Mark IV A - Reverb question

[michelj]

I just joined last week and have been doing a lot of reading. Great stuff!

Last week, I purchased a Mesa Mark IV A short head (serial nr in the 4000 range so circa 91-92 I guess). Cosmetically, it's in great shape and I was able to get some good tones except for some crackling and popping which I cleared up by swapping out some power tubes with ones that I had lying around. The previous owner had the amp for quite a long time and never replaced the tubes so I'm going to give it a complete retube with some fresh ones.

All of the controls and knobs seem to work except for a couple of things that have me puzzled.

First, the reverb seems to act more like a treble or presence control. Does anybody have any thoughts on why that would be? I swapped out the V4 preamp tube and still the same thing. I would expect some really lush reverb especially when it's cranked but it's not there.

Second, how subtle are the presence knobs on the Mark IV? It doesn't seem to have much impact when I twiddle them up or down on all channels.

I'm not much of a techie so depending on what you folks tell me, I may have to take the amp for servicing by a tech to figure this stuff out. Do any of you any good techs around my neck of the woods (Ottawa, Ontario, Canada)

Thanks in advance Michel

 

[michelj]

I just joined last week and have been doing a lot of reading. Great stuff!

Last week, I purchased a Mesa Mark IV A short head (serial nr in the 4000 range so circa 91-92 I guess). Cosmetically, it's in great shape and I was able to get some good tones except for some crackling and popping which I cleared up by swapping out some power tubes with ones that I had lying around. The previous owner had the amp for quite a long time and never replaced the tubes so I'm going to give it a complete retube with some fresh ones.

All of the controls and knobs seem to work except for a couple of things that have me puzzled.

First, the reverb seems to act more like a treble or presence control. Does anybody have any thoughts on why that would be? I swapped out the V4 preamp tube and still the same thing. I would expect some really lush reverb especially when it's cranked but it's not there.

Second, how subtle are the presence knobs on the Mark IV? It doesn't seem to have much impact when I twiddle them up or down on all channels.

I'm not much of a techie so depending on what you folks tell me, I may have to take the amp for servicing by a tech to figure this stuff out. Do any of you any good techs around my neck of the woods (Ottawa, Ontario, Canada)

Thanks in advance Michel

I forgot to mention that I had checked all the connection into and out of the reverb and they seemed fine. Well this morning, I pulled them out and plugged them in several times and voila reverb is back with a vengeance. I'll clean them up with some contact cleaner.

As far as the presence it concerned, it seems to have dropped out of sight. :lol: It doesn't seem to work on any of the channels.

Michel

 

[Shep]

have you tried replacing all the pre amp tubes as well..that might help your reverb..

 

[dancullinane]

I had the same problem on a IIB. bad contacts on the reverb cable were making the reverb knob act like a gain boost (in a bad way). A little cleaner can do wonders

 

[michelj]

have you tried replacing all the pre amp tubes as well..that might help your reverb..

Hi Shep,

the day after replacing v4, I jiggled the reverb input and output cables and the reverb came back in all its glory. Even though this amp is in great shape cosmetically, I think it's probably got loads of gunk, dust, oxide and whatever on the inside. It's going to need a good cleaning.

I still haven't figured out what's happening with the three presence knobs and the pulls for R2 and LD. They hardly seem to have any impact whatsoever. I'm looking at the schematics. Haven't done that in years.

Needless to say, I'm quite impressed with the tones I'm getting, especially with my PRS. That's the guitar that seems to bring out the silkiest and smoothest tones on all three channels. I've also got a couple of Godins as well as Gibson and Epiphone Les Pauls but they don't sound as sweet as the PRS. Maybe if I twiddle with the amp settings, I may find them their own special niche. It's alot of fun.

take care, Michel

 

[michelj]

I had the same problem on a IIB. bad contacts on the reverb cable were making the reverb knob act like a gain boost (in a bad way). A little cleaner can do wonders

Hi, I think I may be spending alot of time cleaning my latest acquisition. Even though, it's in great shape cosmetically, I think it probably sat in a corner for the longest while and gathered dust, oxide and whatever. I was having problems the other day when I was plugging stuff into the effects. Everything would work fine until I pulled the plugs out and then the amp would go dead or would whimper out some barely audible distorted sound ( no matter what channel). I put some contact cleaner on a qtip, cleaned out the sockets and then everything was ok.

Take care,

Michel

 

[metalmm785]

Hey Michel,
Just curious if this affected all your channels or only your lead channel? I have the same problem, but it only affects the lead. I tried putting some contact cleaner where the white cable connects to the amp and still nothing
My reverb unit is covered by some kind of leather material, so i didnt remove it because it seems taped/straped down there. I have the combo version. Thanks for any ideas.
Mike

 

[michelj]

Hey Michel,
Just curious if this affected all your channels or only your lead channel? I have the same problem, but it only affects the lead. I tried putting some contact cleaner where the white cable connects to the amp and still nothing
My reverb unit is covered by some kind of leather material, so i didnt remove it because it seems taped/straped down there. I have the combo version. Thanks for any ideas.
Mike

Hello Mike,

my problem affected all the channels and was solved when I plugged and unplugged both the white and gray cables several times. That must have gotten rid of some oxide that had formed there. The reverb now works great on all channels. In my case, I have a Mark IV A so the reverb is not switchable so I actually use a reverb from a multi-fx in the loop because that's more convenient. There is a fix to use a footswitch with the Mark IV A tocontrol the reverb but I haven't bothered implementing it.

I'm no tech but looking at the schematics for a Mark IV A, there doesn't seem to be any channel-related circuitry that would affect what's going into the reverb, what's going on inside the reverb, and what's coming out of the reverb. So if the reverb works on R1, it should work on R2 and LD and vice versa. The Mark IV A uses preamp tube V4 for the reverb and V3 for the Lead channel whereas the Mark IV B uses V3 and V4 as specified in the owner's manual for both lead and reverb. So if you have a Mark IV B ( i.e. circa late 92 to present), then maybe there's something funny with one of those tubes that is causing your problem.

Just a thought,

Michel

 

[metalmm785]

Thanks for the suggestion bud, I will give it a try when i get back home.
Mike