Anyone modded the OD-1 drive circuit with higher valued Zener?

[Kazinator]

Hi everyone, long time no see.

I've done some simulations of the MP-1's OD-1 circuit, experimenting with different component values, particularly the Zener.

The Zener keeps the drive circuit restrained in its output voltage swing. A larger valued Zener would cause it to hit the tube harder.

Does anyone have experiences with that?

I posted about this on Mastodon in January: https://mstdn.ca/@Kazinator/111814324159005509

[rnolan]

Hey Kazinator, and great to have you back . I haven't played with it myself but it looks really interesting   Great post on Mastodon.  Thanks for sharing that.

[Harley Hexxe]

Hey Kazinator,

   It's interesting to be sure, but the question I have is: Does this mod need to be done?

   Here is where the ears come into play.

   The MP-1 already has a very good range of overdrive saturation when you manipulate the OD-1 level and can push the clean tube voice into fairly high gain territory already. When you adjust the OD-2 with that, there is still a very pleasant gain structure even though it's no longer a clean tube sound. I'm wondering if this interaction between OD-1 and OD-2 will still be as good or will it get more harsh as a result?

[MarshallJMP]

There is more to this then just the opamp circuit. You are simulating without any load on it. I would trow in the rest of circuit like the filter behind the opamp, depending on tube clean or tube distortion will be full or mid boost and at least one tube to get a decent end result.

[Kazinator]

The MP-1 already has a very good range of overdrive saturation

Obviously, some people think so. Then, others have three tubes in there instead of two!

when you manipulate the OD-1 level and can push the clean tube voice into fairly high gain territory already.

But you find that the OD1 drive gets mushy past certain values. This is well known.

Now, see, is that because it's hitting the soft clipping due to the Zener, or is it the tube doing that?

My "main" presets rely on getting most of the gain from OD1; I really like the way it sounds with the tube I have in there. Settings that emphasize OD2 are clarinet-like woody tones with less high end and attack.

Imagine that instead of the op-amp with the Zener, we had another tube. That would put out bigger voltage swings. Tubes don't confine themselves to +/- 8.5V when hitting other tubes!

Also, if you think about how people use tube amps with drive pedals, there is nothing between their pedal and the first tube that would clip the voltage going to the tube's grid. It's whatever the pedal can put out. If you feel that your pedal is weak for the way you're trying to use it, you can throw it out and get one with more output. The ADA guys effectively built a drive "pedal" into the unit, put it on a +- 15V supply but then reduced it down to almost half that swing. A pedal on a 9V battery with a charge pump to get -9V can put out the same swing; why stick to that.

Yet another aspect to this is that your SS Clean goes through that op-amp circuit, so there are implications on its headroom. How the SS Clean would behave if OD1 were unthrottled a bit would also be interesting.

That's what I was asking; has anyone tried this and ... did it rock? Suck? Or just meh?

[Harley Hexxe]

Hey Kazinator,

    I have a 3TM MP-1 also, and the distortion is insane. Todd Langner came up with this mod when he was heading up the service dept. at ADA. The only clean tones you get with that preamp is in the SS voice. The clean tube voice becomes high gain, and the distortion tube voice is even more so. It's not one of my favorite preamps. It's just too over the top with the saturation for my taste.

   I don't know of anyone who has tried this particular method of modification in the MP-1, so I can't say if there were any positive results. MJMP would be more qualified to offer any feedback or advice on this subject.

   The SS voice does go through this circuit too because it is never out of the signal path regardless which voice you choose. The MP-1 is the only guitar preamp of the three ADA designed that does this.

   Yes, OD-1 will get mushy beyond a certain point, but I'm not certain it's from the Zener or the tube, it might be the SS compression affecting it this way, but I'm not certain of that either.

   We all know that ADA did drop the power to the tubes, and that's more than likely to prolong tube life. There is a mod here at the Depot that changes that and it's the MDRT. It does run the full power to the tubes and changes the overall character of the preamp. It's a popular mod here that many people seem to like.

   You sound like you know your way around circuitry like this, so if it doesn't pan out, at least you'll be able to put it back to where it was. I'd be curious to hear what kind of results this mod might yield.

[MarshallJMP]

Most tube amps get mushy when the gain is set high, the more gain the less low end is needed to get a tight sound. I don't think changing the zener diode will change this. But hey no harm in experimenting I guess.

[Kazinator]

Most tube amps get mushy when the gain is set high, the more gain the less low end is needed to get a tight sound. I don't think changing the zener diode will change this. But hey no harm in experimenting I guess.

Right, I was thinking last night along the same lines.

Firstly, the OD1 programmable control is just a level. It's not changing the gain of the drive circuit; only the trimmer pot does that. The OD1 level is not changing how much soft clipping is going on in the Zener drive circuit.

The high pass filter needed for a chunky sound is in the tube circuitry, with the cathode bypass enabled in the Dirty mode. In order to hit the first tube harder without mush, we would need an earlier bass roll off (like in various booster pedals).

(Putting something like that into in the drive circuit is an interesting idea, but with it affecting all voicings and no way to turn it off, it would be bad.)

I have to concur, likely more voltage range out of the drive circuit, all else being equal, would just enable more excursion into mush territory.

[Kazinator]

Yikes, I gaffed in my last post there, forgetting that the OD1 photoresistor level control is before the drive circuit, on the input side.

So indeed, when we turn up OD1, we do potentially bring on the soft clipping based on the Zener diode, or more of it.

[MarshallJMP]

That is correct, the gain of the opamp is also controlled by the vactrol. Gain is equal to R74+ T2 devided by the resistance of the vactrol. The lower it is the more gain you have.