3rd gen Accent/presence

[sonicmojo]

I am working up my design for a 100w 3rd gen with a presence dial instead of the accent switch. My last build used the switch with 1uF electrolytic and I'm pretty happy with it but I am thinking I would prefer a bit more sweep, would a 2kl pot and 2uF cap move me in the right direction? Should I try tantalum?

Thanks,

Bryan

[martin manning]

I would say you only need a 1k pot, and even then it might not have much effect over most of its range. Usually the presence pot is ~5x the value of the tail resistor, which is 100R in your case, I believe. Finding one less than 1k might be difficult, but you can always put a shunt across the element to lower the resistance. On the cap value, doubling the size will lower the spectrum of frequencies with reduced feedback by 1 octave, so you are on the right track there. I don't think the cap's construction matters much because it is only shunting the signal to ground.

[sonicmojo]

Yes it is 100R. This circuit differs greatly in that area from the later ones. Thanks for the advice. I'll probably do what you suggest with the 2k pot and another 2k resistor or so in parallel. I'll experiment with it. Thanks!

[hans-jörg]

Hi,
should the Presence pot be a revers one?. I think I heared someting like that.

Hans-Jörg

[sonicmojo]

Hi,
should the Presence pot be a revers one?. I think I heared someting like that.

Hans-Jörg

Your question got me curious since I didn't know about these and I researched a little bit. Assuming you are talking about reverse audio log pots, it looks like they are not really made anymore. In this case my presence pot like all the Dumble designs that I've seen is linear so it should not be an issue. Since there are various ways to wire current pots for the curve they produce, for example to design a balance type control, I don't think the reverse audio log pot is used much anymore or even obtainable, but I think you can create the effect of it if you want it. Once again, by no means am I that knowlegeable on this subject.....but this is how I understand it.

[Structo]

Yeah, there is no such thing as a reverse linear pot.

[sonicmojo]

I found a 1K pot, a RV4NAYSD. I am going to try it out first once I receive it. I should have my amp together soon too!

[hans-jörg]

Hi,
so from the principe its right that there should be a rev. pot. As lon as its linear (like mine) there is no problem with wireing. thats clear. But it was a principle questione. I´ve no nightmare because of this.

Hans-Jörg

[sonicmojo]

So I used a 1K pot with 1uF electrolytic on the board. I decided I wanted to enhance it a little more so I added a 2.2uF tantalum from the wiper to bus and it added to the effect. There's not much happening until about 2:00 on the dial though. I experimented with adding a 1K resistor to the pot to see what happened if I lowered total resistance and it did not seem to make a difference. It's good enough though!

[Structo]

I've noticed that as well.
The presence doesn't seem to come on until around 2:00.