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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock:

    dynacophil
    dynacophil
    dynacophil


    Posts : 35
    Join date : 2010-10-26
    Age : 66
    Location : Berlin/Germany
    • Post n°1

    My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock: Empty My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock:

    Post by dynacophil Wed May 26, 2021 9:21 am

    Hello!

    After a year on the shelf, I pulled out my original Dynaco Stereo 70 with VTA board and listened a few days using first EH 6CA7 (to be sure not to kill good tubes if fails), after a day I put in new RFT of a rare kind with 2 getterrings, and listened like 2 days with them. Biasing worked fine, I had 2 fun days with it.

    Last night, I thought of trying a set of old Tesla, not JJ. First adjusted right side, while biasing left side towards .40V (showed .29 first) suddenly the V on meter turned to -Zero. Turned of, switched tubes from left to right and vice versa, to exclude tubes. Turned on, for like 20 seconds it showed V again on left, where I was, like before - then - Zero again... and then holy smoke came up affraid between tubes and output tranny. Turned off. Sad.


    I have to clear workbench before I can start  checking, but - do you have an idea what causes this kind of fail? What should I start checking, when I am able to?

    Best, Helge


    Last edited by dynacophil on Wed May 26, 2021 10:05 am; edited 1 time in total
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    Bob Latino
    Bob Latino
    Bob Latino
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    Posts : 3263
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°2

    My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock: Empty Re: My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock:

    Post by Bob Latino Wed May 26, 2021 9:34 am

    Helge,

    One of those "Old Tesla" tubes may have taken out one of the 10 ohm 2 watt resistors that go from pins 1 and 8 to chassis ground. Open the amp up and measure all four 10 ohm 2 watt resistors and see if one of them measures ZERO or a high resistance (something other than about 10 ohms).

    Bob


    Last edited by Bob Latino on Thu May 27, 2021 7:42 am; edited 1 time in total
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    dynacophil
    dynacophil
    dynacophil


    Posts : 35
    Join date : 2010-10-26
    Age : 66
    Location : Berlin/Germany
    • Post n°3

    My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock: Empty Re: My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock:

    Post by dynacophil Wed May 26, 2021 10:04 am

    ok, tubes in drawer usually are tested, but, sure possible. I'll mark them as suspicious, and check this first.
    I will clear my workbench, it looks like a mess, and get back, probably next week!

    Thank you!
    Helge
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    dynacophil
    dynacophil
    dynacophil


    Posts : 35
    Join date : 2010-10-26
    Age : 66
    Location : Berlin/Germany
    • Post n°4

    My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock: Empty Re: My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock:

    Post by dynacophil Wed May 26, 2021 1:33 pm

    Getting further with cleaning the bench, maybe already weekend or monday possible to throw it on the back.

    activated a already 230V Sutherland mod Stereo 70. This drives the tubes w .50V as I read...
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    Bob Latino
    Bob Latino
    Bob Latino
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    • Post n°5

    My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock: Empty Re: My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock:

    Post by Bob Latino Thu May 27, 2021 8:09 am

    Helge,

    See if you can post a photo of the inside wiring of your original Dynaco ST-70 with a VTA board.  One of us may be able to pick something out in the wiring or the condition of the parts that could cause a problem ... Check the sticky post above on how to post a photo on the forum.

    As a side note (and I have said this before on a few other posts) > The original Dynaco ST-70 amp came out in 1959 and some of these amps are now 60+ years old. Just replacing the driver board is many times not enough to keep the amp running reliably. The original choke can leak. The tube sockets lose their grip on the tubes. The electrolytic caps in the amp go bad. The chassis can rust. The original wiring gets brittle. The carbon composition resistors can change their value with time .. It is sometimes difficult to get the amp to be reliable by just changing out the driver board and maybe replacing some of the wiring.

    Bob
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    Jim McShane
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    Jim McShane


    Posts : 237
    Join date : 2011-10-19
    Location : South Suburban Chicago
    • Post n°6

    My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock: Empty Re: My trusted original Dynaco ST-70 with VTA mod showed holys moke :shock:

    Post by Jim McShane Fri May 28, 2021 12:14 pm

    Bob Latino wrote:
    As a side note (and I have said this before on a few other posts) > The original Dynaco ST-70 amp came out in 1959 and some of these amps are now 60+ years old. Just replacing the driver board is many times not enough to keep the amp running reliably. The original choke can leak. The tube sockets lose their grip on the tubes. The electrolytic caps in the amp go bad. The chassis can rust. The original wiring gets brittle. The carbon composition resistors can change their value with time .. It is sometimes difficult to get the amp to be reliable by just changing out the driver board and maybe replacing some of the wiring.

    Bob

    What Bob said! He nailed EXACTLY what we find happening in so many pieces of vintage gear now. I specialize in a different amp, but the advice he gave applies to that other amp 100%. If you end up with a 50 or 60 year old amp you need to be prepared to make the investment in it to thoroughly update/refurbish it. If you don't - then it's not a question of if it will fail - it's when! And how much damage will it do that could have been prevented?

    BTW - not only does the original wiring get brittle, it is often corroded under the old insulation. Sometimes you strip the old wire back and the wire surface is black! And the old carbon comp resistors get noisier when they age and absorb moisture. Both can look okay from the outside but be badly deteriorated under the surface.

    A properly refurbished vintage amp or other gear can be a joy to own. But as Bob said - you need to make an investment in it to get the reliability you want.
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    • Post n°7

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