Vintage Fender Verb amps & their volume pots

[wizard333]

I get this all the time, and I agree: "My <Insert blackface/silverface Fender with verb> distorts too easy". What they really mean is, it distorts early on the pot taper. Usually by the time the volume is above about 2.5 a PAF will be dirty, by 5 you're talking the high side of blues gain. So for people who don't take a vintage Fender and crank it to 8 for their sound but use pedals and play clean or on the edge of breakup, I think I have a solution.

The issue is, how to give more control over the volume using more pot taper?

Here's my thinking, correct me if I'm wrong:

If we consider a pot not turned up all the way as 2 parts (2 parts of the voltage divider)
1) A series resistance with the input signal (between the input and wiper).
2) a load resistance to ground for the next stage (wiper to ground).

Now consider a 1Meg Audio taper (or fake audio taper) pot. At 3 on the knob, it is about 5% of the total resistance between wiper and ground, leaving 95% series resistance with the signal. At 5 on the knob it is about 10% with 90%, and the amp is distorting pretty well. So a 100k pot instead of 1Meg, with a 820k resistor between the output of the treble pot in the input to the 100k volume pot should approximate the range of 0-5 on the knob of a 1Meg pot, but give finer tuned control and more useful knob range. Yes 820k plus 100k isn't 1Meg, but neither are most of those old "1 Meg" pots either.

The load on the output of the treble pot stays about the same, as does the input load on the 3rd stage.

Does this make sense or am I missing something?

[JoeM]

Interesting thought. It's even worse on the RI models, at least on my DRRI. I was wishing recently I could have more control over the volume at the lower end of rotation. It comes up so fast, you feel like just touching the volume control and it changes. Are you going to try this on an amp?

[wizard333]

Yep. I'm tinkering with my mod demo right now and that's going to happen tonight.

[bob p]

The Secret Life of Pots

[wizard333]

Yeah I'm aware of that article but it covers how to make a pot different by using resistors...of which I'm already aware. The issue you run into there is your input and output loads change vs. what an actual pot of that value would be.

[wizard333]

It works as expected. Clean to about 7.5 with moderate breakup dimed. That's about all the range I need out of a Fender clean channel for normal purposes.

[Jazz P Bass]

Nice work!

[JoeM]

Very Cool!

[pdf64]

It's worth checking the actual taper - many vintage / RI Fenders have audio tapers around 25-35% on their volume (and treble) controls.
So they do come up quite quickly - not a whole lot different to linear really.
If that's the case (and it's a problem) then a regular 10% audio taper 1M pot can help a lot, eg standard CTS.
The regular Alpha audio taper is 15%.
So turn the pot to 50% rotation and check the resistance.
Pete

[wizard333]

The other thing would be that these aren't precision devices, so claimed taper is, at best, like an EPA estimate of gas mileage. Just like "1 Meg" varies from about 800k to 1150k in practice given any particular CTS "1 meg" pot. All that said, 100k of swing takes it from clean to Larry Carlton-ish breakup, which is all most folks are looking for.

[pdf64]

Yes, the overall track resistance is usually a +/-20% spec, the accuracy to taper is generally not a specified parameter unless spending >$50 per pot ; so they can be pretty much anything. However, going by the '50% rotation point' the electrical taper seems to match the overall track resistance fairly well. eg if the track is +20% then the 50% rotation point will also be about that much high.
Just to point out that with the common Fender ~30% volume control taper, for a 1M pot, 100k is reached around 3.5-4 on a 1-10 knob.
Pete

[loudthud]

Did you change the way the bright cap is wired? With the bright switch right next to the pot, you can use the switches terminals as convienient places to mount resistors caps and wires.

[wizard333]

I didn't on this one (no bright cap switch) but I've done all sorts of things with those switches in the past. The ones CE sells have two sets of terminals so they can be really useful. You can switch in a bigger midrange resistor, smaller mid cap or bigger treble cap, or add a plate bypass cap with those (requires 2 caps in series, one at either end of the resistor so you don't put high DC across the switch), any 2 combos, which is a nice "crunch" switch.

I ended up changing the 820k series resistor to 470k, I liked it better and it still sounds "right" but gives some warmth and sparkle vs. the larger one.

Think I'm going to be doing this a lot, it really gives a more usable range on that volume pot for a 3 stage Fender channel. The 2 stagers are much more polite as the volume knob goes up and don't need it so much, but even those could probably benefit from a 500k pot; they rarely sound good above 8 on the 1 Meg pot. With the 470k series resistor and the 100k audio pot, it still breaks up nicely at the top of the range but stays clean a whole lot longer; PAFs seem to break up around 7 with a little grit, and crunch with authority on 10.

[JoeM]

wizard333,

What about using a 250k pot with a 680k series resistor? I have access to some of Fenders PCB 250k pots and was considering using one with a 680k resistor in my DRRI. The stock DRRI uses a 1M with 30% taper, so it really comes up fast at the low end.

[pdf64]

For the volume control, maybe a 1M audio pot with a regular 10% taper would help with the resolution at the low end of the scale, whilst retaining the full range of control?
Pete