Running Both Channels On Fender/Marshall Amps (switchable)?

[guitback1]

Howdy,

Many are familiar with the practice of jumping both inputs with a short, second signal cable. Of course you cannot do that with most Blackface amps because there is phase cancelling. However, many tweed, blonde and brown amps allow one to do so, as do some Marshalls (which had their genesis in the tweed bassman circuit). Since I was an idiot and got rid of an old 6G3 Deluxe about 19 years ago, I searched for another one, but wasn't thrilled with how the prices had escalated. Enter Ampeg! I found an Ampeg J20. They are very similar in their circuit to the 6G3. The one I found has most of the typical mods done to correct the misgivings of both the 6G3 and J20 circuits (standby re-configured, bias made adjustable, tweaked bass response, etc.). Regarding the "etc." this amp also has a mod that gangs the bright & normal channels without the need for a patch cable. This is done with a miniature 3-way switch that allows a choice of Bright, Normal or BOTH inputs. There is of course a noticeable boost in volume/gain when I flick from either the Bright or Normal to "BOTH." I'm wondering if it is possible to make the action foot-switchable? What I'd really like to be able to do is to go from either the Bright or Normal channel to BOTH channels (mainly for solos) via a foot-switch. Is this possible to do without having to utilize a relay or some hyper-complex wiring? I'm guessing if it is possible, then it'd be possible on Fenders AND Marshalls. The question is, can it be done relatively easy via a foot-switch?

P.S. I already have a 2-switch pedal which I use to activate the vibrato. The second side of the footswitch is unused.

[The Dude]

I'd assume you could wire the existing switch in the amp to a footswitch. Hard to say for sure without a schematic. If you can upload one, you might get better responses.

[Brad1]

Like this?

www.thetubestore.com/lib/thetubestore/schematics/Ampeg/Ampeg-J20-Schematic.pdf

[pdf64]

The simplest feasible way to implement a footswitchable boost may be to leave the inputs alone but put the 0V return of C7 (2nd stage cathode bypass) on to the spare footswitch line.
That would give about the same degree of gain change as you are getting from linking the channel inputs.
I think you may also need to fit a stereo jack socket for the footswitch, as the existing jack looks to be mono ie it's single channel whereas you need 2 channels, one for each footswitch.
If the above still sounds hyper-complex to you, then as suggested on TGP, an external ABY box (and necessarily reversing the Ampeg's input switch mod) seems the obvious way forward; that would also have the advantage of being able to be used with other amps.

[guitback]

I was hoping the on/off nature of a simple footswitch could be incorporated in place of the on/on/on switch currently in there. The on/on/on switch feeds into the Volume pot. I was thinking that the footswitch might be wired to have one of the channels as default and the other one shunted to ground when switch is not depressed. Thus allowing both channels to be on when the switch is activated?

[pdf64]

Your idea is not feasible without implementing a relay or electronic switching system.
Is it even realistic to replace the footswitch jack with a stereo type? If it's PCB mounted then even that may be 'hyper-complex'.
And without a stereo socket you can't access the other footswitch circuit.

[Chuck H]

I seems like it should be do-able with an A/B/Y pedal.?.

[guitback]

Because the amp is hand-wired with no pcb, the foot-switch jack can definitely be replaced with a stereo version. Sadly, although I have built 7 or 8 amplifiers, I am not well versed in many areas of electronic construction. As an example, I don't really know how the vibrato foot-switch turns the effect on and off. I was assuming it somehow grounds out the effect until the switch is activated, thus bringing effect back into the system. I was hoping maybe the second side of the 2-element foot-switch could be used in this manner. Alas, although I recognize the components and their wiring in a schematic, the "translation" of such things into an actual wiring diagram is a bit hazy for me.

And regarding your suggestion Chuck H, I was definitely able to do this by utilizing an A/B/Y pedal back when the amp still had both input jacks. It does work well. I'm just trying to figure a way to keep the current iteration intact (only one input jack, and the A/B/Y switch basically being an On/On/On mini-switch situated where the 2nd input jack used to be).

[olddawg]

My question is why? Back in the day I jumpered the inputs of a lot of JMP Marshalls and even my blonde Tremolux. At that time good boost pedals weren't generally available so we were all trying to get the most gain out of our preamps (without modding) and finding the hottest pickups to slam them with. Turning the volume control on the guitar down a smidge essentially did the same thing as not jumping the inputs. Later I cascaded the input channels on a couple of amps. I can see maybe footswitching that. But I found later that simply jumpering the amp, using the volume control on the guitar, and having a Rat and/or a TS pedal did the trick even better. That's the old school way countless pros did it

[guitback]

It's an interesting question. I have absolutely been "old school" in the sense of riding my guitar volume knob for solo/rhythm for years. I've also done other things to push the amp: simple boosts (EQ and other); TS, OCD, Rat, etc. I even at one time had a simple switchable pot mounted in a stompbox with a volume control (duplicate of the guitar's volume control, but switchable on the floor). That allowed me to cut volume for rhythms without having to ride the guitar's volume pot by hand. I love the ampeg J20 because it sounds a good deal like the old Brown Deluxe that I foolishly got rid of. The Bright channel alone lets me do rhythm volume with a bright (but not obnoxious) cut to it. For solos, running both inputs adds the perfect push to elevate the amp in the mix live. It also "sweetens" the sound up harmonic-wise. The A/B/Y box certainly works. I simply was looking for a way to eliminate the need for two cords running from the A/B/Y to the amp, but still be able to jump from the Bright channel to both channels for solos.

[Chuck H]

Since the A/B/Y pedal works on the low level guitar signal there isn't much an impedance or noise issue. To do the same thing at the amp requires a relay and a pedal to power the relay. The reason is that once you're in the amp and past the input jack it's harder to manage what makes for a floating lead connected to the first stage tube grid. Basically, it doesn't make sense to run your guitar to the amp and then run it back out of the amp to a pedal and then back to the amp. Lots and lots of lead wire attached to your first stage grid that needs to be managed for grounding, loading, shielding and capacitance. Why? When you can do it on the way to the amp. So what if there's a couple of cables from the pedal to the amp.?. There's still only one from your guitar to the pedal. And the A/B/Y manages grounding so you don't have to. It's actually the shortest path with the lowest consequences. And it's actually easier to change back to two inputs (you can still have an A/B/Y switch on the amp) than it would be to design a foot switch system (that works as well) to replace it.

JM2C

[olddawg]

What I have found works best for me using small(ish) amps in a club situation is a footswitch operated speaker attenuator. I use a Weber Mini mass. It's always on except when I need to get on top of the mix.