Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AZJimC on October 07, 2014, 07:59:16 pm
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I have a dual 500k pot used as PPIMV, and I'm hoping so source something I can't seem to find anywhere.
My idea is to replace that dual pot with another dual pot, but with a switch. Easy enough, right? Here's the hitch, I want a switch that will close when the Master Volume is full up, or at full clockwise position. I want to use that switch to switch in an NFB circuit, so I can use a presence control. I suppose another approach would be to use a presence control with conventional switch at CCW end of travel to switch the NFB on and off? Any thoughts? Anyone know where I might source the dual with switch at CW travel, or hack one from more commonly available parts?
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A dual-pot with a switch on the back will be more-than-triple the normal pot depth. That's likely to be very clumsy in most situations (especially if you're using anything close to stock equipment in most amps). And switched pots rarely (if ever) switch at the top of the pot rotation, though the manufacturer can make those for you if you buy enough (not practical for you & me).
Hoffman sells a CTS 10kΩ pot with a push-pull switch. Depending on the required Presence pot resistance, that may be very close and an ideal solution. If your circuit normally uses a pot above/below 10kΩ, it's possible to adjust the other resistance/capacitance in the feedback loop to allow seamless use of the 10kΩ push-pull.
Additionally, you can turn the NFB on/off with that setup at any position of the Presence pot, which may be preferable in the long run over switching only at fixed spots of dial rotation.
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Additionally, you can turn the NFB on/off with that setup at any position of the Presence pot, which may be preferable in the long run over switching only at fixed spots of dial rotation.
I had thought of that, and I'm beginning to agree with that logic, and readily available parts. I guess NFB would drop out as MV decreases between point NFB injection and power tubes. My thought was that a switch at full vol would make this somewhat intuitive and a non issue, but I'm sure not gonna find that part easily if at all.
I guess it's not very intuitive to have a switch marked NFB on the amp anyway? Maybe a "Clarity" switch? Clarity and Presence, lovely, HAHAHA
Speaking of clarity, I should clarify that all this logic relates to the intended presence control, that would require the NFB, and would be effected by the MV. I want to minimize this interaction.
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I'm not sure how the master volume inside the loop would interact with NFB.
My gut feeling is NFB doesn't change unless you apply such a big output signal to the output tubes that it overwhelms the gain/distortion reduction due to feedback, and the output tubes transition abruptly from reduced distortion (feedback) to large distortion (drive signal bigger than feedback gain reduction).
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If you can solve in one other way, well
if you have interest on finding a dual ganged pot with switch, here it is
http://www.potentiometers.com/Series309.cfm (http://www.potentiometers.com/Series309.cfm)
this only PP
http://www.guitarpartsworldwide.com/BOURNS+DUAL+GANG+500K+PUSH%2BPULL+POT+SOLID+SHAFT+PVFETTRBRE14b0RkalZIWnZKSGM (http://www.guitarpartsworldwide.com/BOURNS+DUAL+GANG+500K+PUSH%2BPULL+POT+SOLID+SHAFT+PVFETTRBRE14b0RkalZIWnZKSGM)
K