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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


    mono/stereo switches

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    rmike

    Posts : 2
    Join date : 2020-10-31

    mono/stereo switches Empty mono/stereo switches

    Post by rmike on Sun Nov 01, 2020 9:37 am

    I need to replace the mono/stereo switch on a stock st70. I am not certain, but I think the original was a SPDT slide switch. The only switches I have in my supplies are DPDT switch.

    Will that work? And if so, how is it wired?

    Thank you.
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    rmike

    Posts : 2
    Join date : 2020-10-31

    mono/stereo switches Empty Re: mono/stereo switches

    Post by rmike on Sun Nov 01, 2020 10:06 am

    Maybe I have to dig a little deeper for the spdt then...

    So one side of the switch would put the two wires together making it 70W mono. The other side would separate the two wire making it 35wpc stereo. Is my reasoning correct?



    Bob Latino
    Bob Latino
    Admin

    Posts : 2970
    Join date : 2008-11-26
    Location : Massachusetts

    mono/stereo switches Empty Re: mono/stereo switches

    Post by Bob Latino Yesterday at 12:24 pm

    rmike wrote:Maybe I have to dig a little deeper for the spdt then...

    So one side of the switch would put the two wires together making it 70W mono. The other side would separate the two wire making it 35wpc stereo. Is my reasoning correct?




    You need a SPDT slide switch. The "change to 70 watts mono" never really worked right on an original Dynaco ST-70. To turn the amp into a true 70 watt mono amp, in addition to flipping that stereo/mono switch, you also had to run a wire between the two ground lugs on the two speaker terminals. You also had to run a wire between the two 16 ohm speaker taps if you were using 8 ohm speakers OR run a wire between the two 8 ohm speaker taps if you were using 4 ohm speakers. When you do this, you mix the two feedback lines of the two output transformers. Doing this, IMHO, will degrade the overall sound quality of any original Dynaco ST-70.

    The VTA ST-70 and VTA ST-120 have a slide switch in this stereo/mono slot but > the switch is a dummy switch just to fill the hole in the original type chassis. If you want to use the 70 watts of an original Dynaco ST-70 on one channel, you should biamp. Most modern speakers have separate woofer and tweeter inputs on the back of the speaker which are wired together with two metal straps. Remove the two metal straps and connect one ST-70 channel to the woofer section and the other ST-70 channel to the tweeter section. You will now have the ST-70's 70 watts going into one stereo channel. If you have another Dynaco ST-70, you could do the same on then other stereo channel and wind up with 70 watts on both stereo channels. If your speakers don't have the straps on the back, then you can't do this unless you take the speaker apart and can get at the speaker's crossover.

    Bob

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