Over 8 dB of gain possible in OD1 (Tube Dist mode), with filter adjustment.

[Kazinator]

It's crystal clear that the designers of the MP-1 were consciously neglecting to optimize for the OD1 circuit to hit the tube as hard as possible. That was not their goal. But don't have to live with all of their design choices.

I've already posted about the Zener mod: removing or replacing the Zener diode in the OD1 op-amp feedback, which clamps the maximum voltage swing.  With that mod, the maximum level rises by almost 3 dB.

After the OD1 op-amp, there is a bandpass filter (CRRC: cap, resistor, resistor, cap) which is activated in Tube Dist mode.  This bandpass filter, at its peak frequency, loses almost 10 dB of power.

The filter does not follow the rule of 10: it uses two identical R and C values: 2.2 kΩ and 33 nF. This means that the second filter stage seriously loads down the first, and we get a low Q (wide) filter which loses lots of decibels.

In the attached diagram, there are three simulations.

• The green curve A is the frequency response of the original filter.
  
• For the blue curve, B, we do nothing but implement the rule of 10: the circuit replaces the second stage of the filter by multiplying the resistance by 10 to 22 k, and reducing the 33 nF capacitor by 10 to 3.3 nF.  The center frequency stays the same, but the signal is significantly louder.
  However, the Q is affected: the filter is now much narrower with a Q of about 0.47 rather than the original 0.33, so there is a tone difference.
  
• The red curve C shows a change in the capacitor values: we increase the first stage capacitor to 82 nF and drop the second one to 1.3 nF: rougly by about the same amount. (Both valuews are away from 33 and 3.3 by the same number of steps around the E Series preferred numbers scale, in the opposite direction: no brainer!)
  

The red curve has almost exactly the same center frequency and Q value as the original, and is louder still. It is over 8 dB louder than the original circuit.


This extra level is not subject to any limiting circuit, other than the tube. You cannot obtain the same effect by increasing the amount of gain on the OD1 op-amp. Even if you remove the Zener diode, the most gain you can get before hitting the power rails is around 3dB.

With this mod and the Zener mod together, you can get around 11 dB more gain. That's 3.5 times the voltage!


The downside of this mode idea is that this actually makes the OD1 drive in all Tube Dist presets louder, rather than just opening more headroom for them. The user may want to peel back the OD1 trimpot to reduce the gain, so that they mostly take advantage of the headroom then (same gain, or only a little more gain, but without clipping out). In that situation then, all the clean presets become quieter and levels have to be rematched.

I suspect that to work this into a mod, we also have to increase the value of the resistor which deactivates the filter in Tube Clean mode, perhaps from 100 k to 500 k or something. I've not looked into it.

Everything here is an untried idea. The filter is being simulated in isolation, but in the real circuit, it combines with impedances around the tube stage.

[Kazinator]

They call him Kazinator

They call him Mr. Gain

They call him insane


I know what I want, and I want it now!

I want tubes

And I want them with more gain

[rnolan]

Hey Kaz, maybe you should get a 3TM, gain city LoL.

[Harley Hexxe]

Hey Kaz, maybe you should get a 3TM, gain city LoL.

  .and THEN some!

[Kazinator]

I don't actually need more gain, but with this change, I could have the same gain, without ever seeing the OD1 LED clip, even a little bit.

Curious what the tone would be like.

[rnolan]

Leon Todd gives the 3TM a good trial and also compares it to the MP-1 which he also uses.  Really suits some music, styles etc.  Personally I always found the MP-1 had plenty of gain (for me), now I use the MP-2 which has heaps more (if I wanted it).  These day's I'm dialling the gain down compared to my old levels.

[Kazinator]

Well, I did the mod!

Wow, this is really in uncharted waters. My mental "map" of the MP-1's gain levels is basically out the window.

I've reduced the potentiometer level in OD1, so that my clean presets now use a programmed value of 2.0 to 2.2 rather than 1.0 to 1.2, yet are about as loud.

Yet, I can get all the same dirty gain as before. Only now, the OD1 clip LED only lights up for a brief instant if I hit an unmuted open E power chord hard. It won't blink for anything else. So it is a more pure tube tone; no pre-tube silicon is going into clipping.

For my main dirty preset, I'm using OD1 OD2 of 6.0 4.0 rather than 10.0:4.0.

Getting the same distortion from less gain means less noise! I've noticed it's quieter if I crank the amp volume with the strings muted.

For fun, I tried seeing how heavy a tone I can get now from a 9.0 0.0 preset, by cranking the OD1 potentiometer. It turns out, a lot --- and it is usable! I was able to get into thrash metal palm mute and pinch harmonic territory. The clip LED was coming on easily, and the tone had a lot of high end, but it wasn't falling apart. Maybe all we really need is one tube? 

One thing is that the OD1 potentiometer changes the frequency response (more gain is darker) because of the small capacitor in the potentiometer circuit which bypasses it for high frequencies. Less gain in OD1 brightens up things slightly. It's not very noticeable on the dirts, but it helps the cleans a touch.

I was going to mention that this mod has very little effect on Tube Clean. In clean mode, the filter circuit is deactivated, and its volume drop does not occur. So in effect, the stock MP-1, for a given programmed OD1 value, hits the tube harder in clean mode than in distorted mode! According to simulation, this mod makes the bass end of Tube Clean have a flatter response. The stock circuit has a light bass cut. It starts rolling off somewhere below 500 Hz, and loses about a decibel at 100 Hz: down to -1.1 dB from a peak of -0.1 dB. The modified circuit is flatter. It has a cut that ranges from -0.2 to -0.6 dB over the 100 Hz to past 10 kHz. There is a slight rise in level in the 100 to 200 Hz range, then a more or less flat midrange, and another rise after 2 kHz or so. But it's all squished into a 0.4 dB range.

Also, the MP-1 Classic doesn't have this gain loss in dist mode issue. It has a similar pre-tube filter circuit, but designed with better values so that it doesn't introduce a volume drop at the center frequency. (That frequency is tuned higher: like 2.5 kHz, IIRC, compared to MP-1's 2.2). So we can think of this mod as bringing a certain benefit of the MP-1 Classic to the original.

[Kazinator]

O. M. G!

I had a chance to crank this up. Wowzers!

I always had this problem of my guitar skills tending to fall apart at volume. It was because of the tone. I would always wince from the tone which would make me hesitate or something. Or the wrong component of what I'm playing would come through the speakers. I can now shred the same way loud like at practice volume with this thing. It's because of the improved high end.

I tried another experiment. This time at practice volume, I cranked the Presence. In my redesigned EQ, what this will do is boost everything past 5 kHz. It's scary!

Previously, before this mod, that sounded like nails on a chalkboard.

Now, ... I can actually shred over it and it sounds fine, in terms of the delivery of recognizable notes. I gradually cranked it all the way to +12 and then kind of my mind wandered and I was actually starting to go over some musical ideas, composing a slightly new lick, forgetting that there is all this high end in there, my brain able to get used to it and block it out.

It's still nails on a chalkboard but different: like ... a baby's nails on a Madagascar ebony chalkboard. :haha You'd never unleash that sound on an audience, but the point is, it's playable. You can ignore the high end and play notes, hearing their tone standing up beneath the hail of high end. Previously impossible. When I tried it, I would take some 15 seconds of it and, OK, such a nasty thing is possible, but enough.