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

[RobinL]

What do you measure at point C and E?

Point C are few milivolts DC
Point E is 13.29 V Dc


I traced the Path. Everything is clean until and including pin 2 of V2. The Plate of v2, pin 1 has the noise. I changed c3, but the part is good, so are the parts and connections at the cathode

[MarshallJMP]

E is quite high, almost at the saturation point, this might cause the high voltage  to be a bit high. Is R10 a 3k3 resistor? if so put in a 2k7 and get back to me.

[RobinL]

E is quite high, almost at the saturation point, this might cause the high voltage  to be a bit high. Is R10 a 3k3 resistor? if so put in a 2k7 and get back to me.

Yes, it is a 3k3, measured good. I replaced it with 2k7. That dropped voltage E to 10.55v

Unfortunately it made no difference to them hum or the switching noise

[MarshallJMP]

Ok E looks better now, I was hoping it would do something.

[RobinL]

Ok E looks better now, I was hoping it would do something.

So shall i leave the 2k7 in? Or should I find out why it is that high with the stock value? I suppose it is due to the transformer change.

I'm waiting for parts and then will continue to search how that hum is sneaking into v2...

[MarshallJMP]

No leave the 2k7 in there. Like you said it's due to the higher voltage comming from the MDRT wich causes the current to rise. Lowering R10 will give more current pass before the HV regulator goes in saturation.

[RobinL]

Ok, i went the extra mile and started breadboarding everything around v2 on an external pcb with all new parts. So far, i get the exact same results. I will substitute more parts from the tubeboard to the pcb (mostly because i started it) but somehow my hope is small to find the cause of the hum on the tube board...

[RobinL]

I´m running out of ideas. I´ll give a short summary of what i found out so far and my conclusions. If I´m off somewhere, please correct me!

- Loud hum in both tube voicings, once the master gain is above 7.5. The hum gets louder with more mastergain and also goes up and down with the master output pot on the front. Solidstate voicing does not have this hum.

So i conclude the noise is generated somewhere in or after the tube board. I traced the signal on this board, see attachment. Green means all is fine, Red means hum is present. When i short v2 pin 7 to ground, hum is gone. When I connect the junction of R18 & R19 to output point F or to an exrernal poweramp (and therefore exclude v2) i get a hum free healthy signal.

I built the v2 parts on an extra pcb. All parts in blue are different and new parts, point-to-point on a prototype pcb and connected to the tube socket and equivalent points. I tried different grounding points as well. Still the same.

So i can rule out all the blue parts, their corresponding traces and pads and the tube socket. Of course i tried some other tube, 5 or 6 of them.

So maybe i´ll just accept it the way it is. I try staying as low as possible on the master gain and try to even out the volume differences with the following effect units or maybe a boost or something in the fx loop. Or use the SS-clean, that would solve the clicking as well...

MAybe i´ll find another mp-1, but the prices are really high right now, 500€ and more are what guys are currently asking. Or i change the mdrt with the stock transformer and see if that does change a thing.

Right now i am out of ideas and some other things need my attention atm...or does anyone have another idea what to test?

[rnolan]

Hey RobinL, I have some ideas, but who knows if they matter... Some of what you are describing "may" be influenced by physical proximity/and/or pickups??.  Ok high gain sounds; have noise, are sensitive. Single coil PUps don't like fluro lights etc. and are also noisy (why God invented humbuckers), and in the early days picked up taxi radio signals.

So have you eliminated those influences ?  e.g. maintain a good distance from the preamp? what PUs are you using?

Given the extent you've gone to, to ensure the components are good, then maybe it's other factors??

From experience I do know that single coil PUs can be a problem, particularly if there are fluro lights.  And also high gain just makes noise worse, no matter what PUs you have.

But hum is is it's own dimension :-(.  Loud hum (both tube voicings) dose seem to indicate something is wrong.

[RobinL]

Yeah, i am aware if that. The hum is present in the output even when i ground the grid pin2 of v2, so the noise is independent of everything before that stage. Of course, when i play guitar other ,,normal" noises occur, like you said. But the hum is present wherever i go and set up the mp1, on my workshop bench, the practice room, two studios...i think it is inherent, caused somewhere around v2 

Good news is, i just bought a supposedly stock mp-1 (330€). So i can compare. Or switch units via my rack loop selector. Either i solve the mystery of the hum or work around it 

[MarshallJMP]

Robin I send you a PM.