MT15 suddenly no sound

[Morgaad1]

Have been playing it just fine and then one day turned it on and just no sound from speaker. I checked fuses. Swapped some tubes around. Re seated all tubes. Just guitar plugged straight in and MT 1 x12 cab with good speaker cable. Lead channel no noise, but a sizzle sound when gain is cranked near output transformer. Also on clean channel cranked makes same noise when I strum guitar. Tubes glow when warmed up. I dont see any burn marks on anything on the board it all looks fine. Any help or ideas what to check next would be great thanks

 

[Bogner]

Tubes glow? Which tubes? Pre or Power or both? How experienced are you at messing around inside amps? Be careful!!

Get a wooden chopstick or pencil and tap the power amp tubes and see/hear what they do. I am guessing it is a tube issue of some sorts initially. Double check fuses. If fuses are good and the tubes don't give you any indication upon the tapping of them (power tubes) and you have rotated Pre amp tubes you may need to send it back for repair. Let me know what you discover and I will do my best to help you once you report back on your findings. Hopefully it is simple and we can get it back up and running quickly without issue.

 

[Morgaad1]

Thanks for replying. I have had a couple electronics classes years ago. I also was a an automation tech on automation machinery and robots. I have dealt with big 440 volt high amp things before so I know enough not to kill myself. But never was very good at trouble shooting amps.
When I leave it one for a few minutes all the tubes seem to glow as they normally do. Nothing is obviously hotter than normal. The three fuses on the board are good. I took them out and checked with a meeter. I have been using an antnuator in the effects loop to get house volumes I played it a fee days ago with no issues, sounds great. Turned it on 2 days ago and nothing. No sound. When I crank the gain i can see a little blue spark at the botton inside one power tube

 

[Bogner]

Thanks for replying. I have had a couple electronics classes years ago. I also was a an automation tech on automation machinery and robots. I have dealt with big 440 volt high amp things before so I know enough not to kill myself. But never was very good at trouble shooting amps.
When I leave it one for a few minutes all the tubes seem to glow as they normally do. Nothing is obviously hotter than normal. The three fuses on the board are good. I took them out and checked with a meeter. I have been using an antnuator in the effects loop to get house volumes I played it a fee days ago with no issues, sounds great. Turned it on 2 days ago and nothing. No sound. When I crank the gain i can see a little blue spark at the botton inside one power tube

Tube arc...tube issue. Try the amp alone without the attenuator into your cabinet. Crank the amp and channel volume/gain and let it run (off of standby) a few minutes and see if the tubes do anything different color wise or heat wise. Make sure your guitar volume is off.

 

[Morgaad1]

The testng I have been doing is with nothing but a cab and one cord to a guitar. When i just let it idle with gain under 3 oclock the tubes just glow as nornal. But when I crank the gain the one tube makes that noise and has blue arc. It stops when I turn the gain back down past 3 oclock.

 

[Bogner]

Power tube issue. Push the volume and gain and let the amp think it is going for it and you should see clearly what is going on with the power tubes. Nothing will explode or anything like that. You may blow a fuse worse case scenario. Which power amp tubes in your amp? 6L6 or 5881?

 

[Morgaad1]

The testng I have been doing is with nothing but a cab and one cord to a guitar. When i just let it idle with gain under 3 oclock the tubes just glow as nornal. But when I crank the gain the one tube makes that noise and has blue arc. It stops when I turn the gain back down past 3 oclock.

Actually new evidence. Both channels make sizzle noises when I strum the guitar at any volume past half way. Blue arc is faintly seen with lights off on the other power tube also.

 

[Morgaad1]

Actually new evidence. Both channels make sizzle noises when I strum the guitar at any volume past half way. Blue arc is faintly seen with lights off on the other power tube also.

Nothing changes when I tap with a wood dowel on power tubes

 

[Bogner]

Bad power tubes. How long have you had the amp? Did you buy it new or used? Where did you purchase from? May be ripe for a return/exchange.

 

[Morgaad1]

I got it about a year ago. I traded a guy for my Marshal DSL 40CR. He said it was not that old it has the 5881 jj power tubes and the ECC83S preamps all JJ. Serial number has a 04/20. Not sure if that means it was made in 2020?

 

[Morgaad1]

Bad power tubes. How long have you had the amp? Did you buy it new or used? Where did you purchase from? May be ripe for a return/exchange.

So you think it is the power tubes? I guess it wouldn’t hurt to get some new ones and try it. Thanks

 

[Bogner]

So you think it is the power tubes? I guess it wouldn’t hurt to get some new ones and try it. Thanks

Make sure you bias the amp with the new power amp tubes installed.

 

[Imgettingcloser]

One thing you can check just to eliminate it as a possibility is your effects loop jacks. I had a similar issue with my PRS SE 30 amp. You might want to do a google search but if I remember correctly if you take a patch cable and jump the send and return jacks and it works as normal you have a bad/dirty jacks. If that is the issue you can spray contact cleaner in the jacks and run the tip of a cable in and out of the jacks several times. This should fix the problem depending how bad they are. In my case it did not so I removed the old jacks and replaced them, but I am comfortable with de soldering and re soldering on circuit boards.

 

[Goran Lorencin]

I found that this site really helped me with tube amp troubleshooting https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm
I think mr.Robinette gave us a really good place to learn/read about tube amps and more.

 

[aamefford]

I found that this site really helped me with tube amp troubleshooting https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm
I think mr.Robinette really gave us a really good place to learn/read about tube amps and more.

Nice and bookmarked, thank you!

 

[andy474x]

I found that this site really helped me with tube amp troubleshooting https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm
I think mr.Robinette really gave us a really good place to learn/read about tube amps and more.

This is a great resource, I’ve referred to it many times!

 

[Lewguitar]

When I crank the gain i can see a little blue spark at the botton inside one power tube

My old Fender amps sometimes have a little blue gas in the power tubes.

But a spark is not good and could take out the resistors on the tube socket.

Time to get it looked at...they might already be fried and you might fry a new tube if they are.

Might be too late to just replace the tubes.

 

[Goran Lorencin]

Found two other sites that talk about it. Have you maybe already resolved your issue?

Why is one of the tubes in my amp glowing blue? - SweetCare

A tube that is glowing blue is often wrongly perceived as a defect, however, it’s really just a side effect of a power tube — a fluorescent glow in the blue spectrum. The tube is fine! It actually indicates that the vacuum inside the tube is very good, which is what allows this phenomenon to […]

www.sweetwater.com

My tube has blue lightning in it? WHAT IS IT?

The other day I had a tube redplate in my Dual recto. I heard some static and then humming and I looked at the tubes and one was redplating. I quickly switched to standby and tube went back to normal. I switched it back on and the TUBE WAS FULL OF BLUE SPARKS AND LIGHTNING. I ordered 4 new...

www.thegearpage.net