Mark V briefly loses power, then comes right back on?

[tele_jas]

The last few weeks, I've noticed when playing my amp that it will "cut out" for about a second then come back on. It's happened maybe 4 times?.... I'm not playing very loud when this happens and it's usually within the first 10 minutes after turning on the amp.

At first, I thought it was my guitar cable or something like a cold solder joint in my guitar jack and I was moving it wrong causing it to short out for a second. Then just last Friday I happened to be looking at my amp when it did it this time.... All my lights went out for a fraction of a second, then came back on. It made a few click (like some internal switching). I played it for another 20-30 minutes afterwards with no more issues? It did the exact same thing just a few minutes ago too?

I'm always on channel 3 when I notice this, running in 45wts in Mark IV mode with tube rectifier. I don't have the footswitch or footswitch cable plugged in.

I'm pretty experienced with tube amps and have had tube issues with other amps in the past, but nothing intermittent like this?

Any clues as to what it could be?

 

[Giz-Tone]

My amp was doing this for awhile. I'm not sure what was causing it, or if I cured it. All I did was to give the tubes a little push, making sure they were all the way in. The amp hasn't cut out since. Maybe the tubes got loose during a car journey, maybe not. All I know is that it's working grand now.

 

[Giz-Tone]

If you try my suggestion could you please let me know how you're getting on? If it's still showing the same problem, or working fine.

 

[tele_jas]

If you try my suggestion could you please let me know how you're getting on? If it's still showing the same problem, or working fine.

Thanks!

I posted this on TGP as well and a couple people suggested that too, so Tuesday evening I:
* Pulled every tube and re-seated them (a couple times)
* Unplugged the power cable from the chassis and re-seated it
* Rotated every knob on the back about a dozen times
* Unplugged the speaker cable and re-plugged it back in about 5-6 times
* Plugged & unplugged a 1/4 cable into every jack on the back several times each
* Plugged in the footswitch cable a couple times

When I played it after doing all of this, above, the amp didn't cut out any....But, there were times before this that it didn't cut out either. It was so sporadic before, that it may take a few weeks to see if this worked or figure out?

I'm running a new set of JJ 6L6 power tubes (maybe 40 hours on them).....My next step is to put the Mesa power tubes back in, then to flip over the solid state rectifier instead of the tube rectifier.

Of course the warranty ran out last month, which is just my luck.

 

[Giz-Tone]

Yeah it was the same for me. Very sporadic. It's good to know a lot of other people have run into the same problem, and that it sounds fairly common. At least both of our amps are working for now. I'll post on this thread if my amp acts up again. Fingers crossed it won't ;-)

 

[tele_jas]

Same here. I'll keep this post updated as any changes occur.

 

[jb's 52]

I had this happen while at rehearsal. For a quick fix, I reseated the
power cable and unscrewed the fuse/cap and put it back. Power
restored.

When I got it back home, I rechecked the fuse. I had previously
replaced it with an All Parts brand, but found that the fuse was
a little bigger in girth than the ones I bought directly from Mesa.

I took the fuse out and there was some build up on one of the
ends. I replaced this with one of the fuses from Mesa and haven't
had a power issue since.

 

[bandit2013]

Try a different power cord, or a different location as it could be related to a loose outlet terminal or connection in the cord.

Does this occur with full power and Variac setting?

I would also stock up on fuses of the correct type. Generally, the slow blow fuse is spring loaded and should become a complete open in the event of a short (hard or momentary).

For starters, CH3 is hard wired to run in diode mode unless you are using the 10W power setting then it will use the tube Rectifier. CH3 options are Pentode or Triode which operates the power tubes differently. However, I too have had similar issues even to the point the amp just cut out completely. Fuse remained intact. Momentary shorts that will occasionally occur in the power tubes can happen but it is more common to have issues with the rectifier tube. If there happens to be a short (momentary or hard) in the rectifier tube, it will pull the entire power supply to a diode level. Note that the silicon diodes run parallel to the rectifier tube and that the rectifier tube is never disconnected from operation. Only the silicon diodes are switched. Intermittent power glitch may be related to the rectifier tube. If your amp is still having this issue, reposition the amp so you can view the tubes while you play though it. Watch for arc flash or lightning inside the rectifier tube or power tubes. If such a discharge is occurring it will cause a discharge on the large supply capacitors which will take a moment to recharge and regain the 448Vdc. In most cases this will also create a pop sound though the speakers. Sometimes it does not. Perhaps you should consider replacing the rectifier tube. It is possible to operate the amp in 90W or 45W modes (set the switches on the back to diode for CH1 and CH2) and remove the rectifier tube. This will work fine to help you debug the problem. If it still happens it may be a wire or contact related issue or one of the power tubes has reached its end of life.

 

[Giz-Tone]

Thanks jb. I'll give that fuse trick a go.

Thanks a million bandit. That's great advice. I've seen a lot of your posts on here over the years, and they're all extremely helpful, well thought out and to the point. Thank you very much for taking the time to help those of us less savvy with tube amps on this forum.

 

[tele_jas]

Thanks for the suggestions.... Looks like I've got a few things to try/eliminate.

When I find out what exactly it was, I'll post back 8)

 

[bandit2013]

Hey no problem at all. I am trying to help. I too have had the Mark V woes. Also had the same issue as described. Amp would go dead. When this had happened, I replaced all of the tubes with known to be good (old tubes I had removed from a used RA100 I bought and saved them for that just in case moment, also had a few 5U4GB rectifiers on hand). Sure enough the old tubes worked but the new tubes I had in the Mark V did not. As it turned out, the Rectifier tube that I bought had a dead short in it. It kept blowing the fuse. After removal of the rectifier tube all was fine. Note that everything was working properly for a while prior to a similar issue.

 

[tele_jas]

[UPDATE]

A few weeks in and I didn't have any problems until last night, when it cut out again. This time I pulled all the power tubes and replaced them with the set of Mesa tubes I had pulled out when I first bought it last year. I played it for a couple hours after that with no issues.... I guess only time will tell if that was the issue?

 

[bandit2013]

I had some issues with JJ tube but they were 6CA7. As it seemed, one of the tubes was cutting out. I could see it happen, the blue glow was there and then not. The same tube was moved to a different location and the problem followed the tube. I blew out the remaining set instantly in my Mark IV. I do like the Mesa 12ax7 (which are JJ preamp tubes) but have not had any luck with power tubes. Perhaps other types would last more than 2 weeks. The 6CA7 really sounded great but for 2 weeks of operation not worth the cost. The Mark V is biased hot especially the center two tubes. It has a hotter bias on the control grid than did the Mark IV (this was based on voltage divider calculation and not measured in terms of plate current. A -43v bias is almost hot enough to run EL34. Best to rely on a current measurement but on paper it seems a bit hot. The outer pair is running at -47v. I have not had a red plate issue ever since I have used SED 6L6GC, TAD 6L6GCSTR, and the Ruby 6L6GCMSTR seemed to be fine (although same tube as Mesa). They seem to operate fine at idle (or quiescent current) but when used in 45W mode they look like 6V6 (red hot plate seams). So far every Mesa power tube I have used has done this.
When it comes to tubes, make not to the seller that you have a mark V and you want something suitable for a hot bias.

 

[tele_jas]

Well, I changed tubes to stock Mesa tubes around a week ago... Played it a few time since the change, no issues.

Playing tonight on channel 3 and it cut out on me, this time a little longer than before.. Maybe 3 or 4 seconds.

I guess I'll be calling Mesa on Monday to see if it's still under any sort of warranty.... I got it used, but sometimes they just go off the mfg date.

 

[bandit2013]

I hope you get the issue resolved.

 

[tele_jas]

Spoke to Mesa today. They walked me through a few basic things and asked a few questions and got the SN to my amp. He informed me that my amp is still under warranty and so I'm shipping it back in on Friday (after the holiday) to be repaired at no charge (other than shipping it in to them).

I've dealt with several amp makers in the past, Mesa and Dr. Z (and maybe one other boutique maker) are the absolute BEST at customer service. Many companies, large and small, need to follow Mesa's lead in this. Make a good product and stand behind it!

I'll report back in 3-4 weeks on their findings, but they suspect some resistors in the power supply or transformer are going bad.

 

[tele_jas]

**UPDATE**

Sent my amp in to Mesa a few weeks ago, they called me Wednesday with an update:

Said they put it "through the paces" and could not get it to replicate the issue I was having.... But, there were some components in the power section that they upgraded/replaced, and they tested and re-tensioned the footswitch jack and another jack (or a few).

He said they tested it for a while after all this and it did not cut out on them, so they are sending it back to me with a detailed repair sheet.

Mesa CS is great and I can't wait to get it back, but it does make me a little nervous that they never could replicate the issue in-house and I got it quite often. Hopefully the power section update fixed it?

I'll keep ya'll posted

 

[Giz-Tone]

I'm not surprised; the **** thing happens so sporadically. My amp cut out once since I last posted, the one time I wasn't watching the tubes like a hawk. Typical. I'm glad your amp is on its way home to you.

 

[Giz-Tone]

Has your amp returned to you? Any issues since if it has?

 

[tele_jas]

Has your amp returned to you? Any issues since if it has?

Yes, I got it last week!

I replied to a post on TGP, I thought I replied here too but I guess I didn't....Sorry.

Yes, got it and have played it several times and have not had any issues..... I think they fixed it.

They called me right after Christmas and said the replaced some components in the power supply/transformer and re-tensioned my footswitch pin connections. I'm keeping my fingers crossed, but it appears to be fixed.

I'm playing a show this Saturday and gonna use my Mark V, that will be the true test