OD1 and OD2

[Moff40]

I have a bit of an operational question regarding OD1 and OD2.  The manual explains their function, but what do they really do, from a layman's perspective?    SS is easy: OD1 is input level, OD2 is compression, but for the tube sounds, do I think of them like Channel 1 and 2 on an old Marshall?  Or is OD1 an "input gain", and OD2 the saturation?  They both affect the amount of distortion, but how?  What does "higher frequency harmonic saturation" actually mean?

I've managed to dial in really good tube tones using OD1 as an input gain, set to 4.2 for every preset, whether tube clean or dist (not sure how I arrived at that setting, but it works), then adjusting OD2 for the desired amount of distortion, and sweetening to taste with EQ.  I've also found that "negative EQing" works best for me - for example, instead of pumping up the bass, just drop the mids (often to -4 or -6), treble, and presence, then set master to balance between presets.

[rnolan]

There's a bunch of posts which talk about OD1/2 settings.  IIRC one of our members experimented with OD2 set to 0.  So basically the 12AX7 tubes are twin triode, each triode is a gain stage.  So you have 4 stages of gain through the tubes.  OD1 controls the level into V1 and OD2 the V1 level into V2.  The different voices are created by some eq in the circuit between V1 and V2 (i.e. around OD2) to get/hone the distortion sounds/tone (I think it's a particular mid boost to give the "higher freq harmonic saturation").  Driving either of the tubes harder will increase distortion but more pronounced by driving V2 harder as the signal has already been boosted by V1 (OD1 level). 

With eq it's generally what sounds good to you.  But keep in mind the gain structure, the general rule with eq is you want it unity gain so it doesn't add or take away gain from the overall signal level, just changes its frequency make up.  So if you boost a bit here, then cut a bit there so overall the signal gain stays the same giving the best signal to noise ratio feeding into the next part of the circuit.  I think the master vol is before the eq.  So in your example, probably better to turn the bass up a bit and drop mids less to get the same effect rather than all cut, which is better gain structure.  That said, with the MP-1 eq, you can pretty much boost and cut to taste without it being a big problem.  Bad gain structure (or more noise from it not being optimal) is more a problem at really high gain settings.

[Dante]

I'm a layman, here's what I find

OD1 is Volume
OD2 is Gain
I know, they're both gains, hear me out

OD1 really high / OD2 really low = Clean
OD1 really low or mid-low / OD2 really high = Super Distortion
In between, is the magic

Personally, I think of OD1 to be a 'preamp' and the OD2 to be a 'saturation' (or, in Peavey terms; Pre and Post)

[Moff40]

Not being argumentative, just discussing here.  The reason I think of OD1 as an "input level" and OD2 as saturation is because I did some experimenting not long after I started using mine, and found with OD1 at zero, you got no sound at all, regardless of OD2's setting, and conversely, you could get a really strong but clean sound with OD1 turned way up and OD2 at zero, that gets dirtier as OD2 is increased.  For me, what works well is with both below 6.0 - as I said earlier, OD1 at 4.2 for a strong signal to manipulate, then OD2 at somewhere from 2.0 to 4.8.  My most distorted sound with OD2 around 5.0).

With respect to EQ, I found that, at least for tube dist sounds, pushing bass higher than 9 in my rig results in a real tubbiness in the ~180Hz range, where a setting of 9 or even 6 is tighter, and then backing off the mids to around -4 the leaves room for the frequencies below the bass centre frequency to speak.  It seemed counterintuitive to get more and fuller bass by dropping the mids, but it seems to work that way for me.  It also takes away some nasal mid honk.  Treble & presence need to be dropped to balance against that.

Now, all of that may be related to how I've been using the ODs, or it may be because I use solid state power amps.  It may also be subject to change once I get into a live situation where I'm driving speakers harder than I can at home or playing through an IR of my cabinets, but I was getting compliments the first gig I did with it (it still had the 3.666 at that time though).  Even so, even then I tried to match the general frequency response of my other preamps (a Kemper and an ElevenRack), that I know sound killer in a live situation.

[rnolan]

All the differences become much more noticeable when you turn it up to stage volumes (as there is a much wider dynamic and also the way our ears perceive different frequencies at different volumes (check out https://www.kmuw.org/musical-space/2015-09-15/loudness-and-the-fletcher-munson-curve).  Although the biggest difference I found was getting the master vol levels right for various patches, that has to be tweaked at stage volume, then you can turn the whole thing down for bedroom etc. 

Eq is very much what works for you and your setup, again that will change as you get louder.  I also like to dip the mids.  There's also a bass mod for MP-1 which lowers the bass eq centre freq, some found it a bit high for their needs.

[Moff40]

Oh, absolutely.  I discovered much of that the hard way 20-odd years ago when I tried to dial up an original Flextone in headphones and then take it to the gig.  There was much frenzied tweaking that night.  :)  But at that point, I had no other pres to A/B against.

My logic in this case was: I know my other pres sound good at stage volume, so it stands to reason that if I dial up the MP-1 to be in the same sonic ballpark when A/B-ing using the same cabinet or IR, it should be pretty close to where I want it to be when I crank it up live.  At the same volume, the Fletcher-Munson curve should, at least theoretically, apply equally to both preamps, no?

[rnolan]

(At the same volume, the Fletcher-Munson curve should, at least theoretically, apply equally to both preamps, no?) Absolutely     
Nothing worse than frenzied tweaking at the gig     I suspect we've all been there at one time or other. 

Your logic makes sense BTW.

[MarshallJMP]

Od1 is actually the gain of a booster before the tube section, OD2 is the gain between V1 and V2. The booster is a mid booster in the distortion voicing. For the clean tube the gain on V1 is lowered except for some high frequencies) and the boost is full bandwith.

[Matthews Guitars]

I'm new here and have a new to me MP-1.   It has a problem and I want to try to get it sorted out. 

OD1 works normally and adjusts normally. 

OD2 doesn't change the level of drive, distortion, or output at any setting from 0.0 to 10.0.

So I can't get that rich saturated lead tone that the MP-1 is most famous for.  And there's no other reason why I acquired it.

I've checked the tubes, I've swapped tubes, it makes no difference. 

I'm a good technician but I'll need to find the schematic or service manual as a starting point.   Where might I find that?

[Harley Hexxe]

Hey Matthews Guitar,

  Welcome to the Depot!

  You'll find all the schematics we have for the MP-1 at the vault.

   https://adadepot.com/index.php?topic=148.75

  I hope this helps

[MarshallJMP]

Can you take a high res pic of the left side of the mp-1 and post it here?

[rnolan]

Hey Mathews Guitars, welcome to the depot .  This probably isn't what's wrong but just in case; The MP-1 has 3 voice modes, Solid State (SS), Clean tube and Dist Tube.  In SS mode OD2 sets compression (not gain), which is only available on the SS voice.  In clean tube and dist tube OD2 should affect gain.

So as you have switched tubes, that's not the problem.

[Matthews Guitars]

I am aware of SS vs tube mode and it is definitely in tube mode.

I assume the request for a photo of the left side of the unit is for a photo of the internal section, with the left side being where the preamp tubes are, with the cover off?

Upon confirmation of that I'll make that photo and post it. 

I did find the manuals and schematics.  I need to spend a little time studying them.

[rnolan]

I assume the request for a photo of the left side of the unit is for a photo of the internal section, with the left side being where the preamp tubes are, with the cover off?

Yes

[MarshallJMP]

I assume the request for a photo of the left side of the unit is for a photo of the internal section, with the left side being where the preamp tubes are, with the cover off?

Upon confirmation of that I'll make that photo and post it. 

 

Yes it is.