master volume: Linear or audio taper ?

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master volume: Linear or audio taper ?

Post by pula58 » Mon Jan 17, 2011 4:05 pm

I am building a Fender AB763 style amplifier and want to include a master volume. What is the consensus of using a linear taper or audio taper pot for master volume?

I am not using it as a way to get significant overdrive, but rather, just to allow me to get the preamp volume high enough (to get just a little bit of hair) without having the amp be too loud. I'm going to put it before the phase inverter (I don't like how the Post PI master volume changes the gain around the PI/output stage feedback loop).

My idea is to use a low value pot to purposely load-down the output of the last tridoe (before the PI) in a similar manner that the 50K tremelo intensity post does (I am not having tremelo on this amplifier).

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Post by Firestorm » Mon Jan 17, 2011 4:33 pm

If you want just a little "hair," I would think audio taper is the way to go. But that's if it's wired in a conventional pre-PI place. Will the amp have reverb? Or at least the reverb mix stage? If so, there's another place to implement a master in the AB763 circuit that doesn't add any new paths to ground.

Look at the circuit following the reverb recovery tube on the AB763 schematic: 100KL pot, 470K series resistor and 220K to ground. All that feeds the grid of the third stage (along with the signal from the 3M3/10pF). Swap out the 220K in that circuit for a 250K pot. If there is reverb, I'd use a linear pot wired as a rheostat (otherwise it will change the reverb mix). If there's no reverb, that's still the location I'd look at to do what you describe.

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Post by Normster » Mon Jan 17, 2011 4:54 pm

I would suggest audio taper. I built something very similar using a 100k linear pot and it was too sensitive at lower settings. Of course, if you're going to be running it fairly loud most of the time, it doesn't matter as much.
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Post by pula58 » Mon Jan 17, 2011 5:32 pm

Thanks folks!

I'm going to use a 250K log pot then. And, put 68k in parallel with it. That way, I get a ~50K load (equivalent to trem circuit 50k intensity pot) on the last gain stage before the PI.

When that 50K load is missing I have found that I did not like the response/feel of the amplifier...too much bass..and other weirdnesses.
Last edited by pula58 on Tue Jan 18, 2011 5:47 am, edited 1 time in total.

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Post by d95err » Mon Jan 17, 2011 9:45 pm

Just look at the hundreds of posts on various forum by people who are desperate to get their Hotrod Deluxes modified, because the linear master volume makes it nigh impossible to play the amp at less than ear-shattering levels. Audio is definately the way to go if you want to be able to use the amp at both loud and quiet volumes.

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