MP-1: What voltage do you see at front input jack?

[Kazinator]

I am seeing around 0.3 mV, which means that 0.3 nA of current are flowing through R91. The obvious source for that could be the JFET. But that's behind capacitor C34, so that cap has to be leaking. Or else stray current along circuit board? If C34 is leaking, it could be more leaky than the current indicates, because it's limited by what the JFET's gate leaks.

It should be obvious by now that I hear things, and so the following should be taken with a huge grain of salt. And it is very preliminary. I put a coupling capacitor into my guitar right at the output jack to block DC. A big value, 1 uF. I would swear it improved the tone, especially around the bass notes on the low frets open positions of the E and A strings. It just seems more spanky with jumpier palm mutes. I haven't done a proper A/B. The busy weekend kicked in now and all, so I won't be able to get back to this for a little while. It could turn out to be nothing.

Nevertheless, I'm curious at what other MP-1s measure at the output jack. Just open, not loaded down by any source such as a pickup.

[Kazinator]

Correction, it was actually a negative voltage, around -0.6 mV. This is something induced in the cable. You have to just insert a plug and measure at the plug.

Opening the unit with the power on, the 1M input resistor R91 showed no voltage, which is what we want. The other resistor behind the cap, R90, showed a small voltage, like -1.1 mV or something. Cannot remember. The gate of the JFET should be more negative than the source. So all looks good.

[Kazinator]

OK, so I did several things.

I ended up swapping out the C34 ceramic cap anyway. I had some suspicions about it. Ceramics do age, and it's close to the tube and all, plus has that white powdery coating that old ceramics do.  I replaced it twice. The first cap, a 82 nF, somehow sounded off in a pretty obvious way. I don't know whether it was the cap or soldering. Rather than reflowing it, I put in a 22 nF film cap that sounds fine. It's less than half the value of the original ceramic, but into the 1 meg impedance that it faces, that hardly makes any difference, like 3.3 Hz high pass cutoff versus 7 Hz: transparent even to a down-tuned bass guitar either way. We can easily use a 10 nF or even lower.  I think they used a ceramic there because it's so close to the V1 tube. If I have problems, I will put in a ceramic.

I plugged a bare 1/4" plug (casing removed, and no cable soldered on) into the input, and with that, I still see a small negative voltage of -0.3 mV, though as I mentioned didn't see one across the input resistor on the circuit board. So there is some leakage from somewhere, involving the chassis or circuit board. Maybe a chassis-PCB ground loop or something with some noise on it that the multimeter rectifies to a DC voltage. It's not worth worrying about, probably, because it's so small.

[Kazinator]

I put in a 22 nF film cap that sounds fine

Ah, but I threw it out and put in a 33 nF monolithic. Wow! Tone more alive. And the cleans "quack" nicely now.

What the heck?

Anyway that original C34 needed replacement, clearly.

This is a part that definitely deserves more attention from MP-1 fanatics: the silly cap between your guitar and the JFET buffer. It's big ceramic disc that sits millimeters from the tube, and is forty years old. It certainly works, but it may be affecting your tone.

On another note, I found that my multimeter probes are swapped! One of the kids musta done it!   The red probe was plugged into COM (common). So the voltage I reported as negative should be positive. Another thing I was working on a couple of days ago (not in the MP-1) where the voltage had me scratching my head now makes sense; and I have an electrolytic that I have to reorient.

[rnolan]

Hey Kaz, maybe you should put a suggested/possible mod sheet together with the pros and cons of each (if you are up for it, no pressure).  Only that you've been going down a bunch of rabbit holes lately and discovering some interesting things (that may or may not suit everyone). E.g. I'm interested in the diode mod as you say it opens up the sound   and also this cap mod, as, as you say, it gets roasted and you can hear the difference.

The Eq mods, which I've found fascinating and very edifying, may suit others.

If you do decide to put a sheet (or 2) together (pls incl pics ), we can put it in the mods section.

[Kazinator]

it gets roasted and you can hear the difference

I'm really kicking myself for not having looked into that input stage and its capacitor years ago. Man, I just played over a list of blues backing tracks, using only crunch level distortion. Fast runs and especially legato notes sound so much better. It's more of a pleasure to play clean and crunch.

I'd say that for someone who is not looking at gain, and likes their stock EQ and tonal profile of the overdrive would benefit from these two mods:

• replacement output jack. I did only my A jack. B is stock. When I compare them (literally A/B comparison), B sounds like ass, and that wrecks anything else you might do to the unit.
• this capacitor C34 replacement

I would say that after this, the improved drive filter circuit (so Tube Dist mode doesn't lose over 8 dB of signal level, while maintaining the same frequency response) might be a priority over removing or replacing the OD1 Zener. But the latter is pretty trivial to do also at the same time. Zener gives you more headroom on cleans, and allows more gain on Tube Dist, but not as much as the tone circuit rebuild (by itself), which on the other hand doesn't do anything for cleans.

I'm super happy with my redesigned EQ; with nothing between the MP-1 and a power amp, it sounds Marshall-like. It's not for everyone. Nobody will believe it without good sound clips, in any case. I also like the change in the center frequency of the Tube Dist drive circuit down from 2.2 kHz to around 1.5 kHz. This is a bit like the response of a Tube Screamer with the tone control near the top (without any other similarities).

Currently I have to concentrate on getting a job.

[Kazinator]

So, I played on this monolithic, (supposedly!) 0.033 uF C34 for close to a week, doing nothing but blues and classic rock type riffs with crunch tones, and some cleans!

Oops: upon returning to da heavies, I noticed problems right away. Something unpleasant in the tone, and a kind of boxiness with thin bass. Like using a crappy treble boosting pedal or something.

In fact, that cap was not passing enough bass like it was supposed to according to its value; that's why it sounded good and tight for crunches and cleans.

The original C34 also 0.047 uF also wasn't passing enough bass, but more than this 0.033 mono. (It's weird; the value is nowhere near where we should hear a difference in bass; the roll off is below the hearing range, even for smaller values. 0.010 uF should still have good bass in the C34 position.)

I put in a 0.033 uF film cap. It's a big sucker, so I left it plenty of leg and mounted it with a bit of a bend and twist so that it's away from the tube as much as possible.

There is no getting away from it; there is just more bass than before with the stock cap, (not to mention much more than than hapless 0.033 uF monolithic).

To get the similar tight crunch sounds on the new film cap that I was enjoying on the bad 0.033 mono, I had to turn down Bass from +4 to 0 or even -2!

But even on a high gain preset, I am finding that it now benefits from going +4 to +2 on the bass. Cleans likewise want to have their bass knocked down. (My Bass rolls down below 195 Hz, not 310 like stock, by the way).

I suspect anyone swapping out the old C34 should be prepared to hear a bit more bass and fiddle with their presets accordingly.

I wonder if I had done this swap first, would I still have needed to experiment with moving the frequency centre of the OD1 Tube Dist bandpass filter. The original motivation was to have more "meat" in the palm mutes. Maybe a C34 swap alone could have done that well enough? One of these days I'd like to hear the good C34 cap with the original band-pass frequency range (but not stock: still modded not to have the over 8 dB drop, of course!)

[Kazinator]

I just noticed something the other day. After this C34 change, I can bring the pickup closer to the strings that before. It sounds great, representing another improvement in tone. I was never able to have the pickup that close before. Whenever I tried it would sound bad and I'd end up backing it off again.

[Harley Hexxe]

I just noticed something the other day. After this C34 change, I can bring the pickup closer to the strings that before. It sounds great, representing another improvement in tone. I was never able to have the pickup that close before. Whenever I tried it would sound bad and I'd end up backing it off again.

Hey Kaz,
Just wondering here... does this same guitar sound as good if you plug it into another amp like this, or is this only with the modded MP-1?