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Old 1st August 2011, 06:18 PM   #1
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Default foot switch safety question

Hi, I am planning on adding a bias switch (cathode/fixed) on my foot switch as have an open hole.

Is this safe? I have -50v running down the line. I also would like to connect the resistor and bypass cap in the foot switch to ground. But in another amp I have that resistor gets really hot.

Here is a drawing:

Thanks,

Daniel
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Last edited by dscottguitars; 1st August 2011 at 06:45 PM.
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Old 1st August 2011, 06:48 PM   #2
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Do you really really want to run all your signal through a foot switch, 3 meters outside of your amp? Remember, the cathode is a signal input too. I m sure you would not like to bring all the electric interference it your powertubes. How about a relay?
If that resistor got hot, it means it is under rated. Replace it with one that's like 5x the wattage dissipated in that point.
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Old 1st August 2011, 06:59 PM   #3
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I second what costis_n wrote about relay. Consider as well what happens to the amp if to disconnect foot switch, or if some wire got broken.
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Old 1st August 2011, 07:01 PM   #4
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Thanks Costis, that's why I asked. I didn't know what it would be like to do that. Does that mean a shielded cable wouldn't work either? There are other signals but they go to and from the preamp, is that different? I put the switch on the amp and I love it, I just thought it would be cool to have it as a foot switch option. Would it be the same if I ran just the cathode line in and out of the foot switch and back to the amp to ground it there?

I will change that other resistor, thanks...
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Old 1st August 2011, 07:04 PM   #5
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Good point Wave... issue settled, I won't do it. Thank you

I have use for another boost switch- one for each channel, so that's what I'll do instead.

I'm sure glad I asked!!

I have another post concerning the amp I mentioned that gets hot in the cathode resistor, would you mind looking at the schematic and my description, for an opinion. I see you are a designer and tech and I would appreciate a professional viewpoint and any criticism or suggestions you may have.

Thanks again,

Daniel

Last edited by dscottguitars; 1st August 2011 at 07:09 PM.
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Old 1st August 2011, 07:36 PM   #6
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Hi Daniel;

if the resistor dissipates fair amount of power it should be hot. Modern resistors rated for the same power as older ones are usually smaller in size, so run hotter. If it dissipates half of rated power (you may easily calculate it), it is fine, just place it such a way it has no less than 1/3 inch distance to other parts/surfaces for proper convection. Also, keep electrolytic capacitors farther from hot air flow from resistors: electrolytics die faster when heated.
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Old 1st August 2011, 07:42 PM   #7
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glad we helped!
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Old 2nd August 2011, 01:40 AM   #8
Enzo is offline Enzo  United States
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For the subtle difference this switch makes, only my opinion of course, it doesn't seem like the kind of thing you;d switch on the fly anyway.

But were you to do this, instead of cabling all that stuff down to the pedal, leave it in the amp and add a DPDT relay to do the job. Then a single line to the footswitch to operate the relay makes the footswitch simple.
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Old 2nd August 2011, 10:48 PM   #9
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Enzo, thanks for your input. I guess I don't know how a relay works, but for now it doesn't matter, and you are right it probably wouldn't be a switch you'd use during songs. But I added this to the back of my Fender Concert Rivera and I was surprised at the difference it makes. It is much spongier in cathode bias mode and it gets a lot of woooaawooooaaw... kind of effect. (it's hard to describe that in words) I've opted for a boost switch on the input tube for both channels and a separate one for channel 2, giving that one a 2-boost possibility.
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Old 2nd August 2011, 10:58 PM   #10
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Relay is like a switch controlled by an electro-magnet coil. When you apply power to the coil it switches forth, when you remove power from the coil it switches back. The same schematic like you drew will work, except you need some power for the coil.
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