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    multiple amps and spreakers, output switch box help

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    paysonmark

    Posts : 6
    Join date : 2018-06-25

    multiple amps and spreakers, output switch box help

    Post by paysonmark on Sat Sep 08, 2018 11:50 am

    Hello all ,

    I have 3 tube amps (st70 vta120) and 3 pairs of speaker. I want to add a switch box to choose any amp with any speaker pair.

    I need a suggestion on how to protect the tube amp outputs so they always see a load with no speaker connected or during the switching period.

    thx
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    Peter W.

    Posts : 888
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: multiple amps and spreakers, output switch box help

    Post by Peter W. on Tue Sep 11, 2018 8:15 am

    paysonmark wrote:Hello all ,

    I have 3 tube amps (st70 vta120) and 3 pairs of speaker.  I want to add a switch box to choose any amp with any speaker pair.

    I need a suggestion on how to protect the tube amp outputs so they always see a load with no speaker connected or during the switching period.

    thx

    Just for clarity, please make any changes where I go off the rails;

    a) You want to have three (3) amps on and in operation and switch between them.
    b) These amps will be passing signal when they are switched.
    c) You wish to avoid thumps and pops when switching.
    d) And you wish the amps to see load when not connected with any speaker(s).

    So:

    You will need substantial resistors, on the order of 8 ohms at 100 watts or so. And an enclosure big enough for six (6) of them.

    https://www.parts-express.com/8-ohm-100w-non-inductive-dummy-load-resistor--019-020

    And a heavy-duty AC- rated switch, as an order of magnitude, five (5) amps.  Two of these - one on the input side, one on the output side. Select the amp, select the speaker.

    https://www.mouser.com/ProductDetail/CK/R20407RR02Q?qs=sGAEpiMZZMvNbjZ2WlReYgfl1vEqXQaLSH0SY3Dz1Iw%3d    

    And a 100-ohm (experiment with values between 25 and 100 ohms) 1-watt resistor across the speaker leads from the amp will handle the switching noise. It's time-in-circuit is momentary, so 1-watt is plenty.

    NOTE: The above gives you a singular amp to a singular speaker pair, but your choice on one-of-three in either case.

    And, why it is that good switch boxes are not cheap, whereas cheap switch boxes are not good.

    The above is strictly seat-of-the-pants design using only a small amount of overkill. One does NOT want to run amps unloaded for any period of time more than a few minutes, if even that. It is even more critical that SS amps do not run unloaded.

    paysonmark

    Posts : 6
    Join date : 2018-06-25

    Re: multiple amps and spreakers, output switch box help

    Post by paysonmark on Tue Sep 11, 2018 2:28 pm

    thanks you for the reply. you are mostly on track. Here is why Im thinking of this. I have 3 primary tube amps, Stock st70 but with new resistors & caps. Heavily modified st70. Finally a VTA-ST120.
    I want to be able to switch amps and speakers to hear the difference / nuances of a particular song with out tearing the systems wiring apart.
    I would not be switching at loud volume, I just want a level of safety incase I do something foolish and not leave a output open.   ( which will happen)
    I thought about using an Arduino and programming a relay matrix but I wanted to see /hear of other solutions that maybe better suited for tube amps by tube amp users.
    Thanks for your suggestion, I like the rotary switch idea.
    mark

    hwaitung

    Posts : 8
    Join date : 2018-05-05

    Re: multiple amps and spreakers, output switch box help

    Post by hwaitung on Tue Sep 11, 2018 11:57 pm

    I am very interested in building such a switch as well. From what I know, there are 2 types of rotary switch. Shorting switch (make-before-break) and non-shorting switch (break-before-make). Which one should we use in this case?

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    Peter W.

    Posts : 888
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: multiple amps and spreakers, output switch box help

    Post by Peter W. on Wed Sep 12, 2018 7:20 am

    hwaitung wrote:I am very interested in building such a switch as well. From what I know, there are 2 types of rotary switch. Shorting switch (make-before-break) and non-shorting switch (break-before-make). Which one should we use in this case?


    You never want to short an amp output. That could be damaging even if momentary, and that is what the 100-ohm resistor serves to do - prevent pops when switching.

    It is going to take some cute wiring to load the amps when switched out. Just keep that in mind. You might need more poles on the switch such that when an amp is "out" to the speaker, it is "in" to the dummy load. BEWARE: It is NOT OK to stack the dummy-load so that, by rating, it could serve two amps. That would be shorting the amps to each other, very nearly always fatal. So make sure that this cannot happen. When stereo amps are strapped for higher mono-output, that option is carefully designed into the circuit.

    hwaitung

    Posts : 8
    Join date : 2018-05-05

    Re: multiple amps and spreakers, output switch box help

    Post by hwaitung on Thu Sep 13, 2018 10:27 am

    Peter W. wrote:
    You never want to short an amp output. That could be damaging even if momentary, and that is what the 100-ohm resistor serves to do - prevent pops when switching.

    It is going to take some cute wiring to load the amps when switched out. Just keep that in mind. You might need more poles on the switch such that when an amp is "out" to the speaker, it is "in" to the dummy load. BEWARE: It is NOT OK to stack the dummy-load so that, by rating, it could serve two amps. That would be shorting the amps to each other, very nearly always fatal. So make sure that this cannot happen. When stereo amps are strapped for higher mono-output, that option is carefully designed into the circuit.

    Thanks!

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