MP-1 - Problem with tube voicings - Loud noise and almost no signal

[Greg]

Hello everybody,

After a few years of searching for an MP-1 in a good shape, I was super excited to finally find one at a reasonable price. It was sold to me as working but unfortunately it seems to have a problem with the tubes. I'm determined to fully restore this preamp back to its former glory and I am hoping you'll be able to help me out. (Warning: this is going to be a long post.)

Starting with what works -- everything except for the two tube voicings. I used the solid state channel to test all outputs, controls, etc. and everything sounds and looks fine to me, as far as I can tell.

As to the problem itself, there is a lot of loud buzzing noise, always present, it doesn't change when switching between the two voicings. And because of how loud it is, first I didn't even notice that there is actually sound passing through. It is barely audible, weak, and sounds a bit like a ring mod.

First thing I did was to swap out the tubes for a pair of new 12AX7s but that didn't help.

For the past few days I've been trying to find anything unusual, hoping it will help me narrow it down further. I took some measurements in several points on the tube board (see attached table) and I noticed that there is a significant voltage drop on the DC side of the rectifier with both tubes in. Based on my somewhat limited knowledge of preamps, 140V feels pretty low, but I was unable to find any information on that so I don't know if it's to be expected. I'm wondering if any MP-1 owners on here can confirm this for me.

I also tried 'jumping' the two gain stages of V2 to see if maybe it is due to excessive current draw with both tubes in. Other than the obvious drop in gain, I haven't noticed any difference really.

Another thing I've checked is the suggestion I found in a letter from ADA to their distributors with some troubleshooting tips (http://www.kylheku.com/mp1-files/mp1-schematics/ADA-MP1-019-027-troubleshooting-tips-1992.jpg). It says: 'Extremely noisy MP-1's are usually FET's (must be SK364) in the signal path, tube cathode by-pass.' I tried measuring the change in resistance around R7 and R23 when switching between the voicings but the readings are symmetrical and I haven't noticed anything suspicious.

I have tested a bunch of other small things but I think this is basically everything worth mentioning at this point. I'm afraid I've reached a bit of a dead-end.

I wonder if any members of this forum have experienced a similar problem before? I have no idea if this a well-known problem or some kind of an unusual issue. I would appreciate any tips, suggestions, anything I can do to help me progress with this issue.

Thanks,

Greg

[rnolan]

Hey Greg, welcome to the depot  :wave: .  Hopefully MJMP can help you get it sorted.  It's good everything else works but obviously it's the tube voices you need/want.

[Greg]

Hi rnolan! Thanks, I'm glad to have joined this awesome community! And yes, while I'm after the heavier tones in this preamp, I have to say I really like the cleans too, but the thing that blew me away during the first 15 minutes with my MP-1 was the chorus. It's every bit as good as an early 80's Boss CE-2!

Going back to my problem, I'm planning to keep updating this thread as I go along and report my observations. I've been experimenting a bit more since my first post and I tried different combinations of the gain settings. Here's what I've managed to establish so far:

- If I set Master Gain to 0.0, there is no buzz.

- If I increase Master Gain and set both OD1 and OD2 to 0.0, the buzz is very much audible.

- I think that setting OD1 to 10.0 and OD2 to 0.0 gives the best signal to noise ratio (especially on Tube Clean); the guitar is almost as loud as the buzz and it sounds relatively healthy. It's difficult to tell because of the noise but it doesn't have that ring mod quality to it.

- While increasing Master Gain seems to raise the level of the signal together with the noise, I think the buzz gets especially loud if I increase OD2. Also, it sounds to me like the buzz stops getting louder somewhere beyond 3.0 on OD1.

That's all for now. Hopefully, I'll be back with more soon.

[Greg]

I am super happy to share some great news! With the help from the awesome MarshallJMP I managed to fix my MP-1. I am documenting my fix for those of you experiencing similar problems in the future, maybe you'll find it helpful.

Basically, all I did was to remove the three high voltage capacitors on the tube board (they are C1, C2, and C8) and replace them with 10uF/350V. The voltage levels are now in the correct 180V-190V range, the signal is healthy, and there is no noise whatsoever.

MarshallJMP, I can't thank you enough for helping me out! All it took was a single email and you were spot on! You have my utmost respect. Hats off to you, sir!

[rnolan]

 :whoohoo!: Go MJMP  @Greg, excellent news.  And any documenting will be greatly appreciated, others will indeed find it helpful.

[Iperfungus]

MarshallJMP, I can't thank you enough for helping me out! All it took was a single email and you were spot on! You have my utmost respect. Hats off to you, sir!

Yes...that's Phil! 

Welcome in The Family.

[MarshallJMP]

That's what I'm here for  ;D