Is my amp is dying ?

[NerNer59]

Hi guys !
I have to say that i really like this forum, really uselfull, but I didn't found any answer to my problem in previous threads so i've decided to create a new one.

So, here is my problem. I've bought a used MarkV 25w 3 months ago.
All was extra, the tone was just perfect, but this last week, i've noticed some strange things.

1) When a metalic part of my guitar, like a tunning key or a string touches the "channel" button of my footswitch, the amp instantanetly starts changing channel at the speed of light, like 10 times in 0.5 seconds doing a noise like " prfsssstprfsssrtprsfftr ". It happened 1 or 2 times, and I do not understand why. I also do not know if it always did that or not.

2)The tone, which was absooolutely great, started to get fuzzy ( not directly because of the first problem. ), I can clearly hear 20% less gain than before and it's very dirty.
The sound is like an overdriven dirty fuzz, nothing to see with what it was a week ago with the EXACT SAME SETTINGS :/
I can also hear some extreamly high "fizzy" frenquency which weren't present in the tone before.

For the second problem, i'm suspecting a dead tube problem, is it possile ? And if it's possible, how to know which tubes are dead or not ?

The "lift/ground" switch doesn't change anything to the problems I've exposed.


Thanks for your help guys !

 

[Monstercastle]

1) Stop touching your guitar to the footswitch. 8)
2) More likely than not it's tube related. Assuming all other things (cables, pedals, everything in chain) are good, I would pull the chassis and reseat the preamp tubes. If that doesn't help, try swapping the pre-amp tubes around (v1 with v4 for example) and see if the problem changes. That will help identify what tubes are bad.

 

[NerNer59]

Hi!
I prefered to be safe and went to see my guitar tech.
He suspected a mass problem + some dead tubes.

If i have to replace it, What tubes would you advice me to choose ?

Thanks for your help !

 

[Monstercastle]

I may be in the minority, but I'm sticking with Mesa tubes in both my Mark V and V:25. I love the tones with those tubes, and see no need to play the tube rolling game I did with my other amps. Can't improve on perfection.

 

[bandit2013]

I would agree with Monstercastle with the brand of tubes. Stick with the Mesa brand if you want to keep your warranty.
I have experimented with all other brands of 12AX7 many times and had best results with the Mesa 12ax7. You could opt for an Mesa SPAX7 for V1 (similar to the 12ax7 but seems to have a slightly different tone than the standard 12ax7 (use this in both my Mark V and Roadster).

As for the EL84 there are a few options. I believe the Mesa brand of EL84 is a Sovtek. There is also Mullard EL84 made by the same company. I prefer the Mullard EL84 in my Carvin V3MC (what I can do with this amp is set the bias, you cannot do this with the Mark V:25 so stick with the Mesa EL84 since it is selected for a gain range that would be suitable for a fixed bias amp).

Reason why you had experienced a bad thing when you touched a metal part to another metal part on the foot switch: the strings of your guitar are not isolated (unless you are using EMG or other active pickup that requires disconnection of the ground wire to the bridge ), in fact they are tied to the chassis ground of the amp once you plug in. For any reason the switch button you touched is not chassis grounded, you are basically shorting out the switch though the guitar which may cause malfunction of the logic chip used to control the channel selection. The ground lift switch will not isolate your guitar from chassis ground. It is used for the cab clone and a means to lift the ground on the XLR cable if you experience ground loop issues. Make sure your wall outlet has earth ground (do not plug into a 2 prong outlet with an adapter as you may end up with line voltage on the chassis vs neutral (which is common to ground but passes current)).

 

[NerNer59]

Hi Guys !!
First : thanks for your reply !

I'm still waiting news from my guitar tech, but I'm very impressed by the speed and the accuracy of your replies guys.
Bandit2013, your explanation about my footswitch problem convinced me. If I undestood what you said in your message ( sorry, i'm not native English, i'm French ) my footswitch " problem " is a normal thing and the amp isn't dysfunctioning at all. And the " solution " would be to " chassis ground " this channel button ( if it's possible and safe ofc ).

For the tubes, I still wait for news from my guitar tech.

Ps: You said that we couldn't set the bias on the Mark V:25, so if I understand well, the tubes which fits the best with that head are the mesa boogie tubes. No ?
And if I put an other brand in the head I have no way to adapt the bias for it, right ? ( these question are important because my tech works with specifics tube brands he advices me to try, but always asks me if I want to replace tubes by a specific model. )

Thank you guys !!! You are the best !!!!!

Ps2: This head is a new model so my guitar tech never touched one before. It's a new model for him, and I fully trust him but would you know where I can find plans of the head to ease and speed up his work ?

 

[bandit2013]

The footswitch should be fine as is. More than likely all metal parts exposed to human touch should be chassis ground (this will also be earth ground). That is a UL/ETL/CSA/and CE requirement. The amp would not have the certifications if it was not. I would first check your guitar to verify that the strings or bridge is connected to the signal ground (multi-meter set to ohms should read a very low resistance value) if one probe is on the string and the other on the metal instrument cable outer metal sleeve. As I mentioned previously, if you have active pickups (EMG) the strings and bridge should not be connected to the signal ground of the guitar.

I do not recall any amp that Mesa has built to have a bias adjustment. Mesa only builds fixed bias amplifiers. Since the Mark V:25 is new on the market, there may not be enough data shared between vendors and the amp manufacturer what bias, current and applied plate voltage is used. Typically when I order tubes for my amps, I request tubes for the specific model I am planning on using them In. Different tubes even from the same manufacturer will have a different cathode current than what was originally designed for. In essence, Mesa tubes are selected such that they will perform well within the amplifier bias settings without tube failure. It is possible to get other brands of tubes if the characteristics are known for the amp. For now I would stick with the Mesa brand. Are you having issues with the original tubes? It is possible that when you touched the footswitch with your guitar string or tuning peg you may have caused some issue with the first preamp tube. One question: do you have active pickups on your guitar? if so, when was the last time you change the battery in the guitar? (this is not applicable if you do not have an active preamp or pickups in your guitar).

Are you in France (or other European Country) or in Canada?, USA? If you are in Europe I would hope the amp was an export model. There should be markings on the back of the amp what the operating voltage should be. I would send Mesa an email for further help in this matter. Also while your amp is with a tech, if you took in the footswitch and cable, have that checked too. The switch should be grounded and that could be determined with a simple impedance check. Also consult Mesa on that issue as well as they have more knowledge on the amplifier. I would definitely have your tech make contact with Mesa if he or she has never worked on such an amp before.