Any tube amp gurus here?

My Mesa Mark V seems to be losing power/sheen for a little bit and then comes back. What do you think? Power tubes? Preamp tubes?
Amp is about 4 years old.
So far did pencil tap test and changed one preamp tube that was noisy.
When amp is cranked up it runs fine for a half hour and then does this thing for a little bit and comes back on.

How old is it? I have a fender tube and it gets weird if there is dust on the inside electronics. It’s an older one. I have seen many photos of people who had mice make nests in there amps and that caused issues as well.

I can’t help you. I had a Fender amp back in the mid 60’s I played for many years. We were pulling a trailer with our group equipment to a wedding gig and the trailer left the hitch and went into the woods. My fender amp was mangled and my ES335 with the neck broken off. I do remember i think the tubes were 6l6gc . Long time ago.
That’s all I got. Good luck

So maybe power tube(s) getting hot? I’m no expert at all. But have you watched the tubes when this happens? Sometimes one will glow brighter or dimmer without making noise. Have you tried replacing some or all the power tubes with ones you know are good? Tubes only last so long, four years of regular use might be it depending how much you use it and how hard you push it.

What is the pencil tap test? It does sound a little familiar, but I’ve never tried it myself.

If you have used the amp a lot and driven it hard, it’s possible you may need some new power tubes. Especially if you had a preamp tube with issues - those tend to last much longer than power tubes AFAIK. I think you’re supposed to replace the whole power tube set, not just one tube. It can throw off the balance and other specs if a new tube goes in with the old ones.

An amp tech would be your best bet, I’m sure you can find one not too far away, and I think they only charge $50 to look at it. Then you can replace tubes yourself if you want, if that’s the problem. And power tubes aren’t as expensive as I thought they might be, but of course it depends on the brand, quality, and specs you want. I believe NOS (new old stock) is a ‘thing’, but that may be pricier (really old mfr tubes that are still new).

Since it’s half an hour before you have problems, it does sound like a heat issue with one or more power tubes. Problems are more likely to show up when everything gets hot - at least if it’s just a power tube starting to go bad.

If you don’t want to hire an amp tech, you could buy a tube tester (meter) and see if you can troubleshoot that way.

Good luck!

You might want to check for cracked solder joints on the bottom of the pcb board. High temp electronics are prone to cracking solder connections. Touch up any bad looking joints with a 60/40 flux solder and a 60+ watt soldering iron.
Be careful due to the high voltages that might still be present even if the amp if turned off due to capacitors storing energy.

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Yeah, I read about that. I got in the habit of putting my amp in Standby mode, turning the Power button off, then switching Standby back to “on” (as if the amp were on and active) and leaving it there for 30 seconds at least. Then turning Standby back “off” (in Standby mode) for the next time I power on the amp. What I read said this will drain the capacitors. Not sure if that’s true, but that’s what I read on a guitar or amp tech forum some years ago.

That’s why it’s probably better to hire an amp tech to look at it, in case he needs to check the board.

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It is a complicated amp - 3 channel voicings per channel. 2 channels, with one master eq and 2 settings for reverb on each, with 2 boosts (one boost per channel), 3 wattage modes per channel…so the schematic is basically not our granpas’ Marshall Plexi.

Mesa suggested I swap tubes first, especially as the EL84s are much smaller compared to EL34s and the amp was most likely due for a tube change anyway.

OK so far retubed and running OK. Swapped most of the tubes - 6 preamp tubes got a little expensive so went for 4 preamp tubes and changed all 4 EL84s (power tubes). One brand new power amp tube crapped out of the box…JJ tubes = low quality control :slight_smile:
It does sound better but I will have to put it thru the stress test in rehearsal.

I did not have any lit tubes before, and beside one preamp 12ax7 being microphonic there were no other issues. The amp does sound better now.

At this point I’ll do a stress test and if all good I’ll stress test it on a few tracking sessions before taking it live, if at all. I use another Mesa combo live, and it’s one with a more Brit voicing, closer to a Marshall. That one runs EL34s and so far has been an absolute champ. If the Mark V has more problems I’ll look for certified Mesa tech.

Thank you all for the help!

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My Marshall DSL-401 (JCM 2000 combo amp) uses EL84’s. I don’t use the amp that much so have never had to replace them, but I’m curious what the EL84 tubes cost, if you don’t mind sharing that. I’m sure I’ll be replacing them someday.

Most things I have read said that JJ tubes were okay. Maybe Groove Tubes are better? They supposedly had (this was some years ago) very stringent quality control, but rather high prices as well.

Mesa amps want you to buy Mesa tubes…but you don’t necessarily have to.

I’ve worked with EH quite a bit and they’re probably my overall favorite EL34s and for 12ax7s there was a certain bright Shuguang 12ax7b (or wb?) that was really good for hi gain amps that are dark. Cleaner and more focused - probably Mullards. The JJ is the lowest quality out there. Probably the testing at the tube store was not as stringent, no idea, but from this place every time I buy there is at least one dud.

JJs EL84s were about $20-25 a piece depending on where you buy.

Groove Tubes were rebranded Chinese afaik as there are a lot of other.

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Yeah, nothing special about the tubes, just their quality control (testing) is rather stringent according to their own marketing info.