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I did it! 8.2V Zener -> 12V Zener swap in the OD1 drive circuit.

Started by Kazinator, September 18, 2024, 04:12:02 PM

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Kazinator

It is fantastic! A lot of that MP-1 "hair" is gone. It still has all the good character I love about the MP-1 tone, but a purer version of it. The "OD1-cranked" patches are now more smooth: they are kind of like "OD2-cranked" patches, but with the OD1 high.

I absolutely love the increased headroom in my SS clean presets, too: the other reason for doing this.

I can highly recommend this 12V Zener mod! Wow!!!

Kazinator

For shits and giggles, I'm trying OD1 = 9.5 + OD2 = 8.0.

It's.

Actually.

Usable.

It has syrupy sustain that goes forever, yet with good note separation on chords. Palm muted chugs are flubby, but clean up a bit if you pick softly. Notes high up the neck articulate superbly.

You know, I'd love to A/B this side by side with the 3 Tube Mod, through all else being the same.

rnolan

Cool Kazinator.  I spose you'll have to buy a 3TM (or find someone with one).
If you can play Leon's reviews through decent speakers you might get a decent comparison?




Leon Todd 3TM pt1
Leon Todd 3TM pt2
Studio Rig: Stuff; Live Rig: More Stuff; Guitars: A few

Harley Hexxe

Hey Kazinator,

 So, you are saying you're getting better clarity by swapping diodes?

  If so, I'm wondering if it would do the same thing to a 3TM.
I only have two brain cells left, ...and I'm saving them for the weekend!

rnolan

Would be good to hear MJMPs opinion on this, my understanding is the 3TM mod changes a bunch of bits in the tube circuit/board, well it get a whole new tube board.
Studio Rig: Stuff; Live Rig: More Stuff; Guitars: A few

iamkrash

Can you post exactly what you replaced?
I'm handy with an soldering iron...

Harley Hexxe

Quote from: rnolan on September 20, 2024, 02:39:04 AMWould be good to hear MJMPs opinion on this, my understanding is the 3TM mod changes a bunch of bits in the tube circuit/board, well it get a whole new tube board.

Isn't the 3TM circuit board posted somewhere in the vault already?
I only have two brain cells left, ...and I'm saving them for the weekend!

Kazinator

Quote from: iamkrash on October 11, 2024, 04:12:44 PMCan you post exactly what you replaced?
I'm handy with an soldering iron...

This thread is about the Zener diode in the OD1 drive circuit which is the basis for it limiting its output swing. This limiting is, in my opinion, pointless, and a big reason for why some MP-1 users look for ways of getting more gain. It limits how hard the drive circuit can hit the V1A tube stage, and when the soft clipping limiting kicks in, it sounds flubby, because there is no bass roll off before it (unlike in a Tube Screamer type pedal).

I wrote about replacing the diode with a higher value, like 12V, which I have done and which works. But I think that a simpler thing would be to just remove the Zener. That will just eliminate the limiting, so that it's possible to just hit the op-amp's limits.

Now as to which exact part that is D45 (hard to read). Circled red in the schematic excerpt posted below.

For where that is located, see the second image. D45 is the middle one of the circled diode triplet.

If you remove it, it will open the limiter circuit (bridge rectifier) rendering it ineffective. I used a 12V zener, so that the swing can go to about +/- 12.8V, which is close to the datasheet-given limit for the op-amp IC I have for OD1 (NE5532).

This mod works well with my original OD1 gain mod I did, which was replacing the OD1 trimpot with an external potentiometer that has a higher value.

I'm using so much gain on OD1 on my "flagship" preset that when I smack a low E chord, the clip light stays on solid for 3 seconds before flickering black. On a stock MP-1 with the 8.2V Zener, that would sound like crap. Now it sounds like a normal tube amp: it has dynamics and clarity, and more gain is possible.  In other words, the LED is not a useful indicator of anything. The mod should ideally be accompanied by an adjustment of the resistors which determine the OD1 clip LED threshold, so that it shows actual clipping. And then, I think, you would never want to (or have to) see it blink red.

rnolan

Hey Kaz, that sounds like a really cool mod.  Can you work out which resistors and the values required to tame the led?  that would be useful as well.  When I get around to getting my 2 MP-1s working, this sounds like a really useful mod.
Studio Rig: Stuff; Live Rig: More Stuff; Guitars: A few

Kazinator

Quote from: rnolan on October 16, 2024, 12:22:45 AMHey Kaz, that sounds like a really cool mod.  Can you work out which resistors and the values required to tame the led?  that would be useful as well.  When I get around to getting my 2 MP-1s working, this sounds like a really useful mod.

I'm now a little confused about something now because I'm looking at the LED schematic again and see that the the OD1, Master and EQ thresholds use identical resistors. By my calculations, the comparators should trigger when the rectified, smoothed (160 Hz LPF-ed) voltage hits around 11.2V. But in consideration of that, it's strangely too easy to get the OD1 LED to light up on a stock MP-1, taking into account the limiter.

According to a simulation I did earlier this year with CircuitLab (see Mastodon post) it's hard for the voltage to go beyond around +/- 9V.  (Why 9V? Because that's about 8.2V + 0.4 x 2.   At the very low currents involved, the rectifier diodes do not drop the rule-of-thumb 0.7V but only around 0.4V.)

Kazinator

I wanted to add a note. When I purchased the 12V Zener, the store had it in two wattages. I can't remember, maybe 1W and 5W? Needless to say, I got the lower one. But that is still beefy. The leads barely fit the through holes of the MP1. I had to apply heat to each terminal, to melt the coating of solder in the trough hole, while pulling on it with a pair of pliers, moving a little at a time. Yikes.

That's one reason why it may be better not to reinstall a Zener. If you really want to install one, and can't easily get your hands on a low wattage one with thin terminals, it may be better to surface mount it to the pads from the bottom side.

(Speaking of which, I did exactly that for those two resistors that are in the OD1 tone filter, and there is a capacitor in the stock MP-1 that is also installed that way out of the factory. You know the one. I can now swap those resistors without having to remove top panel, only bottom. Not to mention removing a tube, which they are close to if mounted the normal way.)

Kazinator

Quote from: Kazinator on October 16, 2024, 11:11:40 AMI'm now a little confused about something now

Yep! The schematic lies! The actual value of R102, the bottom resistor in the divider, isn't 47kΩ but 150kΩ.

This means that the smoothed voltage which triggers the comparator is not 11.2V but around 7.2V.

That makes perfect sense now. Perhaps they originally designed OD1 without the limiter, but then they didn't update the clip LED schematic to account for it. Except that somehow, the right value of resistor made it into production. If the schematic value were used, it would be obvious: OD1 never blinks no matter how much gain you set with the program or OD1 trimpot! It would show up in basic testing, and calibration.

And so, the value we want for R102, if we remove the limiting Zener, is around that 47kΩ value: same as for the Master and EQ. 11.2 is not a bad clip LED threshold voltage for signals based on +/-15V op-amps, which do not go rail-to-rail, topping out at around 13.

Mystery solved.

Kazinator

It turns out, surprisingly, that I have a bunch of 47kΩ resistors, so I did the swap. Yay!

I didn't change anything else; same OD1 potentiometer gain, and program.

The LED now flashes briefly only for loudly strummed notes. Which, isn't that exactly how you are supposed to use a stock MP-1? I had it set right by ear.

My programmed OD-1 gain 10.0, then calibrated with the external potentiometer.

Harley Hexxe

Yes, the LED is supposed to flash as opposed to being on constantly.

I'm glad it worked out though
I only have two brain cells left, ...and I'm saving them for the weekend!