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    by-pass switch for thermistor

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    Captain Coconut

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    by-pass switch for thermistor

    Post by Captain Coconut on Mon Mar 03, 2014 9:00 pm

    I have an off/on switch for my ST-35 in a separate metal box. With this switch is a thermistor wired in series with the 'hot' side of the A/C. If I make a bigger chassis for amy amp some day, these components will be inside, but for now, the outboard set-up works fine. Here's my question: What kind of switch would I need to by-pass the thermistor if I choose to eliminate it from the A/C line after it's done its thing?

    For a second there, I was thinking of an 'on-off-on' switch where I would rapidly switch over to the other 'on' position after a minute or so. Of course that would shut off power for a split second, and then back on again. Not a good idea.

    Is there something simple that would work, where there would be no interruption of power, and by-pass the thermistor?
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    Alan-14

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    Re: by-pass switch for thermistor

    Post by Alan-14 on Mon Mar 03, 2014 9:28 pm

    Assume you use the thermistor as an inrush current limiting device?
    Does it have a negative effect on the sound produced by the amp or are you worried it might impact the power supply's response to transient peaks when needed?
    Does it "sound" better without the thermistor?
    If you can't hear any difference ... maybe just leave it in place.
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    Captain Coconut

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    Re: by-pass switch for thermistor

    Post by Captain Coconut on Mon Mar 03, 2014 9:51 pm

    The thermistor drops 2-3 volts which I don't want in the summer months. I'd like to by-pass it if possible. Rod Elliott shows a schematic for a relay on his site, but I wonder if there's something as-built.

    This has nothing to do with the 'sound'.
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    daveshel

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    Re: by-pass switch for thermistor

    Post by daveshel on Mon Mar 03, 2014 10:51 pm

    Interesting, Captain. I've just been trying to figure out how to add a stepped attenuator to mine. The EFB board uses all the to space so I was considering an external enlcosure. What is it you use?

    Now, really we both ought to be working on a bigger chassis for these things. I figure all you need is a taller bottom piece to fit into the existing top piece.
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    Captain Coconut

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    Re: by-pass switch for thermistor

    Post by Captain Coconut on Tue Mar 04, 2014 1:09 pm

    Dave, here's a picture of my outboard on/off switch. Mains power on top, into the box where I have a thermistor wired onto the switch, and then power leaving from bottom of the box - not visible since I'm working on the amp right now - into the amp. Holes are punched in the aluminum box for ventilation.

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    Luddite

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    Re: by-pass switch for thermistor

    Post by Luddite on Tue Mar 04, 2014 3:08 pm

    Captain Coconut wrote:I have an off/on switch for my ST-35 in a separate metal box. With this switch is a thermistor wired in series with the 'hot' side of the A/C. If I make a bigger chassis for amy amp some day, these components will be inside, but for now, the outboard set-up works fine. Here's my question: What kind of switch would I need to by-pass the thermistor if I choose to eliminate it from the A/C line after it's done its thing?

    For a second there, I was thinking of an 'on-off-on' switch where I would rapidly switch over to the other 'on' position after a minute or so. Of course that would shut off power for a split second, and then back on again. Not a good idea.

    Is there something simple that would work, where there would be no interruption of power, and by-pass the thermistor?


    You may need to change to a rotary switch, but a "make before break" (shorting), three position switch should work. Position (1) is off, position (2) inserts the thermistor for warm-up, and position (3) bypasses (shorts) the thermistor. By using a "make before break" switch the third (bypass) position will be connected before breaking the second (thermistor) position. Hence, no power drop out.

    Best Regards,
    Charlie
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    sKiZo

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    Re: by-pass switch for thermistor

    Post by sKiZo on Tue Mar 04, 2014 3:15 pm

    ... And if you wire it up wrong you end up momentarily passing 240v thru?
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    Luddite

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    Re: by-pass switch for thermistor

    Post by Luddite on Tue Mar 04, 2014 3:27 pm

    sKiZo wrote:... And if you wire it up wrong you end up momentarily passing 240v thru?

    Nope, you can't get 240V from a 120VAC single phase circuit...unless you use a step-up transformer.
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    Captain Coconut

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    Re: by-pass switch for thermistor

    Post by Captain Coconut on Tue Mar 04, 2014 4:29 pm

    This sounds interesting. Make-before-break sounds better than my break-before-make scenario. With just one line coming in, it would always be 120V. (Introducing a second 120V line would give you potentially 240V, but I won't be doing that.)

    The methods described on Rod Elliott's Westhost site seem complicated, but I think he's addressing a bypass for those who won't be manually switching it - something hidden inside the amp that you can't get to.

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    peterh

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    Re: by-pass switch for thermistor

    Post by peterh on Tue Mar 04, 2014 4:34 pm

    Captain Coconut wrote:   This sounds interesting. Make-before-break sounds better than my break-before-make scenario. With just one line coming in, it would always be 120V. (Introducing a second 120V line would give you potentially 240V, but I won't be doing that.)

      The methods described on Rod Elliott's Westhost site seem complicated, but I think he's addressing a bypass for those who won't be manually switching it - something hidden inside the amp that you can't get to.

    Excuse me but; why don't you install a second breaker that shorts the thermistor ? A similar
    as the one you already have.
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    Captain Coconut

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    Re: by-pass switch for thermistor

    Post by Captain Coconut on Tue Mar 04, 2014 4:53 pm

    Do you mean a second single pole switch? I was just thinking that. Turn the switch on for the thermistor, and then turn the other one on, and turn the thermistor switch off, to bypass the thermistor. And when you're done, turn the power off with the second switch.
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    tubes4hifi
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    Re: by-pass switch for thermistor

    Post by tubes4hifi on Tue Mar 04, 2014 5:29 pm

    Capt,
    I would do it the way you just thought of, a 2nd switch to short across it
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    Captain Coconut

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    Re: by-pass switch for thermistor

    Post by Captain Coconut on Tue Mar 04, 2014 5:43 pm

    That's the way I'm leaning, though I would like to find a single make - before - break 3 position switch that Luddite suggested, but I don't think they're readily available at your local hardware store.
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    tubes4hifi
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    Re: by-pass switch for thermistor

    Post by tubes4hifi on Tue Mar 04, 2014 6:28 pm

    Radio Shack probably has something similar to this from Mouser.com
    http://www.mouser.com/ProductDetail/Alpha-Taiwan/SR2511F-0112-19R0B-E9-S-W/?qs=sGAEpiMZZMvNbjZ2WlReYnqYHrQfuERsVVYwe2wXwFk%3d
    $2.77 1 pole rotary switch 1-12 switch positions
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    Luddite

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    Re: by-pass switch for thermistor

    Post by Luddite on Tue Mar 04, 2014 6:49 pm

    Mouser also has a heavier duty (125V, 2.5A) C&K branded switch if you prefer. Here's the link:

    http://www.mouser.com/ProductDetail/CK-Components/A10615RSZQ/?qs=sGAEpiMZZMvNbjZ2WlReYqwnDNdasyIHweYfa0y5NZg%3d

    Best Regards,
    Charlie

    P.S. Either of these switches will come with index rings that can be set for three positions.
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    Captain Coconut

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    Re: by-pass switch for thermistor

    Post by Captain Coconut on Tue Mar 04, 2014 7:06 pm

    Thanks guys!

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