Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 12AX7 on January 11, 2012, 09:39:38 pm
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3 questions on this subject. The example will be creating a 250k pot from a 1m or a 500k. Question one, if you strap a resistor across a pot to change the value, (for example a 470k across a 500k pot to get 250k) does the pot's new value act exactly as it would if it were a 250k pot to begin with or are there compromises? Two, does this work the same with unequal values or will there be a difference/compromise over using equal values? IE: a 1m pot with a 330k resistor to make a 250k pot. And lastly, will it work the same as a pot of the summed value even with a resistor and cap strapped across the input and wiper as a treble bleed? in other words, will that setup be the same as if i took a 250k pot and put the same treble bleed cap and resistor across the in and out lugs? So you have say a 470k across the outer lugs of a 500k pot and a 250k resistor and a cap across the wiper and input. Thanks.
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I've tried this and the taper gets all screwed up it works but not always the way you think it will.
Tony
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Don't have an answer for you
but may be you are interested to read this
http://www.geofex.com/article_folders/potsecrets/potscret.htm (http://www.geofex.com/article_folders/potsecrets/potscret.htm)
Kagliostro
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I haven't done a lot of experimentation with this, but check out the Crunch/Clean switch of the SLO100
http://www.el34world.com/charts/Schematics/files/soldano/soldano_slo100.pdf (http://www.el34world.com/charts/Schematics/files/soldano/soldano_slo100.pdf)
In the clean position it parallels a 39K resistor across the 500K "Normal volume" pot and also switches a 470K resistor in series before it. In that case the value of the paralleled resistor differs greatly from the pot.
Having never played thru one, I'm curious how that volume pot "feels" in the clean setting.
-John
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To me it feels like it changes the tone, not just the amount of gain. I have tried this WITH a 470k in front as you mentioned which is basically a voltage divider in front of the pot, so like two VD's in a row. Right now i just have a 100k strapped across the master along with the treble bleed. With the treble bleed there at all times, lifting the 100k changes the tone in a big way. With the 100k connected the tone gets a lot more jangley. Not just more high end, but sorta like the difference between a strat in the middle/bridge position as opposed to a single p/u alone. Has almost that sound like theres a chorus going which you get with bridge/middle, and the tone is springier feeling. Weird. I like it tho. Pot range also doesn't seem to suffer as far as volume, tho on the other hand the amp is so loud it could lose a lot of volume and you'd never know it. I think any volume loss manifests as less output distortion which is fine because you wouldn't be able to get that unless you're playing arenas anyways. But the taper is better/smoother.
As to geofex, i've known about that page forever but it never seems to answer my questions about it,m or at least my tiny brain can't wrap around it. I just wish i could get the main answer tho....do pots work the same with a resistor strapped across them to simulate a different pot value as a pot thats already of that value unmodded?
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> do pots work the same with a resistor strapped across them to simulate a different pot value as a pot thats already of that value unmodded?
No.