Peavey Audition 30 Modification

[Midwick]

I'm creating a "traveling" guitar with an amp in the case, and used a Peavey Audition 30 for the project. I'm working through a series of modifications to the amp head itself and find myself stumped with a particular issue (Which isn't hard - I'm an architect, so I have a pretty rudimentary understanding of electronics and can stumble my way through a schematic, but I'm definitely at the edge of my understanding with some of this)

The modification that is giving me difficulty is taking the existing "Thick" push button effect on the amp head and adding it to the foot switch. I retained the existing "loop" for the effect, but modified the switching so it functions as I want it to (described more below). I tested the circuit connections after I made the change, and they function as they should, connecting and disconnecting in line with the original schematic. When I plug in a guitar, everything works like it should EXCEPT when the pedal is plugged in and the "Thick" button is pushed, the thick effect works whether or not the foot switch is on or off.

There are two push button effects - Saturation and Thick. Saturation was already connected to the foot switch, but I modified it slightly from the original schematic to give each effect independent control. Because Thick has to pass through a specific intersection on the board before going to ground, I had to keep the two feeds separate to and from the foot switch (and swap out to a double pole switch). I rewired the existing push buttons and used shunted jacks so the effects can be toggled on and off independently, with the footswitch working (when it's plugged in) as a global "engage/disengage" switch for either or both when are turned on at the head.

The only thing I can figure is that pushing the feed through 8+/- more feet cable somehow has the same effect as grounding it back to the appropriate point on the board, but I don't know enough about signals to know if that makes any sense at all... Anyone have any ideas?

Geoff

[Enzo]

Before I wrap my head around it, did you remove the panel switch? And if not, you realize it must be in the OFF position for external switching to do anything?

The think simply switches C11 into parallel with C12. The 100k R15 is just there to keep C11 charged.

A shielded wire from either end of the thick switch runs out to your stomp switch. Neither side is grounded, only the shielding.

[Midwick]

I think I understand what you're getting at - I didn't remove the panel switch, but I rewired it so I can use it to control whether or not the signal even goes to the foot switch. I snipped the center leads (that connect directly to the junction between C11 and R15) and ran new leads off the top. Those go to a shunted jack - when the jack is empty, it closes back to the board at the junction between C11 and R15 - Functionally, when the button is "in" the circuit is closed and the effect is active, when the button is "out" the circuit is "open" and the effect is inactive. So far, so good - I'm opening and closing the connection (which tests with the multimeter) and the effect turns on and off when I run the guitar through it.

When I plug in the foot switch, it's essentially adding another switch to the circuit - if either or both are open, the points don't connect, and if both are closed, the points connect. When I test it with the multimeter, this holds true - the circuit is open in when the board button is out regardless of the foot switch, and toggles with the foot switch when the board button is closed. From a connectivity standpoint, I've just lengthened the junction and added another switch, but that doesn't play out when I test it with a guitar - once the board button is pushed in, the effect is active, whether or not the foot switch is engaged (and whether or not there is continuity between the switch and the junction between C11/R15) Sorry if I'm repeating myself - hopefully this is a little clearer than my first description - I don't think I was using 'ground' correctly.

I used a guitar cable to run the signal to and from the foot switch - the tip makes the connection between the push switch and the foot switch, the ring makes the connection between the foot switch and C11/R15. If I'm understanding correctly, there's no shielding in that scenario. (The saturation connection is just out and back, so I replicated that with the thick connection hoping it would work)

Does that make sense?

[Pappy B]

The SAT switch grounds or un-grounds the circuit. The THICK switch opens or closes the circuit.

If you have mounted a jack to the chassis (which is the typical way to do it) to plug the footswitch into you may be grounding out a circuit. If so isolate the jack from from the chassis.

[Enzo]

COnfused: a "guitar cord" a basic TS cord has no ring contact. It has tip and shield. A TRS cord has separate tip and ring connections, and shield on the sleeves.

Your thick switch is on the signal path, you NEED to have those lines leaving and returning to be shielded. The simple thing would be to use a TRS cord. tip and ring at the amp jack wired to your send and return, and a TRS jack on the footswitch. The sleeve grounded. That way neither tip nor ring would get grounded. And the sleeve insures grounded shield.

[Midwick]

I don't have the jacks in mounted to the chassis yet, so I don't think that's an issue... Yet. That was my plan, so I'll keep that in mind as I continue.

Sorry, yeah - I'm using a standard guitar cable, so tip and shield, which means I'm using the shield to make one of the connections. Shielding is one of those pieces of electronics that I don't fully understand, but I think I'm getting what you both are saying - With the Sat connection, because it's just going to ground, the TS cable can serve as both a grounded shield and a ground. With the Thick, once I engage the push button, it just becomes an 8 foot long unshielded wire off the head.

I'll get a TRS cable and jack(s), give it a try and report back...

Thanks for your help!

[Midwick]

Awesome! That was the issue - I got a stereo cable, gave the signal and return their own path with a separate grounded shield and it worked like a champ... (And yeah - I was using the wrong term - it was originally a guitar cable, so I was using the tip and shield, not tip and ring...)

I did a little poking around online based on you two's comments to get a better idea of how shielding functions (I was aware of it's existence, but didn't really understand how it functioned in a practical sense until now.) Anyway, I think I see where I went off track; the SAT effect works with a mono/guitar cable because the effect is actuated by a connection to ground, so while there is a signal, the grounded shield on the cable serves double duty. Essentially, I saw it as a simple connection to ground, but it was really a little more complex than that. I tried to replicate that with the THICK effect, but because it does not, in fact, go to ground, the signal was effectively unshielded, so once I sent the signal out to the pedal, the unshielded lead created a sound similar enough to the effect itself that I was chasing the wrong problem. Due to the interference, clicking the foot switch at that point made little to no difference at all...

Thank you both - I appreciate your taking the time to help me out!

[Midwick]

Next (and hopefully last) hurdle in this process - I'd like to add a reverb, tremolo and delay effect after the pre-amp but before the power amp. Essentially, I'm just adding a series effects loop and putting a particular combo pedal on it. I've been prowling forums and DIY pages, and so far, I've found three things:
1) A guy who just kinda took a cheap amp and did it. Nothing fancy - he literally just broke the circuit on the board and added a couple jacks.
2) A whole bunch of threads where someone would ask and people responded as though adding an effects loop is some sort of wizardry to be avoided at all costs, and instead recommended various workarounds.
3) One thread that was a little tough to follow because it was from 2010 and the images have long been swallowed by the internet, but they did figure it out. (And this was for a parallel loop)

To that end, I'm looking at a simple series loop similar to what the first dude did. My understanding is that I can literally just grab the signal and return it between the post gain and the power amp with no additional modifications. In his experiment, he actually used the leg of the power amp chip. I did that in my initial attempt, but ended up breaking the leg off and having to replace the chip... This time, I'm keeping the chip where it was and instead removing the legs of C21, R26 and R25. And I'll be using shielded cable...

Am I missing something? I understand there's always some risk involved modifying an amp, but the first dude's experience makes it sound like it's actually pretty straightforward if you're not looking to do anything fancy, which is at odds with the bulk of advice I found in various forum searches.

[Enzo]

Adding an FX loop is not generic. On some amps it IS as simple as breaking into the signal path and adding jacks. On others it is not. Especially in tube amps where there could be 30-40 volts of signal at that point.

Imagine you need to talk to people coming to this country from overseas. Is it easy to talk to them? Well, if they come from the UK, sure, speak English. If they speak Polish, then not so much. So don't add up all the FX loop discussions you find and hope to get a meaningful average.


WHy do you want to remove all those parts? COnsider C20 as the input to the power amp and keep it and all to the right of it. break into your circuit between C20 and the volume control wiper.

C20 is out in the middle with room around it, I'd do it right there.

[Midwick]

That makes sense - I mean, I figured a tube amp would be in a different category than solid state, but I was thinking more about general safety vs electrical load at a given point. That would definitely mess with things.

I like C20 a lot better - I'll give it a go.
Thanks again!