Presence pot question

[Gaz]

Like most people, I've found the common presence circuit (famously in the 2203/4 Marshalls) has little effect until the pot is almost all the way up. I'm referring to the one where the tail resistor is bypassed by .1uf cap in series with a 22K pot to prevent DC on the pot, like this:

http://www.prowessamplifiers.com/schema ... 03_pwr.pdf

It seems like a smaller presence pot (say 10K) would help with the sweep of the control, but it's my understanding that pot is typically made about 5 times larger than the tail resistor so as to not effect the feedback ratio.

In that case, would it be possible to simply make the feedback resistor smaller to compensate for the change in feedback?

For example (and please correct me if I'm all wrong here), the feedback ratio in the 2203 is set by the standard 22K presence pot in parallel with the 4.7K resistor (for about 3.9K) and the 100K feedback resistor off the 4 ohm tap. If I were to use a 10K pot instead and a 4.7K (for about a total of about 3.2K), could I simply use a 82K feedback resistor to keep the ratio the same?

Hope that makes sense! As always, thanks for you time

[Leo_Gnardo]

I think a reverse taper pot would be the solution but where do you find one in that value. Reverse 50K & 3M are available due to use in the Fender vibrato circuit, other values not so much.

Recently I've added presence to a couple stock 70's Fenders, with a 6.8 uF film cap (MKP from CE - cheap!) and 1000 ohm pot, still same "crush" effect at end of travel. Sure could use a RAT.

[Gaz]

Thanks Leo_Gnardo. So is my thinking logical at all? My assumption is that 25K is too much isolation in series, so I was looking to make it smaller and then compensate for the change in NFB. I feel like I'm missing something, or others would already be doing this.

[pdf64]

Yes, a lower value pot would help to smooth out the pot rotation against amount of boost.
I've seen photos of an original 5F6A that used a 4k7 'tail' resistor > 0.1uF presence cap > 5k presence variable resistor.
So it would have had an overall reduced range, some presence boost there even when turned to minimum.
Linear pots would be much better than audio taper.
The track could be modded by scraping away material, to increase resistance (compared to stock) as it was turned down.
Or Small Bear has rev audio 10k pots (16mm alpha type).
Pete

[Reeltarded]

Backdate that circuit to the 5k/.1u just like a 4 holer. It is 1000% more effective. The late circuit is useless to me at least.

There is DC on the pot, who cares? shit works like a mofo

[martin manning]

I agree! The old-school pot-as-PI tail works better.

[Gaz]

Thanks for the replies, but my question that still remains is if I reduce the value of the pot with the 'scratchless' arrangement, do I need to compensate the feedback resistor for the change in feedback ratio (if there even is a change)?

[martin manning]

No. Try a 5k or 10k pot, which will spread the last 20% or 40% respectively of the 25k pot's action over the whole rotation. You'll lose the first 80% or 60%, of course, but if that part wasn't doing much then no great loss.

[Reeltarded]

Oh sure, you can use that late circuit.. if you want to sound like a girl.

Sorry, couldn't think of anything else that sounded as silly. (as the late circuit presence control)

Oh, sorry!

With any luck Jana won't see this.

[martin manning]

I know for a fact that Jana likes the early circuit.

[Reeltarded]

Another great reason to go with the early circuit!

[Gaz]

Ok, ok, I'll toughen up and rock the scratchy punk presence, but Martain you're saying that the presence pot is not seen as in parallel with the tail resistor? What piqued my curiosity is a part from Merlin's preamp book where he says that the pot is made large as to not fudge the feedback ratio. I could only assume that it was because it was in parallel with the tail resistor.

[martin manning]

For low frequencies the FB ratio is set by the FB and tail resistors. For higher frequencies the parallel cap shunts the FB to ground. The pot is there to block (impede?) that path. To block it more-or-less completely, so that all frequencies are fed-back equally, the pot needs to be ~5x the value of the tail resistor.

[Reeltarded]

He called you Martian, almost.

[Gaz]

Ok, I rigged up a switch to A/B between the two presence types, and wow, the 25K is pathetic in it's functionality compared to the old scratchy style. No free lunch I suppose.