MT 15 losing power? Help please!

Jobies8i

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May 4, 2023
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So im new to this forum and there might be the answer on here but i dont seem to see the same problem. So my Mt 15 seems be losing power so i thought power tubes. I change power and pre amp tubes and still didnt change. But i seemed to found that if i plug a pedal into the effects loop it seems to be fine and have full power. I started with the one pre amp tube that was for the effects loop then changed them all just now and still no change. Any insight would be great if not eventually i will just take it to my local shop.
 
If i plug a pedal into the effects loop it seems to be fine and have full power.
Try connecting just a guitar cord to the in and out of the effects loop.

If it seems to have the power you're used to, I would then look at the in and out jacks of the effects loop and see if theres a contact not making contact or that needs cleaning or retensioning or replacing or solder or something.

You might need to get inside the amp and take a look.
 
Yes it works when something is in the effects loop pedal or just cable. Without it very low power/volume. Sometimes it will cut in and out.
 
Yes it works when something is in the effects loop pedal or just cable. Without it very low power/volume. Sometimes it will cut in and out.
Then I'd look at the in and out jacks. There might be a contact not making contact in one of the jacks that's there to complete the circuit when nothing's plugged in..

Or for the time being, just plug a real short guitar cord into those jacks when you're not using the effects loop with a pedal.

The shorter the better.
 
Honestly im ok with just putting a patch cable in it, was just curious if anyone else might have had the same problem. Ive taken it apart a few times pushed on the jacks slightly, tightened up the ends checked all the wires and soldered points and dont see anything its all very clean.
 
Spray one of the jacks and a cable end with contact cleaner and run the cable end in and out a dozen times.

Try that twice for each jack.
 
Take it to a tech otherwise your going to be chasing ghosts, changing parts and not having an amp that works.

I suppose it depends on the person. It's much more convenient for me to just clean the jacks considering it sounds like it's the same common problem that Marshalls have. Cheaper to do too. :)
 
I suppose it depends on the person. It's much more convenient for me to just clean the jacks considering it sounds like it's the same common problem that Marshalls have. Cheaper to do too. :)
It's losing power; it's not the Jacks. Tech or warranty ASAP
 
Most Tube Amp Problems Are Tube Related But Not Always. As Was Said...Warranty Or Return If Possible. If You Aren't Knowledgable And Comfortable Working On Amps It Is Best To Take It To A Competent Tech Who Is. It Can Be Dangerous For You If You Make A Mistake. Be Very Careful Messing Around Inside The Amp.

Is There Anything Else You Can Tell Me About The Amp And What It Is Doing (Or Not Doing)? Are Tubes Glowing Oddly? Volume Cut Outs When Turning Dials? Does It Act Differently When Hot And Cold (Turned On A Short Time). Any Behaviors Of The Amp At All You Can Add To The Already Given Description? That May Will Help To Help You. :)
 
Most Tube Amp Problems Are Tube Related But Not Always. As Was Said...Warranty Or Return If Possible. If You Aren't Knowledgable And Comfortable Working On Amps It Is Best To Take It To A Competent Tech Who Is. It Can Be Dangerous For You If You Make A Mistake. Be Very Careful Messing Around Inside The Amp.

Is There Anything Else You Can Tell Me About The Amp And What It Is Doing (Or Not Doing)? Are Tubes Glowing Oddly? Volume Cut Outs When Turning Dials? Does It Act Differently When Hot And Cold (Turned On A Short Time). Any Behaviors Of The Amp At All You Can Add To The Already Given Description? That May Will Help To Help You. :)
Ive had it for maybe 4 years now the first couple it was fine and ive just let it sit without looking into the problem which i should have but this is not my main practice/gigging amp. Ive said in a earlier post that it sometimes cuts in and out which seems to be after playing it for a bit and warmed up. When i changed the power tubes i re biased it and it metered at what i found online it should be. Sad thing is ive asked around and a compentent tech might be over 2 hours away.
 
It's losing power; it's not the Jacks. Tech or warranty ASAP

IIRC OP clarified that sound cuts out and said it works when the effects loop jacks are jumped with a patch cable. That's typical of dirty contacts on switching jacks. Happens with Marshall loops all the time.

A quick cleaning as I suggested fixes it 99% of the time. The other 1% the chassis needs to be removed and the jacks sprayed on the backside where the contacts are better reached.

If OP is capable and comfortable doing so, it's well worth trying before sending it 2 hours away to a tech and paying $100 for the tech to do a simple 5 minute cleaning. At most it'll cost like $20 if contact cleaner has to be purchased and 20 minutes time.

If that fails, sure, take the 4 hour round trip and pony up the cash for a tech repair.

To each their own though.


@Jobies8i , did you have to remove the chassis to set the bias when you changed the tubes? Do you understand that there are lethal voltages inside of tube amp and what to avoid touching?
 
IIRC OP clarified that sound cuts out and said it works when the effects loop jacks are jumped with a patch cable. That's typical of dirty contacts on switching jacks. Happens with Marshall loops all the time.

A quick cleaning as I suggested fixes it 99% of the time. The other 1% the chassis needs to be removed and the jacks sprayed on the backside where the contacts are better reached.

If OP is capable and comfortable doing so, it's well worth trying before sending it 2 hours away to a tech and paying $100 for the tech to do a simple 5 minute cleaning. At most it'll cost like $20 if contact cleaner has to be purchased and 20 minutes time.

If that fails, sure, take the 4 hour round trip and pony up the cash for a tech repair.

To each their own though.


@Jobies8i , did you have to remove the chassis to set the bias when you changed the tubes? Do you understand that there are lethal voltages inside of tube amp and what to avoid touching?
No i did not have to remove chassis on a mt 15 it has external jacks. But i did remove it just to look around. I am aware of electrical componenents storing lethal energy. I will try some deoxIt D5 later this evening on it.
 
No i did not have to remove chassis on a mt 15 it has external jacks. But i did remove it just to look around. I am aware of electrical componenents storing lethal energy. I will try some deoxIt D5 later this evening on it.

Good idea to not poke around inside a tube amp. The MT15 uses 5881 power tubes which are designed to operate on up to 400 volts, but guitar amps with 5881's might run them higher. The capacitors on the power supply can store that voltage (like a battery) so you could have over 400 volts in that amp after the amp is turned off and unplugged. Eventually the capacitors will drain but it's best not to poke around if you don't know how to test for volts and safely discharge capacitors.
 
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