached the color code sheet of the Transformer from SOWTER's website, sooo if anyone could help me identify which color leads go to what component and what # Pin that would be very much appreciated!

    ive only managed to get as far as connecting the Violet/white and black/white, black and violet to fuse and on/off switch.



    if anyone can help change the colors of the original leads mentioned in the original wiring procedure
    from the dynaco assembly instructions to the colors of the new leads of the "SOWTER 9079 MAINS TRANSFORMER", im pretty sure ill be able to get it sorted from there.



    Any help would great Smile
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    Bob Latino
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    Re: Power Transformer Options - ST-70

    Post by Bob Latino on Tue Jun 20, 2017 10:52 am

    Those colors in your photo are identical to the original Dynaco ST-70 240 power transformer (PA-521) except

    1.  You have in your photo YELLOW wires for the 5 volt line that goes to pins 2 and 8 of the rectifier tube socket.
    On the original Dynaco ST-70 power transformer those wires were WHITE in color. Just run those two yellow wires to pins 2 and 8 of the rectifier tube socket.

    2. The wires in your photo marked green/white and brown white are green/YELLOW and brown/YELLOW on the original Dynaco ST-70. These two wires go to pins 5 and 7 of the 7 lug terminal strip on the original Dynaco ST-70.

    Bob
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    Peter W.

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    Location : Melrose Park, PA

    Re: Power Transformer Options - ST-70

    Post by Peter W. on Tue Jun 20, 2017 1:39 pm

    Remove ALL the tubes.  Install a 3A fuse (lay in a supply of them...doesn't have to be slow-blow).  Plug in the tubeless amplifier to about 100VAC (using your VariAC to reduce the line voltage) and see if the fuse blows.  If it does, suspect the transformer or the wiring to it.

    If it doesn't, install the rectifier tube.  If the fuse now blows, try a substitute rectifier tube.  If it still blows with a known good tube, suspect the filter capacitors (in a stock Stereo 70, the quad cap, if you use separate capacitors, suspect those).

    All:  This is excellent advice and is completely safe IF and ONLY IF the system remains at 100V throughout the first three steps of the test. Point being: NEVER, EVER install a rectifier ONLY in any tube amp unless input voltage is significantly reduced. A no-load condition can possibly blow up the filter caps (even if good) - and no with exaggeration on the "blow-up" descriptive. Until there is at least one set of output tubes in place, voltage must be kept low to protect the filter caps.

    koojinchen

    Posts : 2
    Join date : 2017-06-20

    Re: Power Transformer Options - ST-70

    Post by koojinchen on Wed Jun 21, 2017 10:33 am

    Thanks to Bob, i now have a fully working Dynaco ST-70 with a new transformer Very Happy

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    Bob Latino
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    Re: Power Transformer Options - ST-70

    Post by Bob Latino on Wed Jun 21, 2017 11:13 am

    koojinchen wrote:Thanks to Bob, i now have a fully working Dynaco ST-70 with a new transformer Very Happy


    Glad to hear that you are up and running ...

    Bob

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