21 posts / 0 new
Last post

MM RP 112 low volume and weird distortion


This is my first post here, even if I’ve been on this site many times to get resources and infos (by the way, thanks a lot for this!!).

I’ve bought a MM RP 112 last year, and I’ve still got issues with it.

I’ve got a weird distortion when the gain is past 7, not a « natural » break up but something that comes above this.
As it is hard to describe, I’ve recorded it, you can hear it there : https://youtu.be/C4VRfUD4aFI

Also the amp does not seem to be loud enough, and there’s a volume drop past 7 on the gain knob.

Here’s what I’ve already done to the amp :
- Change the speaker and the speaker wire
- Change driver transistor (for 2n6488)
- Retensioned the tube socket
- Change all the electrolytic capacitors
- Change tubes (6CA7) and biased it (around 25mV)

The amp seems to be a GP1 board, even if I’me not quite sure (it’s not written on the chassis or the board).

Now any suggestion to what I can check? Diodes? Others capacitors? Ic’s?

And sorry for my English, I’m French…hope it’s clear enough.




Clean the Pots

Congratulations for being brave enough to dig in. When the amp was opened up for replacing caps, was contact cleaner spray used to clean and lubricate the volume, gain and other pots? DeOxit is a good brand of cleaner. I listened to the clip but didn't hear any real distortion. Just maybe a little warble from the Phasor circuit. I thought it sounded pretty good. Also, is the Hi Lo switch on Hi? That can affect Max volume. Lower volumes not so much. -mgriffin


Hi, and thanks for your reply

Hi, and thanks for your reply!

Yes, I did clean the pots, board, tube and IC’s sockets with a « standard » contact cleaner, but it didn’t change anything (maybe I should get some Deoxit, as I seems to be more efficient).

The amp actually sounds good in clean mode, expect that I’ve got to increase the master volume to get a good level (especially in gig situations). I get a good level if I increase the gain knob.
It is in Hi position, and the phaser doesn’t work anymore (I’ll take of that later!).

I’ve made another clip : https://youtu.be/xCXH1aDESIc
There’s 3 steps of gain level, so you can hear that strange noise above the sound of the amp.

I can hear the amp overdriving (« natural » break up) behind this weird distortion.
To be more precise, this distortion come earlier depending on the source (stronger pickup output).

I’ve got another MM (HD130) and use to have a 75, both of them has this « natural » break up without this unpleasant distortion (which is louder).

I may be wrong, but I think there’s a relation between the low output and this weird distorted…but I don’t know where to dig…

Thanks again!




I’ve been to older thread on this forum to get precisions about my problem, and to know where I should look at.
I’ve made a few more measures to see if there’s something wrong.

I’ve checked the 2 power tubes, I get 750V on pin 3 (almost 500 on low power) and 22V on pin 5, which is ok (I assume…).

At the wiper bias pot, I get 1,1V.

R68 & 69 are OK at 220 ohms.

All the resistors around the power tubes are OK, except R54, 55, 56, 57 that are 370 ohm instead of 470 as expected. Only R59 is at 470 ohms. Is that a problem?

Don’t know where I should look now…


Is R53 470 Ohms?

Question: What about R53? Is it also 470 ohms? If yes, the circuit will work but why would one make that change? If it's not 470 ohms, there is a slight circuit imbalance. Do the 370 ohm resistors look OEM or were they part of a repair or mod? In short, it's best if R53, 54, 55, 56, 57 & 59 are exactly the same value for a well balanced circuit. The closer the better. If it were me, I would make them all 470 ohms like in the schematic. BTW: The other voltages you mentioned are in spec. -mgriffin


Thanks mgriffin for your

Thanks mgriffin for your advices!

R53 had slightly drifted, so I changed both 53 & 59, just to be sure.

R54 --> 57 doesn't seem to be a mod or a repair. As I found strange that they were all the same value, I measured out of the circuit and it was 470 ohm. So I let them.

I also tried a few more things :
- change Gain and Volume pots
- swap IC's for NE5532 (nothing changed for the sound, but the phaser is back!)
- changed D1 & D2, as I assumed they were clippin diodes (and as the problem occurs when the amp is breaking up)
- as I tried to find infos about ic's in the service bulletin, in #6 I found a suggestion to put a 470k resistor across pin 2 and 8 of the tube socket (to avoid any output tube glitches, that can cause something that sounds like crossover distortion)
- cleaned again contacts and pots

I still have that distortion issue...


I finally decided to change

I finally decided to change R54 to R57, just to be sure.
Now I've got a new problem : as I measured the bias, this was at 25mV at both sides...and then started to increase slowly, to stabilize at around 150mV....

I've got to leave now, but I'm gonna double check my solders tomorrow.

But I'm starting to get completely lost...


Lost and Found

Sorry I was late getting back to you. Yes, go double check the solders and then see if bias adjust comes back to 25mV and holds. Be aware that the bias voltage will drift some while the amp warms up. Be sure to allow 30 to 60 minutes warmup to fully stabilize. Question: Is the foot switch being used to turn off Phaser and Reverb? If not, please do. Getting back to the distortion, perhaps it's time to look elsewhere in the signal path. Here are some ideas: 1) Try bypassing the input stage and plug the guitar into the 100mV Low Level Line In jack. 2) Try a different PUP on the guitar. 3)Try a different guitar cable. 4) Try a different guitar. 5) Try a different speaker. Good luck and don't give up. -mgriffin


Many thanks again for taking

Many thanks again for taking your time to help me!

I've double check the connections, and reflowed a few solders, and now my bias is steady again, just raised a bit to 35mV after 30 minutes, so I think it's ok now!

I've tried to bypass the input stage, I've got the same results.

I've already tried different guitars and pickups, and this distortion occurs earlier with stronger signal. So it really depends on the input signal.

I've already changed the speaker, but with the same results.

Something else that look strange to me : when I checked voltages, I've read around 10V at pin 8 of the opamps.
So I checked the output of R62 and I've also got around 10V (-10V at R63).
I've got around 42V at the positive side of C31.
I assume I should get 16V at the output of R62 (and -16 at R63). Is that can be related to my problem? Should I lower R62 & R63?

Sorry I'm not an expert, so I prefer to get advices before doing something wrong...

Thanks again!


+/- 16V

My bad. I should have advised you to check those voltages early on. +/-10V is not OK. They need to be +16V and -16V. I suspect the regulators have gone soft. I'm assuming this amp is a GP2 version. Replace the zener diodes D10 and D11. Use 1n5353 16V 5W instead of 1n4745 16V 1W. 42V at C31 is OK. It gets regulated to +22V by D9 zener diode. Suggestion: When replacing D10 and D11, stand them off from the PCB by 2-3 mm for better heat dissipation. You're almost there. -mgriffin


I changed D10 & D11 with

I changed D10 & D11 with zener equivalent (my local store didn't have 1n5353, I've got to order them), but I still got around +/- 10V, and it's not steady, it's like oscillating (10 to 11)!
Still got a steady 42V at C31(-42 at C33), and this strange oscillating voltage after R62 and R63 (those two resistors are brand new)....


Low Volts

What is the PN of the zeners? Something else must be loading the + & - 16V power supplies down. Question: Were C30 and C32 150uF electrolytic capacitors replaced? Double check and ensure they were installed correctly with respect to polarity. Although the reference designators may differ, the GP3 and GP3A schematics do a much better job showing the polarities of the power supply caps. The GP2 schematic....not so much. For more troubleshooting, if the IC's are in sockets, remove them one by one recheck +/- 16V after each one. We need to find what is loading them down. -mgriffin


Yes, I've change all the

Yes, I've change all the electrolytic caps. I've double checked orientation and connections, everything seem to be ok.

Then I put the originals LM1458 back in place, and the +/- 16V was back! (but the phaser went...)
I tried different combination of opamp, to bring back the phaser, and to know which IC brought the voltage down. (I think IC7 really needed that LM1458).

Anyway, I've got good voltages, but my problem is still here.

One more thing I tried : I've read somewhere that reducing the feedback resistor (R50 here) could reduce that kind of problem. I reduced it from 150k to 110k but I did not noticed changes.

Now still don't know where to look... Is that could be related to the gain of the opamp? Other capacitors?



It's good the +/-16V are back in spec. Now lets focus on the phasor. Here's something to try to make sure the phasor circuit is not causing the distortion issue. Lets totally disable it. To do that, remove IC3,4,5,6 Phasor ICs. Redo the distortion test and the results can help decide what to do next. Next, why did the phasor work with the NE5532's and not LM1458's? BTW: the gain of the op-amps is controlled by the external network of resistors around them. They don't run open loop so swapping an LM1458 with NE5532 really should not have much audio effect. As for the hiss on cable disconnect, how hi is the Gain Knob cranked? For reference, IC7 is the phase splitter circuit that drive the push-pull power amp stage. -mgriffin


I've once checked my amp.

I've once checked my amp.

I assume that I should search after the preamp? (from the Low lev input?) Even it's really lower, that distortion can be heard a little when plugged in this input.
One strange thing : when I unplugged my guitar from that Low level input, there's a big hiss (a kind of white noise) coming out of the speaker...

One more info, I did not change C23 yet.

Don't know of it can help...




Thanks for your suggestions!
Here's what I've tried this morning :
- remove IC 4/5/6. Without IC3, I had no sound. But following the schematic, I saw that only an half of it was used for the phaser, so I put it back. Still have the distorted sound. I've tried this combination with LM 1458 and NE5532. In any of the combinations, I still got +/- 16V
- I put back the phaser section with LM1458 to verify. Still works and still got +/- 16V. I assume I've got a faulty LM1458 I've got to identify.

I knew about the resistor network around the opamp, but I wanted to check if they were all in specs. I just wondered if a bad resistor in this area could cause this distortion?

I haven't got more time today as I've got to go to work, but I'm gonna try to check more stuff tomorrow.

Thanks again!




I went back to my amp this morning.
I've checked a few more things :

- checked all the resistors values. Everything was as expected, except R43, who was 3,3k instead of 39k (around IC6, the lfo section of the phaser?). I've changed it, but it only affected the depth of the phaser.

- plugged again in the low level input. I haven't got that hiss issue (maybe it was related to the gain pot as you suggested mgriffin). To be more clear, I've got this distorted sound but it's really "behind" the guitar sound, and it's a bit more pronounced with a high output guitar. But it's really not as loud as if I plug in the preamp section.

I've found another thread that really look similar to my problem : http://pacair.com/mmamps3/node/913

In this thread, they talk about R48/49. They are 10k/18k in the GP1 board, and 3,3k/18k on the following boards. That seems to adjust the volume problem. Mine are 10k/18k as on the GP1 board.

In the following posts (#19, feb 20 2011) they talks about an issue that really looks like my problem (most of the gain around 7 to 10, intermittent distortion rather than light distortion, that sounds like a blown speaker).
Unfortunately, that thread didn't last long (maybe I should update on this one?)

Thanks again!


Your amp seems to behave normally, really

Salut Jo,

the early GP boards with 10k/18k in the voltage divider by the phase inverter are not very sensitive. If you have a very loud humbucker guitar you get considerably more volume. Also, the GP-1 does not have an actual distortion circuit - all you are doing is overdriving the op-amp which may or may not sound good (in most cases it sounds pretty bad).
The later versions employ diodes to achieve the distortion. I am in favour of replacing the R48/49 resistors to give the amp a bit more oomph with single coil guitars. You can always turn down the gain knob if the guitar is really hot.

If you can get a loud clean signal out of the amp, I would say your work is done. Use an external OD effect for distortion with these, they sound great with it.

Just my 2 cents,

Lars Verholt


Salut Lars!

Salut Lars!

I’m sorry for my late reply, but I was really busy those days.

Thanks a lot for your message. I finally got the idea that this is the way my amp should sound. Those amps take pedals very well, but I really like the light crunch that comes out of an amp.

But, for my knowledge, I’ve still got a few questions :
I really like the light distortion that come out my HD 130 (and the 65 I used to have).
I know there’s a lot of difference between those amps, but what’s the main difference that make those amps sounds more like « natural overdrive »? The opamps (LM307 vs LM1458)? I cannot see differences about a diode clipping stage
About the diode clipping, I thought that D1/D2 were responsible for that, but I think I’m wrong. There’s an extra stage of diodes on the GP3 board (D3/D4 after the gain pot), is this what you were talking about?

Sorry about all those questions. I’m not just tryin to fix issues, but I’m also very interested in learning how amps work. And thanks to all the advices on this site, I’ve already learned a lot!


Compression Stage

Hi Jo, Glad your amp is working better. It's great that Lars chimed in to help. He and Mike (mm210) have worked on many MM amps and we value their experience and advice. Getting back to "light distortion", the later models of most MM amps have a compression stage. This is what Lars referred to. The RP GP1 and GP2 have no compression stage. The RP GP3 has one compression stage with Q1,Q2,D3 and D4 in the feedback loop of IC2. The GP4 version has two compression stages. As far as LM307 vs LM1458, there is not much audible difference between them. Hope this answers some questions and always feel free to ask. Good luck, -mgriffin



Once again, sorry for my (very) late reply. But I've been really busy few last months.
With more than 40 gigs with this amp, I finally managed to get the sound I wanted and really enjoyed the sound.
Many many thanks again for your precious advices and help!



P.S. : unfortunately,I've now got another issue, I'll make another thread about this one...

Log in or register to post comments