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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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    7 lug terminal strip wiring

    steve46
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    steve46


    Posts : 8
    Join date : 2017-12-14
    Age : 77
    Location : Dallas
    • Post n°1

    7 lug terminal strip wiring Empty 7 lug terminal strip wiring

    Post by steve46 Sat Dec 16, 2023 4:37 pm

    I am about to replace the neg bias caps on my ST-70. I was just looking at the wiring diagram and noticed a jumper wire from terminal one to terminal six, on the seven lug strip. I suppose I overlooked this wire when I built the kit.

    This amp has served me well, with no hum or noise, for over fifty years without this wire!

    I suppose I should go ahead and add it while I'm replacing those caps. I just wonder what purpose the jumper serves, if it works great without it?
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    Bob Latino
    Bob Latino
    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°2

    7 lug terminal strip wiring Empty Re: 7 lug terminal strip wiring

    Post by Bob Latino Sun Dec 17, 2023 9:49 am

    steve46 wrote:I am about to replace the neg bias caps on my ST-70. I was just looking at the wiring diagram and noticed a jumper wire from terminal one to terminal six, on the seven lug strip. I suppose I overlooked this wire when I built the kit.

    This amp has served me well, with no hum or noise, for over fifty years without this wire!

    I suppose I should go ahead and add it while I'm replacing those caps. I just wonder what purpose the jumper serves, if it works great without it?

    On the 7 lug terminal strip lugs 1 though 4 are for the bias system for the original Dynaco ST-70. The two 50 uF capacitors store DC voltage in a reverse polarity. The positive side of both caps are grounded by the wire that goes from pin 1 to pin 6 and then pin 6 is grounded to the main grounding lug next to the quad cap. The two 10K resistors set the range of the two bias pots. The "double .02 cap" is placed across terminals 5, 6 and 7 to ground the power transformer's two secondary wires. The green/yellow wire grounds the two green 6.3 volt AC wires for the left channel and the brown/yellow wire grounds the two brown 6.3 volt AC wires for the right channel. The use of grounding these two wires through the .02 uF cap prevents any AC from getting to the chassis.

    Bob

    7 lug terminal strip wiring Dynaco-ST-70-pictorial-color-coded
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    steve46
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    steve46


    Posts : 8
    Join date : 2017-12-14
    Age : 77
    Location : Dallas
    • Post n°3

    7 lug terminal strip wiring Empty Re: 7 lug terminal strip wiring

    Post by steve46 Sun Dec 17, 2023 11:31 am

    Bob, Thanks for your thorough explanation of the circuit. I suppose the wire in question must be redundant, but I can't see how terminal one would be grounded without it. I will definitely correct my omission!

    Edit: After actually removing the two big caps, I found the wire between 1 and 6. Mystery solved! I apologize for speaking too soon, but I still appreciate your description of what is going on along this terminal strip.


    Last edited by Bob Latino on Sun Dec 17, 2023 4:22 pm; edited 2 times in total (Reason for editing : Solved)
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    • Post n°4

    7 lug terminal strip wiring Empty Re: 7 lug terminal strip wiring

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