Bottlehead Forum
General Category => General Discussion => Topic started by: Laudanum on January 21, 2017, 04:28:02 AM
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Hey gang.
I have a guitar amp that has a rotary switch that selects between different resistor and/or resistor/bypass cap. combinations from cathode to ground on preamp tube. Problem is that there is a large thump when switching between positions so the volume needs to be rolled of first. It's a non-shorting switch. My thinking was to replace it with a shorting switch. My understanding is that a shorting type selector would make contact with the next position before breaking contact with the current position. In other words, swithching from position 1 to 2, before breaking contact, both position 1 and 2 would be active (in parallel). If I have that correct, it would seem to me that this would be better than the open circuit and the big thump between positions and should be ok to implement.
Please set me straight or otherwise add your input.
Thanks
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It's a pretty bad idea any way you connect it. If the switch opens between steps at least the tube stops conducting, but you have found out what happens. If you parallel two resistors together with a make before break switch the tube will draw excess current as it goes from one step to the next. That's not good either. Using a variable pot for the cathode resistor might be a little better, but it will still make noise when you turn it.
Your best bet is probably to leave the circuit the way it is and figure out a way to put a momentary mute switch in the signal chain after the preamp tube, if possible - like maybe at the grid of the next stage if it is cathode biased. Or simply pick your favorite setting for that cathode resistor switch and stick with it through a song. I would assume that is the intention of the designer.
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Makes perfect sense Doc. Thanks. That's why I ask before I do dumb things. ;)
It's got a master volume which I roll down before switching and that mutes it. Probably almost as easy as adding a momentary switch. It's not a gigging amp so it's not a big deal. Think I'll leave it be.
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Put the highest value resistor from the cathode to ground, then use the rotary switch to swap in different paralleled resistors. This will be quiet. IMO, I'd use the largest value of cathode bypass capacitor between them all and put that in permanently between cathode and ground with the highest valued resistor.
This will reduce the effect of this rotary switch, but it will switch quietly.
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Put the highest value resistor from the cathode to ground, then use the rotary switch to swap in different paralleled resistors. This will be quiet. IMO, I'd use the largest value of cathode bypass capacitor between them all and put that in permanently between cathode and ground with the highest valued resistor.
This will reduce the effect of this rotary switch, but it will switch quietly.
Why didn't I think of that ;D. I'll try it. Thanks Paul!
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Put the highest value resistor from the cathode to ground, then use the rotary switch to swap in different paralleled resistors. This will be quiet. IMO, I'd use the largest value of cathode bypass capacitor between them all and put that in permanently between cathode and ground with the highest valued resistor.
This will reduce the effect of this rotary switch, but it will switch quietly.
Thinking about this some more ... Capacitors are parallel with resistors and switched in 3 positions. So there is no bypass cap in the other 3 positions. Does switching the capacitors bring me back to the switching noise issue even though there will be a fixed, non-switched cathode to ground resistor?
Thanks
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Can you post a drawing of this arrangement?