A Few Mark III Questions (Speaker Load, Reverb)

Hey guys, first post on Boogie Board.

My primary gigging / jamming rig is a Single Rectifier Solo 50 Head through a Laney IRT212 cab. It's a great rig but a fairly cumbersome beast so I typically leave it at the rehearsal space, it's far too much of a stage beast for practicing in an apartment anyway, so last week I set out to find a quality practise amp for at home and ended up scooping a simul-class MESA Mark III Combo for $800 off a local on Craigslist.

As most Craigslist purchases tend to, the deal came with a few (hopefully minor) concerns. The guy let me demo the amp in his garage at full power a bit before sealing the deal, I only cranked the master volume up to about 3 - 4 (which is pretty **** loud on these Mark Series Combos as I'm sure a lot of you know). As I expected I was immediately won over by tone options (only a small fraction of the many I have yet to hear) and was quick to purchase the amp and bring it home.

So I spent a couple more hours dialing in as many tones as I could at the lowest possible volumes with the power stepped down to the ~15W Class A setting, familiarizing myself with the controls. The amp performed very well and I was able to find the tones I was hoping for (or at least close enough considering how low the volume was set).

This morning I was looking over the pictures I took of it yesterday after the purchase and realized that the previous owner has had the Electro-Voice Force 12 speaker (that he threw in some time ago) jacked into the 4 ohm slot, this is an 8 ohm speaker.



And now the questions have started brewing in my head:

This speaker has been here for a long time and he has likely be been playing it this way since the day it was installed, I don't know if he used the amp primarily for practice in his bedroom or for gigging. As far as I know none of the tubes are in bad shape but I haven't thoroughly checked yet, I know for sure he has replaced every tube at least once, the 12AX7 tubes more often than the EL34s and 6L6s, all of his replacement tubes are JJ's. There's isn't an unusual level of noise coming from the amp when it's idle (no noise at all on low volume cleans, minimal noise at low volume leads even when I'm sitting in front of the amp).

I have noticed that the reverb setting is completely unresponsive at practice volumes but the guy insisted it only kicks in at rehearsal volume levels, and that it was normal that way, that raised a bit of an alarm in my head but I have no experience with these Mark Series Combos so I didn't say otherwise. I haven't fully cranked this thing in a band setting yet so I don't even know if what he said is accurate, maybe the reverb never kicks in. Is it possible that having the speaker jacked into the 4 ohm impedance rather than 8 ohm has fried the reverb tubes and he just hasn't clued into it?

Should I have full reverb functionality at low volumes?
The amp seems to be in full working order besides the reverb question, is it likely that his mismatched impedance hookup has caused underlying issues I should pay extra close attention to?

Thanks a lot, sorry for the long-winded first post, but I figure Boogie Board is the best place to go for questions like these.

I would have to say the reverb not being on at low volume is a problem,but the spkr impedance mis match is not the cause of it,nor is it a problem.I have used my 8ohm spkr in the 4ohm out jack for over 25yrs in my MKIIC+.The reverb should work at any volume,I would have it checked out.

That is odd that the reverb is not working at lower volumes. Question: in the photo there is a cable going out of the send jack. Where is it going to? perhaps this may be the cause of your reverb issue.

In any case, budget a couple hundred for some upkeep. That amp was built in the 80's and if it has not been maintained it likely has a few issues that should be addressed.

I remember the "Force 12" speaker. I bet you would love the amp more with a EVM12.

Mesas don't really care about impedance mismatch. Check your preamp tubes, I believe one of them is dedicated to the reverb. For what it's worth, the reverb on my Mark III head is actually pretty weak even with a good tube.

The reverb tube (12ax7) is the leftmost looking from the back.
BTW the reverb is a little unusual on a mark iii in that is is never off unless the reverb pot is set to zero.
If it is working normally it also works differently with the footswitch unplugged.
I presume you have a manual and have noticed that the f/s actually switches between 2 different levels.
On mine reverb level 3 (f/s unplugged) is about the same as 5 with the footswitch and full reverb selected. I tend to run mine at 7 to give Peter Green levels of reverb on full with just a hint on low reverb.
If yours is really weak there are a couple of possibilities:
1. The reverb leads have been accidentally reversed. On mine, grey is nearest the front.
2. There is oxidation on the reverb RCA phono plugs/sockets. Try a squirt of deoxit and a plug wriggle.

As for the speaker jacks, in the owner`s manual is is written the original speaker, EVM 12L can be plugged carelessly in all three jacks, even more, they suggest to experiment.........
I don`t know if this is outdated or still valid, but I think so. I use them all correctly ever since.......
Techs in general in the past always advised me to use the jack with the correct impedance at any rate.

As for the reverb, my both IIIs, always crancked up fully, work on the same level regadless of the dialed in loudness.
But what makes me still wonderous, crancked up fully on both, the reverb intensity on my first is about 1/3 or a bit more weaker than on the second.
As I bought the second used and didn`t ask about maybe there had been done a mode on, or as MESA always claims, while building one amp after the other there always had been done modes from time to time by their own.
But what you are telling seems NOT to be O.K. at all with your amp.
As already said above it`s one of the best advices to remove the can and the grey/white cable and clean all the jacks and plugs thoroughly, as like I did the experience they look TERRIBLE after a long time having been uncleaned.
So it`s very likely that this might help.
Tube sockets also should be cleaned with a non aggressive deoxite spray.

Threw a new 12AX7 in the reverb slot (furthest left while looking in the back) and it is certainly functioning now, thanks fellas.

Just in case.....

This may help depending on what you find:

The reverb on my Mesa Mark III went out. Changed tubes, check connections, tried changing rca cords on the reverb, pulled the tank and checked for wiring problems, cleaned the eq/reverb jack that I don't use.... Suggestions, recommendations?

gts said:

You probably have a grounding problem. Happens all the time.
All works fine, then poof it stops for no apparent reason.
Do this:
Then report back.

Click to expand...

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Also fwiw the reverb in Mesa Mark series amps is pretty weak as compared to many other amps.