Herby's attemt @ SS Mod, Noise Mod, MDRT Mod

[MikeB]

I'll try the trim pot adjustment before I replace any components. But I won't get a chance this weekend.
It's funny, it never even occurred to me to adjust the output pot during my testing. I have mine pretty high, about 3 o'clock. I'll have to do some more playing around,  because I have plenty of headroom on the gmajor2 input pot.

I don't have any spare tubes to roll with.

[Iperfungus]

I'll try the trim pot adjustment before I replace any components. But I won't get a chance this weekend.
It's funny, it never even occurred to me to adjust the output pot during my testing. I have mine pretty high, about 3 o'clock. I'll have to do some more playing around,  because I have plenty of headroom on the gmajor2 input pot.

I don't have any spare tubes to roll with.

Yep!

You could try to lower the output pot until the squeal disappears...and see at which level...

[rnolan]

Hey Mike, presence is probably anywhere between 2.5khz to 6kHz.  In audio (mixer) land it's generally 5 or 6kHz, in mics (eg SM58) the presence hump is around 5kHz, and in guitar gear maybe a little lower (and varies on different gear), but 3kHz is definitely in the presence ball park (and hard on the ears when it squeals  ).

@ Max, interesting results with loop send and headphone out with output off.  So the loop send is before the chorus (IIRC after the eq, MJMP did post the signal flow a while ago), the headphone out is after the chorus (as is stereo) but before main outs (I think, or what you did wouldn't work), the headphone out also has rudimentary filters to reduce top end (similar to a cab sim).  So if you have lowered your trim pots to minimum (means with MDRT the tube stage is running much hotter  >:D ), this could be overloading the eq or chorus input ? (a guess  ).

@ herbyguitar, the bass trim pot mod lowers the bass cut/boost centre frequency down to (IIRC) around 80Hz (bottom E in concert pitch is around 82Hz) from (again IIRC) 127Hz, there's a post MJMP put up that lists the centre frequencies of the eq bands, they're not listed in my MP1 manual.  But this is not low mid although harmonics (halving/doubling) happen. So 3kHz > 1.5kHz > 750Hz > 375Hz (375Hz is in the low mid/mud zone), or conversely 80Hz > 160Hz (always a problem frequency) > 320Hz.  I don't recall that others who did this mod had any introduced "hum" issues, it should be a problem, check you install though, make sure all is good and no solder or component issues, maybe trim pot needs a clean  .

Interesting tit bit, when AKG D12 was made it was intended for male vocals and has a 80Hz boost (hence good on kick drum (add 2 or 3 kHz for click/cut), also the Bayer M88 was similar and a useful kick mic.

[MarshallJMP]

Frequency and phase graph of the presence.

[Iperfungus]

In the meanwhile I will manage to add the 100K resistor in series to R17, so we'll see what happens.
Should this be not enough, I still have the U10 OP AMP and 4 caps (C56,C57,C48,C49) to replace.

But I think that a "step by step" approach would be the best.

I'll have a look to my other MP-1's tubes board before I do anything inside my main one. 
If there's a reason for everything, maybe that MP-1 with all its issues could be here for this purpose.

Hey, that's the fun of being part of this place!  :wave: :wave: :wave:

[Iperfungus]

@ Max, interesting results with loop send and headphone out with output off.  So the loop send is before the chorus (IIRC after the eq, MJMP did post the signal flow a while ago), the headphone out is after the chorus (as is stereo) but before main outs (I think, or what you did wouldn't work), the headphone out also has rudimentary filters to reduce top end (similar to a cab sim).  So if you have lowered your trim pots to minimum (means with MDRT the tube stage is running much hotter  >:D ), this could be overloading the eq or chorus input ? (a guess  ).

Hey Rich, yes...internal trimpots have been set that way already by Phil when he modified my MP-1, due to new MDRT voltage I assume: OD1 and OD2 trimpots are set to their lower value and there's plenty of gain still... :headbanger:
What do you think could be overloading eq or chorus? By the way, chorus is 100% off on my "gainy" patches.

By the way, the more I think about the issue, the more I'm convinced that the squeal could be an internal oscillation somewhere between the output of the tubes board and the input of a next stage, probably due to an overload under certain conditions: MDRT gives higher voltage to tubes, tubes are running hotter (and that's f**king good!!!) and maybe the stronger signal coming from there could be too high for some next stage when I rise gain and eq up.
The issue seems to be related to final output level and this is the reason why MJMP suggested to replace U10 and C56,57,48,49 at first.
Then we had some test comparison with MikeB and Herby and find that similar mods lead to the same issue...and here we go with the idea of adding a resistor.

Let's go on and I'm sure we'll come to a solution, so that we can run our MP-1s hotter than Hell and enjoy the fabulous tones coming out from them thanks to MDRT and other mods.
Should be this the problem, MJMP's idea of adding that 100K resistor could be the solution.

[rnolan]

Hey MJMP, not quite sure how to interpret the graph, comparing phase and frequency (not sure I quite get it ?).  Could you draw us a diagram/schematic like the one on page 3 of the MP2 manual of the MP1 signal flow (I know you posted it in words but a picture would be nice)

Hey Max, the next stage after the tubes/ss is eq, the loop send/retrn is after that and then the chorus stage.  The chorus stage is always used as this is where the signal is split into pseudo stereo by inverting (180 deg) the signal into B.  If the chorus is IN it also applies very short delay (depth setting, more depth = more delay) to the B channel and rate (sweep modulation, varies the pitch (slightly) of B compared to A) between the 2 channels. So the eq stage could be overloaded at its input, and/or the chorus could be overloaded (depending on eq settings e.g. if you boost everything = more level).  However, I was intrigued by what you said about turning the main output to 0 making the squeal go away (and monitoring the signal elsewhere (loop send/phones out), this indicates (to me) something to do with the output section  .

[Iperfungus]

Hey Max, the next stage after the tubes/ss is eq, the loop send/retrn is after that and then the chorus stage.  The chorus stage is always used as this is where the signal is split into pseudo stereo by inverting (180 deg) the signal into B.  If the chorus is IN it also applies very short delay (depth setting, more depth = more delay) to the B channel and rate (sweep modulation, varies the pitch (slightly) of B compared to A) between the 2 channels. So the eq stage could be overloaded at its input, and/or the chorus could be overloaded (depending on eq settings e.g. if you boost everything = more level). 

However, I was intrigued by what you said about turning the main output to 0 making the squeal go away (and monitoring the signal elsewhere (loop send/phones out), this indicates (to me) something to do with the output section.

Yes, I'm quite sure it's something there as well...and for the same reasons you said.

@MJMP: Phil, when you said that this squeal could be related to some too high voltage to digital pots....which digital pots were you talking about in the signal chain/path? 

[MarshallJMP]

@R, this is the frequency response of just the presence section. As you can see it has a quite big range (but it's also a simple filter) and the center freq is around 6kHz.

@Ip, these are U13 (TC9176), as you can see in the schematic the output of the tubeboard goes straight into pin 3 of U13.

[Iperfungus]

@Ip, these are U13 (TC9176), as you can see in the schematic the output of the tubeboard goes straight into pin 3 of U13.

f*ck yes! Got it on the schema.
So..how the Output Level pot is related to U13?

Forgive the silly questions...but solutions to issues could arise from shred them in smaller pieces! 

[MarshallJMP]

It's actually not directly related, this controls the master volume (which you can program).

[MikeB]

Ok, so I turned down the output volume pot and the squeal went away.  I have been pretty busy and then got sick so I haven't been motivated to do another test of the thresholds.  I also haven't yet adjusted the trimpot.  More to come.  Got to sort out all my levels again by tomorrow for the jam.  Should be good.  I was able to play with OD1,OD2 and MV on 10 each.  Output pot was way down though.

[Iperfungus]

Ok, so I turned down the output volume pot and the squeal went away.  I have been pretty busy and then got sick so I haven't been motivated to do another test of the thresholds.  I also haven't yet adjusted the trimpot.  More to come.  Got to sort out all my levels again by tomorrow for the jam.  Should be good.  I was able to play with OD1,OD2 and MV on 10 each.  Output pot was way down though.

Hey Mike!
So...it looks we're on the same side of the road! 
Where can you set Output pot with OD1 and OD2 at 10.0???
I set OD1=OD2=7.5 and MV=5.5 and I can set the Output pot around halfway before the squeal starts...and a little over halfway if I decrease highs and presence...or ODs, of course.

By the way, I've the 100K resistors ready but I've been busy as well and then my welder broke 2 days ago and it needs to be repaired...or I need a new one...but this is NEW and f**king expensive! 

FYI, my internal trimpots for OD1 and OD2 gain are set to minimum (and there's a lot of gain anyway...).

[Iperfungus]

f*ck.
As I wrote to MJMP, I've got my welder repaired...and now I'm struggling with resistors!

R17 is 430K but I can only find 470K resistors around...and putting too many resistors in series seems to be not a good idea, when you've to put them all together on the tubeboard... 

Stay tuned.

[herbyguitar]

Been a while.
I had a chance to examine the OD trim pots today. Both pots were set to about 60%. I'm assuming this is 'way' too hot. I'm going to run some tests this weekend. I'm going to start by setting both pots to 0% and see what happens.