Loud pop/click when changing to a preset with clean tube voicing

[RobinL]

Hi,

i do have an issue with my MP-1.
I get a loud pop/ click noise when I switch presets with different tube voicings. When changing from a Dist- or SS-Patch to Clean-Tube voiced one there is a loud noise. The Overdrive 1 & 2 controls amplify the noise to some extend, but even with both set to 0.0 it is still very present. The Master Gain value has no impact on this sound, with all three at 0.0 it has the same volume, even when the Mastergain is set to 10.
When switching to a Dist voiced patch, there seem to be a slight fade, but no noise. Which is much better, as it doesn´t trigger all reverbs and delays in my clean signal chain ;-)

It is a top switch v1 unit with the big noise mod and MDRT. I don´t know if the problem had been there before the mods, i just bought the unit and installed the mods before I dialed in my sounds and integrated it into my rack.

Changing Tubes makes no difference.

I heard that this issue was discussed here before with possible solutions and parts to check, but I can´t find a helpful thread.
Does anyone here has an idea what causes these noises? And even better, what parts to check to solve this problem?

Thank you all so much!!

cheers from germany,
Robin

[rnolan]

Hey Robin, welcome to the depot and hi from Australia .  I have a very vague recollection of this issue but it was a long time ago.  Hopefully others may recall it better, it may have been on the previous version of the depot.  I get a (sort of) similar issue with my MP-2 when I change to a compressed clean sound, like the clean patch acts on the tail end of the previous patch and I get a really loud thump.  It's not a problem if I'm careful with the patch change timing, but this is different to your issue.

[RobinL]

Thank you!

Maybe the switching noise might be just one symptom of some other problem. The clean voiced patches do have a significant hum as well. When i was testing the unit it was faint as i had the Output volume low with my workbench poweramp. But in my rack i have it at noon and it is really humming. I thought it was a tube but i changed them both with no difference. Them hum seems not to be present on the distortion voicing. With these, there are other noises like a whine and i can here a noise when pressing a frontpanel button but let´s focus on the clean first :-)

The patches i use mainly are:

Dist  OD1- 6.0  OD2- 6.0  Master- 3.0
 
Cln  OD1- 2.0  OD2- 4.0  Master- 10.0

With these extreme Master Gain settings both have equal volume. The hum gets lower with lower master gain.

I measured some voltages on the tube board:

AC In : 252v AC

Between rectifier and R1 & R4 : 242v DC

V1 pins:

1- 158v
2- 0v
3- 1,12v
4- 11,56v
5- 0,51v
6- 158v
7- 0v
8- 1,05v
9- 5,78

V2

1- 147v
2- 0v
3- 0,85v
4- 11,56v
5- 0,46v
6- 146v
7- 0v
8- 0,8v
9- 5,78

I found shorting Pin 7 of v2 to ground did cut the hum significantly, so i do believe the fault is somewhere around the first gainstage.

Does anyone here can tell me if these are regular voltages whith a MDRT? I feel the plate voltages seem low... Thank you all so much!

[MarshallJMP]

Voltages look ok to me, normally you should get 240Vdc.

Are you talking about tube clean or solidstate clean?

[RobinL]

You mean 240v right after the rectifier?

The tube clean. The solidstate is fine 
All 4 cathode bypass caps (c16, c12, c6, c7) are 10u BP and not 33u, due to the Noise mod. I don't know if that causes the hum. I am pretty sure the hum wasn't this loud before the mods and the mdrt. I'll try replacing it and check the components around v1 for now. Maybe a coupling cap went bad

[MarshallJMP]

yes after the rectifier.

Caps are not the problem, they are smaller so hum should be less.

Do you have hum if nothing is plugged into the mp-1 on the input?

[RobinL]

Yes. The hum is present when i short the Input to ground, it does hum when i short the tube board input to ground.

The hum does stop when when i ground pin 7 of v2. Yet all the usual tests i know when troubleshooting tube amps are fine, no dc or ac in the wrong places, no leaky caps or open resistors. The fets in v1 cathode are working as they should, all grounds are where they should be.

Maybe my settings are a bad coincidence? Gain 1 & 2 make no difference at all to this hum.  But when i raise the master gain above 7.0 it is really loud. Below that it is acceptable. But i would think you could use the whole master gain range.

I test and double check some more and will post my results.

[rnolan]

But i would think you could use the whole master gain range. 

Yes you should be able to.

[MarshallJMP]

Do you have hum on the SS clean ?

[RobinL]

No. The hum is in the tube voicings only. Solidstate is fine.


What i tested in the meantime:

The hum is indeed on both tube voicings. Same thing with the Dist as in the clean -> The hum gets really loud when the Master Gain is above 7.5. The Gain 1&2 settings make no difference. Changing tubes makes no difference. Turning both trimpots concerning the tube board make no difference. Changing the EQ settings alters the sound of the hum.

Shorting Point F on the mainboard cuts the hum. Removing the tube board cable F and injecting a signal directly to Mainboard Point F produces a good clean signal, without any strange noises at all. Connecting the tube board out (cable F) to a clean and silent power amp carries the hum with the guitar signal over to the new poweramp.

So i think that the hum creeps into the guitar signal somewhere in the tube signal path. The Master gain section (u13) is clean, as is the guitar signal in the solid state path. I need to test the output of U4, but I can turn up the Solidstate Mastergain to 10.0 without the annoying hum.

Maybe it´s just the nature of the beast? After installing the MDRT i had to turn both OD trimpots all the way down and just barely got it within the factory specs. Maybe there is just a lot of gain in the tube circuit now, which results in a louder base hum level, which gets annoying with a loud Master gain?

All parts i checked in the meantime are good. I don´t know about the C -Ref and E-Drive section and especially the parts on the mainboard belonging to that section. What does this part do?

And still,  all of this doesn´t cure the popping when switching 

Really thankful for your help

[rnolan]

Hey Robin, the hum shouldn't be there, sure, high gain brings some noise as does old age of caps etc., but the MDRT should reduce noise albeit it opens up the dynamic range of the MP-1. So my take is something isn't quite right in the tube stage (so not  Maybe it´s just the nature of the beast? ) .

The pop is another question, it also shouldn't happen.  Maybe the 2 issues are related? but I don't think so.

So do you always get a pop when switching? As I said previously, I get it with my MP-2 but only in particular circumstances (i.e. going from distortion patch to fairly heavily compressed/chorused clean patch too quickly).

Anyway, hang in there, MJMP will help get you sorted .  Maybe it's time for a new tube board??

[RobinL]

Yeah, i agree with the tube stage, but i am not sure if the board is to blame... on monday, the holidays here started, schools are closed for the next 6 weeks and my two kids and wife constantly draw me away from caring about my poor mp-1   

So I´ll inch my way forward trying to circle the problem. I started scoping the signal through the tube path. It enters healthy at the grid of v1, still healthy at grid 7 of v1.

Stay tuned on what i can report next time, after some more minutes in the workshop. Of course the mp-1 decided to misbehave once it gets busy at home 

[MarshallJMP]

What do you measure at point C and E?

[David Drake]

I don't want to be a simple Simon here but just incase I'll be Simon if it fits. The settings,

Dist  OD1- 6.0  OD2- 6.0  Master___((( 3.0)))_____
 Cln  OD1- 2.0  OD2- 4.0  Master___  (((10.0)))____

As I remember an inherent flaw of Dist.tube switching due to varying master volume settings. It had a very audible Click or (whishichk sounding) type of swell that had to be addressed when you actually switched between Dist tube channels, you had to hide it. This is when I abandoned the tube clean for cleans and fell in love with the S.S. Channel.

Sorry I totally missed  "The Master Gain value has no impact on this sound" in this post :(

[RobinL]

I don't want to be a simple Simon here but just incase I'll be Simon if it fits. The settings,

Dist  OD1- 6.0  OD2- 6.0  Master___((( 3.0)))_____
 Cln  OD1- 2.0  OD2- 4.0  Master___  (((10.0)))____

As I remember an inherent flaw of Dist.tube switching due to varying master volume settings. It had a very audible Click or (whishichk sounding) type of swell that had to be addressed when you actually switched between Dist tube channels, you had to hide it. This is when I abandoned the tube clean for cleans and fell in love with the S.S. Channel.

Sorry I totally missed  "The Master Gain value has no impact on this sound" in this post :(

Yeah, that's what i feared in the first place. I still have hope to find the cause of the hum. Then i return to the switching issue, either solving it or work around. I also have the feeling that when switching presets, not all digital pots change instantly or at the same speed. Especially when switching from clean to dist, i hear what you describe. Small short fade with kind of morphing into the new preset. But even with both masters at the same value, it does make some kind of switching noise...