Brand new Mesa Mark V 25 all of the sudden operating at 1/3rd volume

[Denver Cole]

Hey guys!

 

I recently bought a brand new Mesa Mark V 25 head and a Mesa 1 x 12 mini recto cab. I used it for several gigs and have probably put nearly 20 hours on it with zero issues.

 

Today I plugged my guitar straight into it, the amp operated at normal volume, and then I unplugged to plug into a tuner on my pedal board (which was not plugged into the amp), tuned the guitar, and plugged back into the head. It worked, but only about a third or less of the volume it has always operated at.

 

I think when I first plugged it in, I had cables plugged into the FX loop send and return that were running into nothing. I had no signal and just unplugged them and there was my full signal at full volume. The I used the tuner, then the problem started. Are you not supposed to mess with the FX loop jacks when the amp is running? Did me doing so mess something up?

 

Does anyone have any experience with this sort of issue? It's as if I'm running an attenuator, but I'm not.

 

Halp!! Thanks!

[onelife]

There is a half power switch for each channel of that amp. Is it possible you switched one of them to the 10 Watt position?

 

Some of those lower powered Boogies will still run on one of the two EL84s - if one of them stops working. The Boogies that I have used have a tendency to eat EL84s so my next thought would be to check the power tubes.

 

 

 

 

[Denver Cole]

There is a half power switch for each channel of that amp. Is it possible you switched one of them to the 10 Watt position?

 

Some of those lower powered Boogies will still run on one of the two EL84s - if one of them stops working. The Boogies that I have used have a tendency to eat EL84s so my next thought would be to check the power tubes.

 

 

 

 

[uSER=62663]onelife[/uSER] Thanks man! There is a 10/25 watt switch for each channel but even on 10 watts this amp gets really loud, usually... Right now it's at maybe 1/4 of the volume it was when I first plugged in tonight. I checked the EL 84s and they look fine both are glowing the same with no strange colors. I tapped them with the tip of a pencil and didn't hear any pings or weirdness.

 

It's a great amp but the layout is interesting in that the two channels have independent masters, as opposed to one master volume to control both channels. Whatever's going on with it, the clean channel's master has barely any effect on the volume, while the gain channel's master has some and seems to be in the same "normal ratio" (if that makes sense).

 

Maybe could be the preamp tube that runs the clean master volume is dying/dead? I base this speculation on no knowledge whatsoever.

[WRGKMC]

Since the only thing you did was unplug the guitar, it sounds like you have a partial guitar cable short.

Try the easiest solutions first. Swap the guitar cord with a known good one. If the problem goes away, open the plug covers on the cable and see if you have any cats whiskers shorting out. Make sure the hot wires are completely separate from the ground.

 

Also check and see if the tip and sleeve of the jack isn't loose so you can twist it. Cheap connectors are riveted together and it only takes one tug and they crappy plugs loosen up and start having connectivity issues. I only buy high grade non riveted, military grade plugs any more. I have some that date back to 1950's I've used all my life. The cables can go bad but the plugs wont.

 

When I make my own cords I use hot glue over the connections then wang the cap on there. It makes the connections last 2~3X longer and works better then heat shrink tubing ever could. This way if I trip on the cord and give it a tug, the cable doesn't pull the wires out.

 

Of course the cord can have an internal short anyplace. Cheap wire can be at fault. the worst cable killer is using a guitar cable for a speaker cord.

Guitar cords are designed for less then 1 volt. Amps put out enough voltage and current to cook the cable from the inside out making it completely unreliable. It melts the insulation away from the wire letting oxygen get to the copper. Then the copper oxidizes and then the cable can become noisy and crackle, loose conductivity or just fail expectantly. I've even seen cords that become so microphonic that when you bounce them off the floor you can hear it in the amp.

 

Check your speaker cord too. Make sure its a two conductor speaker cord that can handle the wattage, and never use a guitar cord to connect a head to a cab. You will slow fry the cable and take a chance on damaging the head.

[onelife]

The Master controls should simply be called Volume on those amplifiers.

[Denver Cole]

Since the only thing you did was unplug the guitar, it sounds like you have a partial guitar cable short.

Try the easiest solutions first. Swap the guitar cord with a known good one. If the problem goes away, open the plug covers on the cable and see if you have any cats whiskers shorting out. Make sure the hot wires are completely separate from the ground.

 

Also check and see if the tip and sleeve of the jack isn't loose so you can twist it. Cheap connectors are riveted together and it only takes one tug and they crappy plugs loosen up and start having connectivity issues. I only buy high grade non riveted, military grade plugs any more. I have some that date back to 1950's I've used all my life. The cables can go bad but the plugs wont.

 

When I make my own cords I use hot glue over the connections then wang the cap on there. It makes the connections last 2~3X longer and works better then heat shrink tubing ever could. This way if I trip on the cord and give it a tug, the cable doesn't pull the wires out.

 

Of course the cord can have an internal short anyplace. Cheap wire can be at fault. the worst cable killer is using a guitar cable for a speaker cord.

Guitar cords are designed for less then 1 volt. Amps put out enough voltage and current to cook the cable from the inside out making it completely unreliable. It melts the insulation away from the wire letting oxygen get to the copper. Then the copper oxidizes and then the cable can become noisy and crackle, loose conductivity or just fail expectantly. I've even seen cords that become so microphonic that when you bounce them off the floor you can hear it in the amp.

 

Check your speaker cord too. Make sure its a two conductor speaker cord that can handle the wattage, and never use a guitar cord to connect a head to a cab. You will slow fry the cable and take a chance on damaging the head.

 

[uSER=103821]WRGKMC[/uSER] Thanks for all the info man! I really want to start making my own cables and the hot glue/military grade plug tips are really good.

 

Here's the update:

 

Took the amp to my guy yesterday and he very graciously took the time check it out for me. The first thing we noticed was that the amp was only outputting 1 watt when set for 25. This amp is 3 weeks old and I've put < 20 hours on her. We first swapped out the power tubes with a brand new matched pair of EL84s and that had no effect so the original ones went back in. Then we started checking the 12AX7s, and bingo, the phase inverter tube was blown. We replaced that and the amp put out 11.5 watts, < 1/2 of the 25 it's designed to run at. Everything else appeared fine but we stopped there as to not void any potential warranty work (it has a five year warranty). Mesa is closed Fridays but we're going to give them a call on Monday and see what we can do. I've never bought an amp off the showroom floor, and while I understand that all tube amps require care and maintenance, this is kind of a drag. It sounds so great!

[Denver Cole]

Yeah it's an interesting layout for sure. I always played Blues Jrs, which is why I think I like this amp with the same power tubes, it's got awesome tone.

[WRGKMC]

If you bought it from a dealer/music store you would bring it back to them, the same way as you would take a car back to the dealership you bought it from (or an on line dealer is they have a return policy) Its then the dealers responsibility to repair the unit or replace it.

 

If you bought it directly from Mesa, then the warrantee would be handled through them directly. You should go to their site and see if tubes are covered.

 

Also I'm curious to know how you came up with the wattage. This isn't something everyone knows how to do and its only guess work if you try and use your ears. To calculate wattage you need to feed the amp a fixed frequency test tone while watching the output on an oscilloscope until it just begins clipping. Then you'd measure the current and voltage through and across a fixed resistance.

 

Also, Most amps manufacturers use 12AT7 tubes for inverters including Mesa, not 12AX7.

A 12AX7 may work but there can be a difference with how well some amps work with those as a substitute.

One of the best 12AT7/s inverters you can use are the NOS Philips military grade which you can buy here. https://www.tubedepot.com/products/12at7wc-jan-philips

 

The ratings on these blow all others away without busting your wallet. The site I posted has a preamp tube comparison chart so you can compare them yourself. Use the drop down here. https://www.tubedepot.com/tube-comparison-tool

 

[onelife]

If the rsistance is known, then measuring the voltage is enough to determine power (and it's much easier than measuring current).

 

The Mesa/Boogie amps that I have seen with EL84s use a 12AX7 as a phase inverter.

[onelife]

Have you noticed a bit more hum coming from the amp since the problem manifested itself?

 

According to documentation from Mesa, one way to test for a noisy EL84 is to (put the amp on standby) remove one of the tubes then run the amplifier. Mesa says it is safe to run the amplifier this way for a short time.

 

Perhaps there is a problem with one of the sockets for the EL84s - effectively removing the tube from the circuit and causing the loss of half the power.