Tone Stack Bypass Choice

[luthierwnc]

I've got too much time on my hands.

I'm working on a design for a friend who wants a Princeton Reverb head with something to kick for the occasional solo. This goes over a 2X10. He was talking about a master volume but my thoughts are this design loses too much of a pretty limited gain structure.

I was thinking about a partial tone stack bypass -- recovering some of what is lost rather than losing more. There are three designs I'm playing with. This guy usually plays Tele's and slide 1/3 of the time so just lifting the ground is probably too much. In each of the three examples below, there is an extra pot and possibly a switch (prolly a push/pull pot). All of these are at least mostly stolen from better designers than myself so thanks to those who went before me.

The first one is basically a really big mid pot with a parallel cap to take a little bite off the highs. Pretty simple. The middle example is just a big mid-boost with a pot to control the increased signal and possibly a bleed resistor to not overwhelm the next stage.

Contestant #3 is similar to the Dumble PAB except I'm using a pot to separate the treble from the bass pot. The mid is set at half the stock value in that design. Usually this is a switch to separate the treble and bass pots but the bass and mid still bleed to ground. In the larger versions of this amp, the bass wiper and #3 lug are on a dpdt footswitch. I'm doing it here as a variable resistor.

None of the values are written in stone and I'm sure I'll leave a couple extra lugs on the board for tweaks. Still, if anyone wants to wade in with comments, improvements or questions, I'd love to hear from you.

Thanks, Skip

[Chuck H]

Well I personally don't care for the tone stack bypass systems. They are clunky macro things that change the amp too much. Not that it's wrong for everyone or every amp. But some amps don't respond well to it.

Another option could be something like the Torres "mid boost" mod. It's basically just like the Randall era Fender "boost" switch that jumpers a bigger value cap across the stock 250p treble cap. Mesa does the same thing and calls it "fat". Randall and Mesa both used a 750p cap added to the stock 250p treble cap in parallel. I liked the Torres mod because it used a rotary switch and gave options for 6 different cap values. I only ever used two of them so IMHO a three or four position switch would be fine using progressive values like 680p, 1500p, 2200p and 3300p (2200p across the treble cap is very cool). The effect is boosted upper and middle mids. So you don't get ugly top end or boomy bottom like a bypass switch. I think it's worth consideration. It also has the benefit of not nulling the use of the tone stack. If the effect is too dramatic it can be made more subtle by adding a series resistor (22k to 100k) to the switch input. To make it foot switchable you would only need a spst relay with a large value resistor across it (5.5M or 10M)

More versitile than the "bypass", you can still use the tonestack and it's a variable frequency effect too.

If this didn't provide enough gain I would consider a bypass mod.

Food for thought.

Chuck

[chipaudette]

I agree with Chuck H on this one...I've got a 750p cap on a switch on my amp and it provides a pretty decent perceived volume boost and perceived "strength" to the tone. It should be great for leads. I've not tried the other values...but they would certainly be interesting.

I've got my cap wired in Mesa Boogie style where you've got a 10M resistor in series with the 750p cap. That resistor+cap are then wired in parallel to the standard 250p treble cap. Here's the key: the switch is then wired just around the 10M resistor. With the switch "open", the resistor effectively keeps that new 750p cap out of the circuit. With the switch "closed", the 10M resistor is shorted, which brings the 750p cap into play.

The advantage with this arrangement (versus using the switch to directly cut the cap into and out of the circuit) is that there's no "popping" sound when switching. Without the resistor, the cap has to suddenly charge from zero whenever you bring it into the ciruit. BANG! The resistor prevents this. It's nice.

Give it a try. This could be a great solution to your need.

Chip

[Chris / CMW amps]

It all depends on the voicing/gain-structure of the whole amp.

In most circuits just "UNgrounding" the tonestack works but does add muddyness and other changes to the sound.

The mid-boost (using a larger treble-cap, total value between 680p and 0.002) works more subtle and makes the sound bigger/fatter.

Combined with my latest Fender-mods (designed during a couple of years) both mod/switching-options do work excellent, even did make the tonestack-bypass-switching with a boost-level-control but it ain't no stock black/silverface Fender anymore.

[luthierwnc]

Thanks guys. All of these ideas are set up to dial-in. I am leaning towards the dumble version. I've made several of those amps and it is a useful part of that design. Each one has needed tweaking and I have ended-up with smaller and smaller bleed resistors. That's why I just used a 1m pot.

Those amps usually incorporate a separate mid-boost by using an outsized treble cap (.002 or season to taste) and a switch that puts a 330 puff cap in series. The series connection is normally-closed relay position for capacitance in the 350 pf range. Bypassing the little cap gives it some giddy-up but I don't use mine much.

What makes this boost set-up a little different is that the mid resistance is not in series with the treble and bass pots. When the boost is on, the mid is unchanged and the bass is dimed. The switch/pot between the treble pot and the bass wiper denies the easiest path for the highs since they don't have to fight through the slope resistor. This is based on one of the older Dumbles which is much closer to the Princeton.

The in-between on this is that the bypass pot wiper is on a switch so the player can dial in how much boost he wants in advance and just stomp it for 12 bars in the spotlight. I've attached a more complete version below -- leaving out the relay power and LEDs.

I'll keep an eye on this thread to see what comes up,

Skip