Bypass Cap switch

[retrograde]

I'd like to put a switch on the .68 mf second stage bypass cap on a marshall 50 watt (1987). Its a mid 70s model which was originally not bypassed. I added the cap. Are any special considerations involved? Can I ground it by the switch or is it preferable to take the return leg from the switch back to the ground side of the cathode resistor?

Thanks
Geoff

[scottl]

I have bypass cap switches on all my amps. Love them! But, in my amps I alter the size of the already bypassed cathode. Not sure about how to do it on yours. It probably will pop no matter what. I'd ground it with the switch.

[Swarty]

A push/pull switch/pot would be a good way to keep the amp looking stock.

[scottl]

Use a 3 way switch. That way you can have one other bypass size! Maybe a .33 or 1uf for an in between tone.

[hasserl]

You could do it wither way. It still will pop when engaged. You can wire it up in series with a large value resistor to ground, that you connect to switch to bypass straight to ground. The resistor will effectively keep it out of the circuit until you switch it out, that way you won't get a pop.

[retrograde]

You could do it wither way. It still will pop when engaged. You can wire it up in series with a large value resistor to ground, that you connect to switch to bypass straight to ground. The resistor will effectively keep it out of the circuit until you switch it out, that way you won't get a pop.

Thanks, I actually hadn't thought about the pop. My concept was to run the ground leg of the cap to the switch in series and then run a lead back to the ground side of the cathode resistor (or gound it by the switch, I'm not sure which way is preferable). Hasserl. I'm not clear on where you are suggesting to place the large value resistor to avoid the pop.

Geoff

[hasserl]

Yeah, that reads really rough, doesn't it. If you put the resistor in series with the cap it will allow the cap to charge, but it won't allow it to effect the tone. The switch then would bypass the resistor and go straight to ground, this way the cap is already charged and there will be no pop.

Just think about it for a bit, you'll see what I mean. If needed I'll try to draw something up and post it.

[donnyjaguar]

The time-constant of a 1M resistor and u68 is less than one second so that's what I'd go with.

[retrograde]

Yeah, that reads really rough, doesn't it. If you put the resistor in series with the cap it will allow the cap to charge, but it won't allow it to effect the tone. The switch then would bypass the resistor and go straight to ground, this way the cap is already charged and there will be no pop.

Just think about it for a bit, you'll see what I mean. If needed I'll try to draw something up and post it.

I got it. Very cool idea.

Thanks
Geoff

[hasserl]

I went ahead and drew something up anyway. Here's one idea, though you could do it other ways.

http://img425.imageshack.us/img425/1646/bypascapswitch1js.jpg

[retrograde]

http://www.retrogradestudios.com/images/amp%20stuff/bypass%20cap%20small%20pic.bmp


Or this from a Mesa Boogie? Although the "big resistor" is only 47k

[hasserl]

There ya go. I was thinking, now that we've posted this Randall Smith will probably patent it. He probably already has.