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Lets get Technical > All Things Tube All about preamp and power amp Tubes.

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Author Topic: Tube mode sounding awful, need help!  (Read 461 times)

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Layupe

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Tube mode sounding awful, need help!
« on: Time Format »

Hello everybody! First post here but I am a long time reader of the forum.

I have a problem with my new MP1, like the tittle says, the tube mode is sounding really like shit, specially the distortion, the clean tube mode sounds so so, if the gain is low it can be more or less ok, nothing to write home about but better than the tube distortion mode anyway, the solid state mode sounds amazing.

It is the first version with the switch on the upper side, I changed electrolitics on the power supply and the tube board and I also tried different tubes with no luck.

I have a second MP1 that is working good, I measured with a multimeter pretty much every IC on the board and compared the readings between the 2 MP1 and did not find something strange, also the voltages on the power supply seem to be what is expected...

On the tube board I think there is too much voltage drop at Q2 or maybe coming to Q2 but I don't know, the tubes readings on the other hand seem correct to me. I am attaching the image with my voltages along the tube board, the red ones are from the faulty MP1, the black ones are from the working one.

What do you think? Can be a faulty Q2?


http://imgur.com/a/3VCXXzp

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MarshallJMP

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Re: Tube mode sounding awful, need help!
« Reply #1 on: Time Format »

Don't think the Q2 is bad, voltages of the "faulty " look good, you should have 190V on R1 and R4 so that looks good. On the good one the voltage is a bit low at 185V.
You get a difference in voltage because the output of the transformer is different, 209 vs 190 Vac so the regulator has to drop more voltage over Q1 and Q2. This is why you have those different voltages.

On the tubeboard did you also swap C6,7,12,13,16 ?

Check Q3,Q4 these should be 2SK364.

Have you tried to swap the tubeboards, this way you know if it's the tubeboard or not ?
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Layupe

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Re: Tube mode sounding awful, need help!
« Reply #2 on: Time Format »

Hi JMP thanks for answering!

I understand (I think) about Q2, I just thought that almost 30 volts drop there in comparison with 10-20 volts in other areas (due to one of them being the European version and the other the USA version) could be a bad signal.

On the tubeboard did you also swap C6,7,12,13,16 ?
No I did not! Could be related?

Check Q3,Q4 these should be 2SK364.
I will check that they are the right type tomorrow in the morning. When I was with the multimeter I could only read something from them while at solid state mode, is this normal? Should I be able to get voltage readings from them also when switching to tube mode?

Have you tried to swap the tubeboards, this way you know if it's the tubeboard or not ?
No I did not yet, I thought about it but I was hoping to find something obvious elsewhere, It will be my next step.
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MarshallJMP

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Re: Tube mode sounding awful, need help!
« Reply #3 on: Time Format »

Well you have 19Vac difference between the 2, that translates to about 26Vdc, add 5Vdc difference between the MP-1's (190Vdc vs 185Vdc) and you get a difference of 31Vdc. The transistors rarely fail unless you have a shorted cap. I have only seen it once.
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Layupe

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Re: Tube mode sounding awful, need help!
« Reply #4 on: Time Format »

Well, i swapped the tube boards today but still sounding like crap... I was hoping this to be the problem and now it is clear that it's not the case.

Q3 and Q4 are the 2sk364 type.

Any ideas of what else to look for?  :(
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MarshallJMP

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Re: Tube mode sounding awful, need help!
« Reply #5 on: Time Format »

To make sure, you swapped the tubeboard but the MP-1 is stil sounding bad?
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Layupe

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Re: Tube mode sounding awful, need help!
« Reply #6 on: Time Format »

Yes, that's it, I swapped the tube board but the mp1 is still sounding bad in tube mode, in ss mode sounds good.
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MarshallJMP

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Re: Tube mode sounding awful, need help!
« Reply #7 on: Time Format »

The only thing left that is different between tube and SS besides the tubeboard is C31, 29, R69, 70, 73 Q5

Is Q5 a 2SK364?
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Layupe

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Re: Tube mode sounding awful, need help!
« Reply #8 on: Time Format »

Is Q5 a 2SK364?

Yes it is a 2SK364

Today I was measuring voltages and I accidentally shorted pins on U4, now there is no sound and OD1 led in front panel stays on all the time  :facepalm: I feel so stupid!

I guess I need to get a new U4 now, I hope I just f*cked this up and not something else...
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MarshallJMP

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Re: Tube mode sounding awful, need help!
« Reply #9 on: Time Format »

Is only the OD1 led on, the rest is not?

Measure pin 1 of U4, is there a DC voltage on this pin?
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Layupe

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Re: Tube mode sounding awful, need help!
« Reply #10 on: Time Format »

Hello, yes only OD1 led is on.

After this happened I measured 13.5v more or less (I don't remember exactly but was 13v and something, I can check again) on pin 1 on U4.
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MarshallJMP

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Re: Tube mode sounding awful, need help!
« Reply #11 on: Time Format »

Strange that it died when shorting pins, which pins did you short?
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Layupe

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Re: Tube mode sounding awful, need help!
« Reply #12 on: Time Format »

Well I am not sure that it is the U4 that died, but after shorting pins 3 and 4 I think (could have been 2 and 3) the sound went away and OD1 led keep on all the time. I can still hear some hiss if I put volume all the way up by the way.

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Bazza

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Re: Tube mode sounding awful, need help!
« Reply #13 on: Time Format »

I could be totally wrong but my MP1 sounded like a cheap practice amp till a local tech recapped the power section now sounds killer. He said he found a couple of caps that where dead. I’m no tech but that’s my 2 cents.
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Layupe

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Re: Tube mode sounding awful, need help!
« Reply #14 on: Time Format »

Yes, it makes sense, I just changed the big caps on the power supply and tube board but I would not be surprised that there is a bad one between the smaller ones, I need to order some of them and try out.

I just worry now about the new problem, on weekend I will have some time to put on the mp1 and let's see  :thumb-up:



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