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Driving both inputs - Mesa 400 Plus

Foz

Foz

Jul 26, 2008
2,585
1,164
Jax FL USA
throatdiscs.com
#1
Several folks describe getting beneficial tonal variations by driving both inputs on the 400+ and I gather from comments this is typically done with a Y cable or ABY box of some sort.

My question is how might one hard-wire the amp for this "feature".

I would appreciate any input from someone with E-Tech skills [or Ninja skills for that matter] who might take a look at the attached schematic and comment.

Can one achieve the ability to drive both inputs by simply adding a jumper wire from the "tip" of Input 1 to the tip of Input 2?

Will this cause both sides of the V1 to be energized and folded into the gain structure - adjustable by manipulating one of both of the Input Gain Knobs? Any Comments on how to optimize this mod?

Can I safely add a switch to the jumper to turn this feature on and off at will - even whilst playing?

Thanks in advance.

+1 kudos if you blow up a couple of amps exploring the question and post pics! :hyper:

PS: more legible copies of the schematics here

Invalid Link Removed
 
#2
It seems the simplest solution is a cheap Y-connector. Several advantages:

First, it appears you haven't actually heard what others have described. Hook up a Y-connector and decide whether it makes any difference for you.

A y-connector doesn't modify the amp in any way. It's instantly removable if you later decide the tone difference isn't worth it. No soldering inside the amp. No defeat-switch installation.

If you later wish to sell the amp, you haven't modified it at all.

That said, I don't see how two identical circuits in parallel will make any difference except perhaps to have a bit more gain. True, you can run one channel "bright" and the other normal, but I doubt this will make a noticeable difference.
 
#3
Just get the Y cable. It will cost you under 10 dollars and is a workable long term solution. I used it for nearly two years that way without problems, although I did carry a second Y around since it was so small and easy to do.

And yes there is a very discernible difference in tone when running both channels.
 
#4
Thanks for the reply. I understand the obvious means of testing the mod with a Y cable, and I won't argue the disadvantages of adding "Y" cables or extraneous stomp boxes permanently to the rig... these are also obvious.

I take your point - and have no problem with the idea that you would not pursue this mod - but that aint what I asked. Nor am I looking for input on whether solder is involved or how mods might effect selling an amp. I respect you views, but I don't share them, or need help with any of that.

Back to the topic - the circuits aren't identical. Attached is a blow-up of the relevant part of the schematic... note the asymmetry. This in part is why i am asking for some informed input on how one might best wire up to use the other half of the V1 as it doesn't look to be a straight forward matter of adding a precisely equivalent parallel gain stage.
 
#5
The schematic is, I think, a little bit wrong in that it does not show the grounding contact that silences the inputs (connects to tip contact with no plug inserted).

Simplest way of doing what you ask it to remove the ground from that contact from CH2 and wire it to the tip contact of CH1. That way if you plug into CH1 you get both channels working. If you plug into CH2 as well, the two channels will be independent of each other. With nothing plugged in, CH 2 is silenced by CH1.

Paul
 
#6
That said, I don't see how two identical circuits in parallel will make any difference except perhaps to have a bit more gain. True, you can run one channel "bright" and the other normal, but I doubt this will make a noticeable difference.
Click to expand...

in a nutshell, more available gain *is* what you get. however, it is a little more complex than that, believe it or not. first, with the extra gain stage, there is (duh) more available gain--- but the biggest benefit of this IMO is that you can keep the preamps cleaner while still supplying plenty of signal to the eq driver stage. this actually allows the amp to play somewhat cleaner and louder and with less noise. you also gain some "tone"--- the two channels are *not* identical, even if V1 happens to be perfectly matched (which it probably isnt). IME with this amp, that manifests itself by producing richer harmonic content and a fatter tone with more character than with just one channel driven, regardless of the amount of drive. doing this, coupled with the 12AU7 in V4 completely changes the gain structure in the amp, and is very worthwhile to do, *particularly* if cleaner tones are your game. YMMV and all of that.
 
#8
...it is a little more complex than that, believe it or not.
Click to expand...

I believe!

...even if V1 happens to be perfectly matched (which it probably isnt).
Click to expand...

I reckon it aint even it is... V1a and V1b are different circuits so I'm thinking a perfectly balanced tube still encounters asymmetry when you try to run both sides together.

coupled with the 12AU7 in V1 completely changes the gain structure in the amp
Click to expand...

I thought the 12au7 went in the V4? I better go read that one again.
 
#10
So hold on a second...say you have a bass with two outputs. Can you plug each output into each input? That won't hurt the amp at all?

I've got a couple basses that I've tinkered with so that each pickup has it's own output jack completely independent from the other. I've run across some guitar amps with multiple inputs where you can get more gain by plugging into both inputs. I've been wondering about this on the 400+, but frankly, that ****'s expensive. I don't want to mess my amp up. ;)
 
#12
Played my first gig with my "new" 400+ last night. Tried the dual input thing after i got set up. I had WAY too much overdrive going. I then plugged straight to channel one, sounded incredible. Then tried channel two, big time distortion even at low settings. :eyebrow: I'm wondering if channel two is going to need a little work in the near future.

One thing to note, both my basses have a pretty high output, Modulus Q5 and Ritter Classic 5. Could the fact that they're both active be the culprit in channel to? Thoughts and suggestions are appreciated.

BTW, did the gig with just channel one and it sounded incredible. Fat, thick, punchy. Forced a couple of overly loud guitarists to lock with the rythym section once again,...as it should be! ;)
 
#16
strke-fender said:
similar question to the dual output bass question here.

if i take my EXH bass big muff and run it into channel two and dry out into channel one would i get a blended tone?

would it just be all kinds of fuzzy?
Click to expand...

I tried that with my BDDI a couple days ago and to be honest, I didn't really dig the results. I could sorta control the fuzz by cutting or boosting bass or treble on the BDDI, but I think it sounds fine just using channel 1 or 2 seperately. Not to mention I really can't stand the distortion on the BDDI, sounds like...yuckiness in audio form to me :(
 
#19
I've done it with an ABY switch. For some reason I thought it would be really cool. I don't think it effected my tone much, maybe made it a hair thicker but probably not in a way that made it through the mix. I ran through a few 9V batteries powering my ABY and haven't touched the thing in years.

I currently run into Channel 2 (the higher gain one for passive basses) with my active (18v) bass, at around 3 1/2 for the brighter aggressive tone that I like. Channel 1 is way more mild (and I run it at about 6-7 to get loud volume without OD).

A more substantial improvement in my tone was obtained by running my outboard pre through the FX loop.
 
#20
voodoobassist said:
Played my first gig with my "new" 400+ last night. Tried the dual input thing after i got set up. I had WAY too much overdrive going. I then plugged straight to channel one, sounded incredible. Then tried channel two, big time distortion even at low settings. :eyebrow: I'm wondering if channel two is going to need a little work in the near future.

One thing to note, both my basses have a pretty high output, Modulus Q5 and Ritter Classic 5. Could the fact that they're both active be the culprit in channel to? Thoughts and suggestions are appreciated.

BTW, did the gig with just channel one and it sounded incredible. Fat, thick, punchy. Forced a couple of overly loud guitarists to lock with the rythym section once again,...as it should be! ;)
Click to expand...

I'm not sure if the 400+ is set up the same as the 400, but according to the Mesa user manual, Channel #1 on the 400 is meant to handle active electronics while Channel #2 is for passive. Not that you can't plug an active bass into Channel #2, but you will overdrive that Channel more dramatically in doing so because of the hot signal from your bass. Check your manual?
 

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