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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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    Stereo/Mono Switch....

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    wolverine


    Posts : 59
    Join date : 2010-02-20

    Stereo/Mono Switch.... Empty Stereo/Mono Switch....

    Post by wolverine Wed Mar 10, 2010 10:29 am

    I would like to get some opinions on the removal of the switch. My basic question is...."should I remove the switch from the circuit first of all, and if so then what is the proper method of doing so for a stock board". Thanks to all who reply, your experience is greatly appreciated by me.... :}

    wolverine
    Bob Latino
    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    Stereo/Mono Switch.... Empty Re: Stereo/Mono Switch....

    Post by Bob Latino Wed Mar 10, 2010 3:30 pm

    wolverine wrote:I would like to get some opinions on the removal of the switch. My basic question is...."should I remove the switch from the circuit first of all, and if so then what is the proper method of doing so for a stock board". Thanks to all who reply, your experience is greatly appreciated by me.... :}

    wolverine

    There is no real reason to remove the stereo/mono switch other than maybe a couple of less solder connections between you and the music. Removal of the switch will make no audible difference in the sound of your amp.

    What many Dynaco ST-70 users don't realize is that when you JUST flick this switch to mono you do not have now a 70 watt mono amplifier. What you have when you just flick the switch to mono is two 35 watt amps carrying the same audio signal. In order to truly have a 70 watt mono amp you must also, in addition to flipping the switch to mono, run a wire between the two output transformers. If you have 8 ohm speakers you must run the wire between the two 16 ohm taps. If you have 4 ohm speakers you must run a wire between the two 8 ohm taps. Now you have a 70 watt mono amplifier and you can connect your speaker to EITHER transformer output. You can connect your input interconnect to either input jack.

    If you do want to remove the switch wiring do this ...

    1. Remove the wire that runs from lug #3 of the stereo/mono switch to the positive terminal of the right channel input jack.

    2. Remove the wire that runs from lug #2 of the stereo/mono switch to the positive terminal of the left channel input jack.

    3. Remove the end of the 470,000 ohm resistor connected to lug #1 of the stereo/mono switch and reconnect it to the center positive lug of the left input jack.

    Check the photo below ...

    Bob


    Stereo/Mono Switch.... Stereo_monoremoval
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    wolverine


    Posts : 59
    Join date : 2010-02-20

    Stereo/Mono Switch.... Empty Thanks Bob... :}

    Post by wolverine Wed Mar 10, 2010 4:39 pm

    I have for some time envisioned a different chassis for one of my ST-70's. I have seen some really nice ones done by DIY'ers and I thought I would enjoy designing my own chassis. Since I NEVER use the switch I though why in the world make a cut-out for it in a new chassis build. I really had no clue as to whether or not this little switch was integral to the workings of the amp at all, though I cannot see where it has any function other then to send the same signal to both sides of the amp.

    Bob I truly appreciate your time. Thanks for your support, and thanks to all you other folks who have contributed your time and energy in answering stupid questions from dumb old farm boys like me!.... :}

    wolverine
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    GP49


    Posts : 792
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Stereo/Mono Switch.... Empty Re: Stereo/Mono Switch....

    Post by GP49 Thu Mar 11, 2010 11:36 am

    But even after doing all that, you don't have a PROPER 70 watt mono amplifier.

    For one thing, if you put it on the test bench, you will find it won't make 70 watts, anyway.

    And...worse...the two amplifiers will then share the same negative feedback signal to the input stage, which is incorrect because it is virtually impossible that the error correction needed by each amplifier channel will be identical in both. The combined negative feedback signal won't match the errors in each channel, since it will be a mongrel composed of both feedback signals, combined.
    lorne
    lorne


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    Post by lorne Fri Aug 12, 2022 3:52 am

    This is an old thread. I've read over Bobs's instructions on how to remove the mono switch several times. Also the late Bill Thomas, who many of us knew, contributed virtually the same described process years ago on another forum. Deleting the mono switch is not a complicated thing.

    But, I am baffled by one detail that never, ever comes up in all the millions of stuffs I've read about the ST-70. Here goes:

    • Page 5 #46 of the original ST-70 assembles manual states this:

    Observe two wires extending from the printed circuit board just under the input socket. Connect the extended wire on the right to the ground (short) lug of the right input socket. Connect the left extended wire to the ground (short) lug of the left input socket .

    OK ... DUH ... not hard to follow. But my Vintage Electron board I bought many years ago in anticipation of a rebuild some day does not (of course) have these wires. (I populated the stock-type board myself .) And by the way, my mono switch is broken and I am not going the replace it.

    Q1: What are these two wires doing? Are they grounding those short lugs?  

    Q2: So all I have to do is ground these short lugs somewhere. On the chassis ... say on on a screw where the mono switch was. Or — to the universal star ground — is that better? (A long wire though).

    Cheers ... Lorne
    lorne
    lorne


    Posts : 21
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    Location : Sendai, Japan

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    Post by lorne Fri Aug 12, 2022 7:08 am

    ANSWERING MY OWN QUESTION
    I feel rather foolish.Embarassed The wires on the short lugs go to two respective 10R's. My bad planning: I have to pull two resistors I previously installed on the board. Then I have to install two new resistors with hooks on the ends of one lead so I can solder on the wires.

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