# Turning a simple single channel amp into a simple two channel amp...



## screamingdaisy (Oct 14, 2008)

Actually, all I want is two gain settings... more and less. All the other controls can be shared.

The back story is that I have an amp that I really like the tone of. I have it setup for clean through rhythm on my guitar's volume knob. I'm fighting with the lead tone as I've tried various overdrive and fuzz boxes and none have me totally sold, and if I push the amp's gain up high enough for a great lead tone, I totally use the ability to clean up as much as I'd like.

Yesterday I was playing guitar, not quite rockin' out as I was somewhat frustrated with the results I was getting from my pedalboard when I did the old 'reach back and grab the amp's gain knob' thing for one of my solos. It sounded great with great cut and compression. Exactly what I had been trying to achieve but failing at getting with an OD pedal. However, I can see this trick being being a pain in the ass and extremely inconsistent to pull off live.

So.. I got to wondering. What if I installed a second (footswitchable) gain pot?

If I went this route I'd prefer to not have the blatantly messy looking pot sticking out from the chassis look, and I (rather conveniently) have a slave out pot and jack I don't think I'll ever use.

Thoughts?

Shot showing the slave out and jack;









Slave out and jack in top right corner, next to the speaker outs. The front pannel gain knob is second from the right;


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## Guest (Dec 7, 2008)

screamingdaisy said:


> Thoughts?


I don't know why they made any of the other Charlie Brown Christmas specials after the first one. Their all lame after the first one. And very pieced-together -- almost like a series of vignettes, and not one coherent story.

And I think you could re-purpose that jack and pot. Why not? Too bad you don't live near Wild Bill...


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## screamingdaisy (Oct 14, 2008)

iaresee said:


> I don't know why they made any of the other Charlie Brown Christmas specials after the first one. Their all lame after the first one. And very pieced-together -- almost like a series of vignettes, and not one coherent story.
> 
> And I think you could re-purpose that jack and pot. Why not? Too bad you don't live near Wild Bill...


I live near Ripper.


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## bcmatt (Aug 25, 2007)

I'm wondering if another option might be to use just a boost pedal or equalizer pedal in front of the amp. You could crank the gain knob and the have equalizer pedal set to quieten. Or maybe go the other way around. You may have already experimented with that and didn't like the result though...

Depending on the circuit used, the passive tonestack can be bypassed by pedal for a nice boost. Tweed era fenders allowed you to do that. If some of the guys checked out the schematic of your amp, they could tell you if it would work. 

You'll get better ideas than mine though; maybe the two separate gain knobs (your idea) and using relays to switch between them....


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## screamingdaisy (Oct 14, 2008)

I never thought of bypassing the tone stack.

That would provide an increase in volume (output), but no increase in gain, would it not? Also, my limited experience with bypassed tone stacks is that it produces a very bright/trebly response. It's an option, but I don't know if this is the way I want to go.


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## Wild Bill (May 3, 2006)

screamingdaisy said:


> I never thought of bypassing the tone stack.
> 
> That would provide an increase in volume (output), but no increase in gain, would it not? Also, my limited experience with bypassed tone stacks is that it produces a very bright/trebly response. It's an option, but I don't know if this is the way I want to go.


Have you talked to Ripper about a switch to disable the negative feedback loop? He'll know what I'm talking about. It's not normally on a footswitch but I don't see any reason why it couldn't be.

Even without a footswitch, most guys find that if they dial the guitar's vol knob to 8 or so things are pretty clean and then they can max it with their pinkie to 10 and get an amazing boost.

The nice thing about the mod is that in one switch position things are still totally stock. You lose nothing!

If you haven't considered it I really suggest you talk to him.

:food-smiley-004:


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## gtrguy (Jul 6, 2006)

The 'anti-boost' might do the trick...

gtrguy


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## screamingdaisy (Oct 14, 2008)

Wild Bill said:


> Have you talked to Ripper about a switch to disable the negative feedback loop? He'll know what I'm talking about. It's not normally on a footswitch but I don't see any reason why it couldn't be.
> 
> Even without a footswitch, most guys find that if they dial the guitar's vol knob to 8 or so things are pretty clean and then they can max it with their pinkie to 10 and get an amazing boost.


How does it work? Feel free to go reasonably nerdy (I work with, maintain and break radios and other electronics... I just don't go inside and fix them).

Also, won't disabling the negative feedback loop only affect the performance of the power amp? I'm looking for more front end gain...


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## Guest (Dec 7, 2008)

bcmatt said:


> I'm wondering if another option might be to use just a boost pedal or equalizer pedal in front of the amp.


I was about to suggest that but he specifically asked about the mod. I, personally, would try this route first before mucking with the amp. A good quality EQ+Boost can hit that front end with enough power to make it go from mean to scream. And my recommendation is, of course, going to be the empress effects ParaEQ+Boost. Ultra quiet operation. Powerful EQ. And a huge load of gain on tap. Run it last in the chain right before the front of the amp. Boosting the front end gives you pre-amp clipping, not unlike (but not exactly alike) the gain knob.


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## mhammer (Nov 30, 2007)

The standard Fender/Marshall-style tonestack can be bypassed by lifting the resistor to ground that is often replaced with a pot in amps that have a 3-way tonestack. 

In this example - http://www.ampwares.com/schematics/super_reverb_aa763_schem.pdf - a blackface Super Reverb, you can see that there is a 6800 ohm resistor to ground off the bass control on the Normal channel, and a 10k midrange pot in the same position on the Vibrato channel. The tonestack works by selectively bleeding off signal. That means that even without any sort of change in the gain, if you disable the tonestack and eliminate the passive loss, that second tube (in this instance) is hit with a hotter signal, which is functionally equivalent to having the preceding stage possess more gain.

This mod was suggested in an issue of Polyphony magazine some 25 years ago by Steve Morrison (now overseer of the Music Electronics Forum and former webmaster/overseer of AMPAGE). I did the mod to an old blackface Tremolux I had at the time, and am pleased to report it works great.

Of course, by lifting the ground connection, you not only eliminate the passive loss, but you also change the tone. The result is a sound that is not only hotter but "fuller" and meatier. To a very limited extent, the controls can still provide some tonal adjustment (the treble control works more than the bass), but nearly as much as before. In that sense, the tonestack bypass works like a "solo" switch, raising both level, and breakup-potential, as it also reshapes the tone to be more foreground.

It's not for everyone, or for every amp, but it works, and is dead easy to do. I used a jack normally reserved for an extra extension speaker. You could also use an extra input jack. Just make sure that the cable out to the footswitch (a simple SPST that grounds and ungrounds) is shielded or you'll pick up hum.

Alternatively, you can simple pick up one of those soon-to-be-released EHX pot in a box pedals that provides exactly that: an adjustable preset level. Set the amp for breakup at full guitar volume, and use the reduced level from the pedal to ease below that critical point.


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## screamingdaisy (Oct 14, 2008)

mhammer said:


> The standard Fender/Marshall-style tonestack can be bypassed by lifting the resistor to ground that is often replaced with a pot in amps that have a 3-way tonestack.
> 
> In this example - http://www.ampwares.com/schematics/super_reverb_aa763_schem.pdf - a blackface Super Reverb, you can see that there is a 6800 ohm resistor to ground off the bass control on the Normal channel, and a 10k midrange pot in the same position on the Vibrato channel. The tonestack works by selectively bleeding off signal. That means that even without any sort of change in the gain, if you disable the tonestack and eliminate the passive loss, that second tube (in this instance) is hit with a hotter signal, which is functionally equivalent to having the preceding stage possess more gain.


Hmm.. that's thought provoking. I've only experienced tone bypass with stomp boxes... usually fuzz, and it usually results in a pretty nasty ass fuzz tone (nasty in a good way, for a fuzz). So my view is definitely off. I do have a spare input however...

How easy is it to reverse this mod? The tone stack in this amp is an Orange style F.A.C. plus Baxandall tone stack with a presence and top boost. Unfortunately I don't have any blueprints.



> Of course, by lifting the ground connection, you not only eliminate the passive loss, but you also change the tone. The result is a sound that is not only hotter but "fuller" and meatier. To a very limited extent, the controls can still provide some tonal adjustment (the treble control works more than the bass), but nearly as much as before. In that sense, the tonestack bypass works like a "solo" switch, raising both level, and breakup-potential, as it also reshapes the tone to be more foreground.


Definetly got my interest...



> Alternatively, you can simple pick up one of those soon-to-be-released EHX pot in a box pedals that provides exactly that: an adjustable preset level. Set the amp for breakup at full guitar volume, and use the reduced level from the pedal to ease below that critical point.


Puts me right back in the position where if I add enough gain to get the lead tone I want I can't get the the amp to clean up as much as I'd like (plus it feels different).

Basically, I value my rhythm tone more than my lead tone. I'd rather have a ok lead and great rhythm tone than a great lead and ok rhythm tone.


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## screamingdaisy (Oct 14, 2008)

iaresee said:


> I was about to suggest that but he specifically asked about the mod. I, personally, would try this route first before mucking with the amp. A good quality EQ+Boost can hit that front end with enough power to make it go from mean to scream. And my recommendation is, of course, going to be the empress effects ParaEQ+Boost. Ultra quiet operation. Powerful EQ. And a huge load of gain on tap. Run it last in the chain right before the front of the amp. Boosting the front end gives you pre-amp clipping, not unlike (but not exactly alike) the gain knob.





screamingdaisy said:


> I'm fighting with the lead tone as I've tried various overdrive and fuzz boxes and none have me totally sold


I've tried various OD's and fuzz boxes. I've also tried clean boosts, treble boosts, not-so-clean boosts, compressors, wahs, wah with built in clean boost, mild fuzz, 



gtrguy said:


> The 'anti-boost' might do the trick...


yup, tried that one too, crazy fuzz, lots of fuzz, distortion pedals, stacking pedals, unstacking pedals, and a volume pedal.

None have me totally sold. I think the amp sounds better on it's own.


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## screamingdaisy (Oct 14, 2008)

I've decided that listening to the amp at home is putting the thing under the microscope, and that I'm not going to do anything until I've heard the amp in a live mix a few times.


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## Guest (Dec 9, 2008)

screamingdaisy said:


> I've decided that listening to the amp at home is putting the thing under the microscope, and that I'm not going to do anything until I've heard the amp in a live mix a few times.


A incredibly wise decision.


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## Greg Ellis (Oct 1, 2007)

Speaking of bypassing the tone controls...

I recently did this, sort of by accident, in one of my guitars.

I was changing pickups, and reducing the pot count from 3 to 2, and I was in a rush, and it's a huge pain taking a strat apart and putting it back together again. I ended up with the neck pickup running straight to the output without any tone control in that signal path.

It was a really great sound - very warm and rich, although I missed the ability to roll off a touch of treble now and then.

Now that I've worked on the guitar some more, and re-wired so both pickups are now running through a master tone circuit, I miss the sound. Even with the tone pot dialed up to max, it's not quite the same as bypassing that tone circuit entirely.

I'm thinking of maybe changing the pot to a push-pull type, so I can switch the tone circuit out of the signal path.

Sorry for the tangent, back to your regularly scheduled programming.


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## mhammer (Nov 30, 2007)

Stupidest thing Fender ever did was stick with a one-piece pickguard for the Strat instead of a 2-parter like the Tele, Jaguar, and many others.


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## Ripper (Jul 1, 2006)

Hey Screamin have you talked to Joel about what you're looking for? He might have something cooked up all ready that we could implement or if nothing else he might shoot you a copy of the schematics and we could go from there. If I were you though I would try and hear it out live a few times, you might be surprised at the results.


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