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Bypass cap selector switch wire type?

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  • Bypass cap selector switch wire type?

    Hi again!

    I am planning on using a 1 meg audio push/pull volume switch (I wanted to use the tone control but the volume is closer to the cap locations on my board) to switch 2 capacitors next to the 1st cathode resistor on my 5F2-A project for some tonal differences. The two values I have decided to use are 10uf and .15uf along with the 1.5K resistor. I am trying to keep wiring tidy, short and thought out for a change! My question to you fine folks is should I just use my 20g solid copper for the wire runs to the switch or a few pieces of my leftover RG174? The length from the volume switch to the 2 caps is about 4.5-5 inches and about 6.5 inches if I do dare try to run all the way to the tone control.

    Happy Sunday all,
    Dale

  • #2
    Fender has made amps for what 75 years now? Have we ever seen a piece of RG174 in any of them? You are switching bypass caps on cathodes, not a high gain input to start with.

    If you want to switch between 0.15uf and 10uf, try it this way. Install the 0.15uf permanently across the resistor. Now we can just switch the 10uf in or out in parallel. When the 10uf is on, that extra 0.15uf won't matter a whit. Tie the 10uf to the cathode, then switch the ground connection off or on as needed.

    Now that will likely pop when you switch it. SO add a 1 meg or 470k or something resistor across the switch. When teh switch is open (off) then the large resistor is in series with the 10uf cap, making it essentially disappear. Then the switch shorts across the resistor when you turn it on, and that grounds the cap putting it into the circuit.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Thanks again Enzo. I had read that the shielded tends to help with a longer run on the inputs, I wasn't sure if it would be a benefit on the cathode or if that area is susceptible to picking up noise.
      Appreciate the advice!

      Dale

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      • #4
        Now that you have mentioned it Enzo ...that resistor across the switch ...is there a minimum that works? Ive seen it used a few times in various schematics since you brought it to my attention. It is used in that Prince o Wails build I will try to complete in a bit for all the push/pull switches but with varying values to the resistor. Is there some math/formula to what Is the ideal resistance in different circumstances?

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        • #5
          All the resistor does is provide a path to keep the cap charged. The value isn't at all critical. You just don't want it so low the cap takes effect when it is off.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            I've got a bypass cap switch on my BJ conversion I recently did.

            I made it front panel and foot switchable. The "FAT" switch.

            12 feet of non shielded cable to the foot switch and no issues.

            Click image for larger version

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            The resistor (R8) in series with the cap and shorted with the switch is the way Fender did it originally,
            but they used a FET to do the switching.
            If it ain't broke I'll fix it until it is...
            I have just enough knowledge to be dangerous...

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            • #7
              Originally posted by galaxiex View Post
              I've got a bypass cap switch on my BJ conversion I recently did.

              I made it front panel and foot switchable. The "FAT" switch.

              12 feet of non shielded cable to the foot switch and no issues.

              [ATTACH=CONFIG]49650[/ATTACH]

              The resistor (R8) in series with the cap and shorted with the switch is the way Fender did it originally,
              but they used a FET to do the switching.
              Safer that way with a FET?

              nosaj
              soldering stuff that's broken, breaking stuff that works, Yeah!

              Comment


              • #8
                Originally posted by nosaj View Post
                Safer that way with a FET?

                nosaj
                Dunno, fender did it that way cuz... well... I think that's how they do all their switching on these PCB amps. (FET)

                The cathode only has a volt or so on it, so I don't see any danger.
                If it ain't broke I'll fix it until it is...
                I have just enough knowledge to be dangerous...

                Comment


                • #9
                  Cathode circuit is a low impedance point, not particularly susceptible to pick up from capacitive or magnetic coupling. Since you are switching capacitance then the capacitance of the wire is not an issue either.
                  Just run a pair of plain wires. Twist them if you want to get "fancy".
                  Cheers,
                  Ian

                  Comment


                  • #10
                    Originally posted by galaxiex View Post
                    Dunno, fender did it that way cuz... well... I think that's how they do all their switching on these PCB amps. (FET)

                    The cathode only has a volt or so on it, so I don't see any danger.
                    Ah, but what if a tube shorts? HV on the pedal switch! But I wouldn't worry about it for a home brew amp. Fender might sell twenty thousand amps with that boost circuit though. And for a company with their profile it would only take one severe shock or (at worst) electrocution to bury them for negligence in court.
                    "Take two placebos, works twice as well." Enzo

                    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                    "If you're not interested in opinions and the experience of others, why even start a thread?
                    You can't just expect consent." Helmholtz

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                    • #11
                      Using the JFET to switch has a couple of benefits at low cost.
                      1)The switch wires just carry DC so you caan run them as far as you want with no problem
                      AND
                      2) you can easily incorporate it into channel switching by diode OR of the switch voltage (if it is a common gain element of a multi-channel design), that is have it switched in for Clean and out for LEAD automatically .

                      Cheers,
                      Ian

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                      • #12
                        I don't worry about a tube short. A plate to cathode short is so unlikely. If it did happen, then the HV only gets there through the plate resistor. It also has to contend with the cathode resistor. That would be less than 3ma assuming 100k and 1.5k, with a B+ of 300v. 3ma through 1.5k means there would be 4.4v on the cathode. Add to that that the cap is finding ground through our switch, so the 4v on that cathode has to get through the cap to see the outside world. That HV short to the cap/switch is shunted by the 1500 ohm cathode resistor, which then has to dissipate only 13mw, so it won't burn out.

                        So if the tube shorts plate to cathode, my pedal wires might have a cap - with 4v on the other end of it - exposed. No direct connection. I am not too concerned over that.

                        Though still very unlikely, I think a plate to grid short would be a more likely fault. That would put that 100k resistor onto the grid. Fender did that for decades at the input jacks, not to mention the foot switch that shunts a grid on the reverb. I have never seen an instance of HV appearing on the input jacks. SO not hot guitars other than in ungrounded amp, but that is not a shorted tube, that is a hot chassis. Likewise, I have never seen B+ on the FS for reverb.
                        Education is what you're left with after you have forgotten what you have learned.

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