Bad ears? Pointless mod? or unsupportive rig? A BMP tale

[Mark Hammer]

A buddy was generous enough to give me a NYC Reissue BMP that a customer had left off a half dozen years ago and never claimed.  I thought I would use the presence of all that chassis space to play with some mods.  I installed a sort of mid-lift mod, with a toggle that would raise the rolloff on the bass side of the tone circuit from 720hz to 1500hz, and that yielded some nice sonic options.

Then I thought I would finally do what I'd recommended to several folks to do and install a diode-lift switch in the first clipping stage, to mimic the Colorsound Supa Tonebender.  So confident was I that it would make an audible difference, that I used an on-off-on toggle to select between the Schottky-type it comes with, a silicon pair, and no diodes.

And I cannot for the life of me hear ANY difference at all.  I'm pretty sure I've installed it right, and that the switch and wiring all do whatthey are supposed to.  The forward voltages all read exactly what they should in the different switch positions, and I can't see anything shorting out anywhere.  Were it not the case that EHX took the Supa circuit and added the diodes (a move contrary to what you'd expect 70's era MIke Matthews to do), I would have shrugged it off and thought "Meh, I guess the diodes don't do anything".  BUt why would anyone include additional parts when they don't seem to make any tonal difference whatsoever?

So, my query to fellow members:  if you have ever tinkered with a BMP and removed the diodes in that first clipping stage, DID YOU HEAR A DIFFERENCE?

[GibsonGM]

Might be worth probing around with a scope, Mark, with your unit and a stock one, huh?  There HAS to be a reason, like you said...

[kingsac]

is it possible that you are tinkering with the 2nd stage? in my experience, diode lift doesnt change sound much (if at all) in 2nd stage, but Ive gotten big changes when done in the 1st stage.

[antonis]

if you have ever tinkered with a BMP and removed the diodes in that first clipping stage, DID YOU HEAR A DIFFERENCE?

partial answer: I've never heard any difference between different diode pairs (Si, Ge, LED) even between symmetrical and asymmetrical clipping BUT I've heard some observable difference with diodes removed...
(more "clear" distortion - according to my taste...)

[Mark Hammer]

Both kingsac and Antonia have some good ideas.  I *think* I've done the mod to stage 1, but it is worth confirming to put my mind at ease.  I'll pay closer attention to the diodes-vs-no diodes comparison.

[Mark Hammer]

So, following up on Antonis advice, I paid special attention to the no-diodes/diodes contrast, and managed to hear a difference.  But boy was it subtle.  A little more evident with humbucker than with single coils, but still subtle.  What was interesting was that if I had the toggle set to diodes, slammed a string, and then quickly switched to no diodes, I couldn't hear any difference.  But if I had it set to no diodes, slammed a string and then quickly switched to diodes, I could hear a slight increase in sizzle and a slight drop in level.

What that tells me is that:
a) The second clipping stage is likely doing the brunt of the dirty work, and the first stage simply does some tone-shaping ahead of what the second stage does.
b) The difference between diodes and no diodes in the first stage is really all about how it responds to the initial attack transient.  That's why switching from diodes to no diodes after the first 250msec made no difference (the attack transient was already gone) but quickly going from no diodes to diodes DID produce an audible (the initial peak was gone but there was still enough signal left to yield additional clipping for a little while).

It also tells me that there was at least some rationale for EHX maintaining the use of two cascaded clipping stages, rather than going the Supa route.  On the other hand, different issues have had slightly different characters, so maybe the diodes vs no-diodes contrast was greater in earlier issues.

Interesting tidbit: if you DO plan on installing a Supa/Muff switch to lift the diodes in stage 1, the first time you connect the diodes to the cap - successfully bridging the path from collector to base - it will take a moment for that cap to charge.

Thanks for the idea Antonis.  Always good to verify that one is neither incompetent or crazy.  

[antonis]

Thanks for the idea Antonis.  Always good to verify that one is neither incompetent or crazy.  

The pleasure is all mine, Mark...

(but now you drive me crazy about how did you manage to strum a string and tongle the switch in a 250mS time interval...!!!)

[Mark Hammer]

I'm guesstimating the time  but it's not that hard.  Lean the guitar against something.  Get one hand ready at the toggle, and use the other to pick an E or A string hard.  As soon as you pick, flick the switch.  Reaction time generally isn't all THAT fast in someone my age, but that presumes a decision being required prior to reacting.  If the decision is already made (i.e., flick it UP), reaction time can be pretty quick.  FWIW, this has been extensively studied in older typists.  It's also why Les Paul could still play like a speed demon well into his late 80's: he knew what he was going to pick.

[antonis]

My bad....

I thought that you firstly strummed the string and then you tongled the switch - by the same hand...

[duck_arse]

.....  you tongled the switch ......

can we have this added to the lexicon of pedal jargon? like when E-B gave us "very neck".

[Ripdivot]

Hi Mark, if you change the diodes in the second clipping stage from schottky (stock in the NYC) to standard silicon you may notice more of a difference with your switching in the first stage. I have modded a few of these NYC reissue pedals and the first thing I do is lose the Schottky diodes. I find they compress the muff circuit too much.

[Mark Hammer]

I may try that.  Another thing I could try is using a bigger emitter-resistor value than 390R to reduce the gain.  That may actually have a bigger impact on the 1st clipping stage than swapping diodes or lifting them.

[Les Paul Lover]

Not sure if that's going to be helpuf but I built a clone BMP (ram' head as a base), and put an on off on switch for the 2nd stage clipping, with 1N34A Ge( 3 of them so asymmetrical), no diode, and 2 standard 1N4148 Si the other side.

The differences in tone between the 2 diode types isn't drastic, but audible enough, and I prefer the Ge. Without the 2nd stage clipping diodes, the sounded is a bit "rougher" and clearer, less compressed. as striking difference is the varying volume. No diodes at all is soooo much louder than Si, and Si is much louder than Ge.