Mod V-Twin to use both tubes?

[sumitagarwal]

The V-Twin is a great preamp that I think sounds awesome, but I feel like it could easily have been even better.

It's well-known that the pedal uses only one tube at any given time, with the other tube reserved for the other channel (try it: pull out a tube and one channel will still work; put the tube back in and pull out the other one and the other channel will still work).

On the high-gain channel diodes are used for gain/clipping. It sounds good, but does anyone know why Mesa didn't simply use the two gain stages available in the other tube?

Could the V-Twin be modded to cascade the four tube stages?

Also, does anyone know if the tone stack on the V-Twin is identical to a Rectifier preamp tone stack?

Thanks!

 

[94Tremoverb]

"Could the V-Twin be modded to cascade the four tube stages?"

No. It would probably be easier to design and build a new unit from scratch. I've tried modding mine in a couple of simple ways and even that was very tricky, it's just not an easy thing to work on. The density of the PCB layout does not lend itself to extensive mods.

"Also, does anyone know if the tone stack on the V-Twin is identical to a Rectifier preamp tone stack?"

No, not quite - it's very close to the Orange channel but the slope resistor is different (33K vs. 47K). Both the slope resistor and the treble cap (500pF vs. 680pF) are different from the Red channel. In fact, it's actually the same as the classic *Marshall* tone stack.

"On the high-gain channel diodes are used for gain/clipping. It sounds good, but does anyone know why Mesa didn't simply use the two gain stages available in the other tube?"

Actually - and this may come as a nasty surprise - the V-Twin is suspiciously similar to a Marshall JMP-1 but without the programmable control. The basic audio path circuit is alike to the point that I think it's unlikely that one is not loosely copied from the other - they both came out around the same time, but given the tone stack values I have an idea which way round it is... and this may also explain the use of the clipping diodes, since they were a standard part of Marshall design at the time. I can't prove this but I'm sure that anyone else who knows how to read a schematic would reach the same conclusion.

Mesa did this before too when they based the first part of the Rectifier preamp on the Soldano SLO...

 

[sumitagarwal]

Very cool and knowledgeable feedback! I sort of suspected the diode thing was related to the popularity of the JCM 900 and maybe the JMP-1 (though I hadn't done the research to be certain the JMP used them).

So is the V-Twin really more Marshall in character than Mesa? Is the perceived Mesa quality from the fact that some people run them into Mesa poweramps and cabinets?

Would changing those resister values basically make the overall character more like the Red channel?

Aside from Mesa's own stratospherically-priced pre's, are there other pre's better suited to Recto tone? Soldano SP-77?

Thanks so much for your insight and knowledge!

P.S. It's not surprising that Mesa pulls designs for sections from other manufacturers considering the company's origins...

 

[94Tremoverb]

I certainly think it sounds a bit 'Marshally', although the tone stack is not by any means the only factor. I still think it sounds like a Mesa rather than a Marshall, overall.

Changing the tone stack values will revoice it a little bit further in the Mesa direction - I haven't tried it, but those components do affect where the frequency centers are, and the Rectifier's 47K slope resistor is certainly a part of why it's deeper-sounding than a Marshall... a Marshall Lead model anyway, the Bass and PA models do actually use a 56K resistor here. (As well as a 250pF treble cap, so less top end.)

I've never tried a Soldano preamp, nor even the Supercharger pedal. A friend of mine has an SLO though, and I would say it's somewhere in between the Marshall and Mesa voicing, maybe a little closer to the Marshall. I don't know of anything comparable that sounds *more* like a Rectifier than the V-Twin - which is why I use one, for when I'm not using my own amp.

 

[sumitagarwal]

Awesome feedback. Would you recommend changing the slope resistor? Would this be particularly difficult?

Would it really be that hard to send the high-gain stage through the other tube instead of the diode?

I'm not worried about resale value or anything like that. I got the V-Twin used in pretty poor cosmetic condition for a flat $300 including delivery.

 

[94Tremoverb]

It should be easy to change the resistor, if you can find it on the board - or at least as easy as modding anything on this pedal is, ie not very! - I don't know where it is, sorry. Mine is bolted to my pedalboard at the moment so opening it up is not something I really want to do .

I suppose in theory you could run one tube into the other, but first it's going to be an immense amount of work to get the switching to still operate correctly even if you can break the circuit in the right places without causing other problems, and secondly it may not actually sound at all good unless you tweak a whole lot of other stuff as well - you're really getting into R&D'ing a completely new design, and a finished PCB is not the right place to do this! I wouldn't get too hung up on the tube vs. diode thing - it is what it is, and actually multiple tube stages have problems too that diodes don't... like adding noise.

If you're interested, what I've done to mine so far is to move the Blues To Solo switch and the clean gain control to chassis-mounted parts under the tube cover at each end - there's just enough space to fit a mini-pot and a little switch - because with it bolted to the board you can't get at the stupid little things underneath! This was fairly easy in electrical terms but needed some very careful measurement and drilling. And, I converted the Remote Switching jacks (which I never used) to be an FX loop, after the preamp section and before the speaker emulator circuit, so I could run delays etc there where they need to be. But this was not successful because for some reason I can't work out, it adds a lot of extra noise - only when the loop is in use, luckily, and I've never bothered to put it back. So be careful what you do, and that it's reversible if it doesn't work out...

 

[sumitagarwal]

Hrm, I think I'll pop the lid off to get a good look at it and see if it would be easy to change the resistor. I'll drop the cascading tube gain idea... it really would be reengineering the whole thing (considering that Mesa bills it as 'sired by the Recto' why didn't they just do this to begin with?). And you're right, the low-noise performance is very nice... I run Duncan Pearly Gates which are very traditional PAF's that can be somewhat noisy, and which require pretty high gain to really chug.

Being that I only run the V-Twin either straight into a power section or straight into a computer, I have absolutely no use for the bypass footswitch. Think it would be hard to turn the bypass switch into a clean/blues switch so I can get at all 3 channels using my feet?

 

[94Tremoverb]

Should be fairly easy - remove the old switch and connect the wires together (to make it power up in non-bypass mode, if yours powers up in bypass - mine does), then fit a DPDT switch instead, remove the little push-switch from the board and connect the pads to the new switch... which is the tricky bit, you'll need a good desoldering tool and a lot of care to get the switch off cleanly.

Or if you want to keep all your options open, fit a miniature momentary switch in the hole where the Clean/Blues switch was and use that to operate the bypass...

 

[sumitagarwal]

Well crapola... turns out I can't do any of that. It seems the idiot owner before me stripped the threads on the right footswitch so I can't unscrew the nut from that.... and without that, I can't lift off the upper casing.

Now I want to do the mod AND check to see what else he or she did inside there before I got it!

 

[94Tremoverb]

Dremel the nut off - cut two slots into it and break it in two. If you don't have a Dremel you can probably do it by splitting the nut with a fairly sharp screwdriver and a hammer - it's brass, so it's softer than a steel screwdriver - crude, but if you're careful and determined you should be able to do it. Aim sideways against the switch barrel, not down onto the pedal. Use many light impacts not one big one!