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    Power Transformer Options - ST-70

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    Kentley

    Posts : 296
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: Power Transformer Options - ST-70

    Post by Kentley on Wed Jun 15, 2016 9:46 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

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

    Re: Power Transformer Options - ST-70

    Post by GP49 on Thu Jun 16, 2016 12:50 am

    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.

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Thu Jun 16, 2016 1:55 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

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

    Re: Power Transformer Options - ST-70

    Post by GP49 on Thu Jun 16, 2016 2:21 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.

    Bob Latino
    Admin

    Posts : 2376
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Power Transformer Options - ST-70

    Post by Bob Latino on Fri Sep 02, 2016 5:52 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

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: Power Transformer Options - ST-70

    Post by Aljaheejus on Fri Sep 02, 2016 6:33 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.)

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