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Author Topic: MP1 No Overdrive/Distortion  (Read 2657 times)

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Billy Penn

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MP1 No Overdrive/Distortion
« on: Time Format »

Hi all. First post here and I was hoping someone could help me out with an original MP1. It seems that the overdrive/distortion is not working. Both 12AX7 tubes are new and known to be good. I notice if I pull both tubes the unit still works but obviously no overdrive/distortion. Has anyone ever had this issue before? Any thoughts on what it could be? Thanks in advance!!
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Harley Hexxe

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Re: MP1 No Overdrive/Distortion
« Reply #1 on: Time Format »

Hi Bill,
  Welcome to the Forum. You say it works without the tubes in the sockets? That would have to be a solid state voicing then. Is that what the front panel indicates?
  That seems very strange that you get no overdrive. I assume you raised the levels of OD1 and OD2 and got the same result?

       Harley 8)
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Billy Penn

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Re: MP1 No Overdrive/Distortion
« Reply #2 on: Time Format »

Thank you for the reply Harley Hexxe! Ok, I just tried it again and when the OD led is lit I do have control over the overdrive (OD1 & OD1). The overall gain and distortion seems low to me. In other words it should have more gain in my opinion. Another note is that the Clip led's do not light up when I strum hard on the guitar. Could it be that the lack of gain is an issue with an op-amp... U4?
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MarshallJMP

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Re: MP1 No Overdrive/Distortion
« Reply #3 on: Time Format »

I agree with Harley,if you pull the tubes it shouldn't work on the tube voicing,only the SS voicing.

What settings do you use,and maybe if possible could you post a soundclip?
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Billy Penn

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Re: MP1 No Overdrive/Distortion
« Reply #4 on: Time Format »

Ok, I just tried it again and when the OD led is lit I do have control over the overdrive (OD1 & OD1). The overall gain and distortion seems low to me. In other words it should have more gain in my opinion. Another note is that the Clip led's do not light up when I strum hard on the guitar. Could it be that the lack of gain is an issue with an op-amp... U4?


I agree with Harley,if you pull the tubes it shouldn't work on the tube voicing,only the SS voicing.

What settings do you use,and maybe if possible could you post a soundclip?
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MarshallJMP

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Re: MP1 No Overdrive/Distortion
« Reply #5 on: Time Format »

If the OD1 clip goes on the U4 opamp is fine.
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Billy Penn

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Re: MP1 No Overdrive/Distortion
« Reply #6 on: Time Format »

The OD1 clip does not go on. None of the clip led's go on. Should I suspect U4 opamp then?

If the OD1 clip goes on the U4 opamp is fine.
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MarshallJMP

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Re: MP1 No Overdrive/Distortion
« Reply #7 on: Time Format »

Ah you said that the OD1 led is lit?

You can can try to raise the OD1 trimpot,there's a pic on the forum with a list of the trimpots.Just do a search.
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Billy Penn

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Re: MP1 No Overdrive/Distortion
« Reply #8 on: Time Format »

The OD LED I was referring to is actually the Dist LED. Sorry for any confusion. I am not familiar with this unit. I will check out your trimpot suggestion. If U4 was bad would I get any sound at all? Thank you very much!

Ah you said that the OD1 led is lit?

You can can try to raise the OD1 trimpot,there's a pic on the forum with a list of the trimpots.Just do a search.
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MarshallJMP

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Re: MP1 No Overdrive/Distortion
« Reply #9 on: Time Format »

Don't think U4 is the problem, else you would have no sound at all.

I attached the pic,quality isn't great but just search for the OD1 trimpot,you can turn it up and see if that helps.If you turn it fully CW and you set OD1 at 8.0 or more the OD1 clip led should go on when you play.
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Billy Penn

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Re: MP1 No Overdrive/Distortion
« Reply #10 on: Time Format »

Thanks for the photo. That does help. Adjusting the OD1 gain at 10 and turning up the OD1 trimpot gets me a little more gain but does not light up the clip LED. The trimpot doesn't affect the overall gain or volume much at all. When I first power up the unit all four clip LED's do blink. Any more thoughts on this? Thanks again..!

Don't think U4 is the problem, else you would have no sound at all.

I attached the pic,quality isn't great but just search for the OD1 trimpot,you can turn it up and see if that helps.If you turn it fully CW and you set OD1 at 8.0 or more the OD1 clip led should go on when you play.
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MarshallJMP

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Re: MP1 No Overdrive/Distortion
« Reply #11 on: Time Format »

It's normal at startup that the 4 clip leds blink.

What input jack do you use? Front or rear?
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Billy Penn

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Re: MP1 No Overdrive/Distortion
« Reply #12 on: Time Format »

I'm using the front input jack. This unit just seems ot have like half the overdrive gain it should have for some reason. Any more help is appreciated. Thank you!


It's normal at startup that the 4 clip leds blink.

What input jack do you use? Front or rear?
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MarshallJMP

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Re: MP1 No Overdrive/Distortion
« Reply #13 on: Time Format »

Does it have the topswitch?
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Billy Penn

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Re: MP1 No Overdrive/Distortion
« Reply #14 on: Time Format »

Yes. One position is low output and the other position is higher volume output. Does not change the gain however.

Does it have the topswitch?
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