First post here so please allow me some slack, hope to be able to pay it back in the future. I have a Mark iic+(signed) 60/100 no reverb. I'm a Mesa fan 40 years now, this one is by far the best. When it was prime, wow. Played out with it for 20 years but now it's not happy. New JJ tubes today (yeah don't judge me). I don't think it's a power tube problem. Problem is that it has less volume than I would expect (both channels), and then it will drop to nothing at all, and just now sounded ok at 5's across the board and then without warning opened up to a full scream (which is what I would expect at those settings). Preamp tubes? I hate to continue to toss $ into tubes if it's something else. Any help would be much apprectiated.
Mark iic+ misbehaving
- Thread starter RussellG
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That’s sounds like a failing preamp tube in a cathode follower position. Cathode followers are mostly used to buffer an FX Loop. Do you know your preamp tube layout?
Dom
That’s sounds like a failing preamp tube in a cathode follower position. Cathode followers are mostly used to buffer an FX Loop. Do you know your preamp tube layout?
Dom
Dom, thanks for getting back and thank you for the welcome. I have a II-C Maintenance & Repair doc. As far as I know it's all stock. So in the Front Row it lists "Junction: Preamp to Pwr" (no tube listed), then it has a12ax7 in each of V4 (B:Reverb Driver, A: 1/2 Lead Drive Amp), V3 (B: 1/2 Lead Drive Amp, A: Rev Return Amp), "Dual Footswitch" (no tube listed), V2 (B: Mixer Amplifier, A: Effects Return), V1 (B: Tone Control, A: Input Amplifier), V5, (Phase Inverter/Output Driver). It sounds like you might be referring to V2? I guess I could well replace them all, probably overdue, but I'd sure like to find the exact cause.
R
R
A good preamp tube will last a very long time, sometimes a lifetime. But when it stops working it shows things like whooshing sounds, buzz, and volume fluctuations. This usually comes on out of nowhere and just gets worse.
Power Tubes wear over time it can be hard to notice the slow changes. I change mine yearly, but I play about 35 weeks a year (practice mostly once a week 4+ hours and some 45-90 min gigs thrown in).
I would buy a good 12AX7 or two, and starting with V2 replace one at a time with the new tube until I found the culprit.
Yes, many players will just ‘shot-gun’ it with an entire set. Your choice.
Whatever you do don’t use any New Sensor brands with a spiral filament in cathode follower positions, they can’t take the higher voltages used in that circuit and give up the ghost rapidly. Those common tubes would be branded Electo-Harmonics, Mullard Re-Issue, Tung Sol Re-Issue, & Sovtek, as well as the Mesa branded Russian 1 (Sovtek), and Russian 2 (EH). Chinese, JJ (which is the current Mesa 12AX7) and just about any good condition VOS/NOS tube should work fine. I’ve had good luck with JJ’s in my Roadster’s CF, and late ‘70’s Mullard CV4024’s in my TC-100.
Hopefully someone with more experience with the tube positions and functions of your amp chimes in.
Dom
Power Tubes wear over time it can be hard to notice the slow changes. I change mine yearly, but I play about 35 weeks a year (practice mostly once a week 4+ hours and some 45-90 min gigs thrown in).
I would buy a good 12AX7 or two, and starting with V2 replace one at a time with the new tube until I found the culprit.
Yes, many players will just ‘shot-gun’ it with an entire set. Your choice.
Whatever you do don’t use any New Sensor brands with a spiral filament in cathode follower positions, they can’t take the higher voltages used in that circuit and give up the ghost rapidly. Those common tubes would be branded Electo-Harmonics, Mullard Re-Issue, Tung Sol Re-Issue, & Sovtek, as well as the Mesa branded Russian 1 (Sovtek), and Russian 2 (EH). Chinese, JJ (which is the current Mesa 12AX7) and just about any good condition VOS/NOS tube should work fine. I’ve had good luck with JJ’s in my Roadster’s CF, and late ‘70’s Mullard CV4024’s in my TC-100.
Hopefully someone with more experience with the tube positions and functions of your amp chimes in.
Dom
Thanks, yes that all makes sense, I've had to replace power tubes many times over the years but never pre-amp tubes. I guess it's about time for that too. I'll get back to this thread for an update once I try that out. Thanks again.
Blessings everyone. There are no cathode follow circuits in the preamp. Did it start after inserting new JJ's? No worries, I use them too 
. If you can find Jay Omega here, he's very knowledgeable on C+'s. Have the tube sockets been cleaned? Power and preampies? If choosing to troubleshoot w/ preamp tubes, although if its tube related, any tube can be the culprit but always start w/ V1. And with amp powered up, gently tap and gently wiggle the power tubes. Have a finger on the power switch when doing this . If you hear any signs of noise thru speakers, immediately turn off. Cud save your fuse and perhaps 470ohm plate resistors. Of course the same applies for preampies though just check for microphonics, noise....Use a pencil or something gentle to tap preamp tubes.
. If you can find Jay Omega here, he's very knowledgeable on C+'s. Have the tube sockets been cleaned? Power and preampies? If choosing to troubleshoot w/ preamp tubes, although if its tube related, any tube can be the culprit but always start w/ V1. And with amp powered up, gently tap and gently wiggle the power tubes. Have a finger on the power switch when doing this . If you hear any signs of noise thru speakers, immediately turn off. Cud save your fuse and perhaps 470ohm plate resistors. Of course the same applies for preampies though just check for microphonics, noise....Use a pencil or something gentle to tap preamp tubes.
Thanks Henz, it was happening before I replaced the power tubes with JJs. For those following the thread, there actually is no V4 tube, I'm guessing because there's no reverb with the amp. Which means 100% of the Lead Drive is from V3, which is interesting in itself.
I swapped a known good 12ax7 into each of the sockets and the the last swap was V2 and the tube was an old Sylvania! So far that seems to have fixed it. I thought I did that swap earlier without success but whatever, it's working now. And it's become incredibly loud with the new power tubes, hope they mellow out a bit.
Thanks everyone for the help, much appreciated.
I swapped a known good 12ax7 into each of the sockets and the the last swap was V2 and the tube was an old Sylvania! So far that seems to have fixed it. I thought I did that swap earlier without success but whatever, it's working now. And it's become incredibly loud with the new power tubes, hope they mellow out a bit.
Thanks everyone for the help, much appreciated.
Great!! Always check preamp tubes first. You are correct regarding a " missing" preamp tube. Reverb. I think having one dedicated 12ax7 for verb and lead is a blessing. Much easier to troubleshoot. The MK III and MK IV have dedicated tubes for lead and verb. Maybe others. When Randall, Mike and Doug designed the earlier ones, I don't know why they split the tubes. Someone can chime in as to why. But, to shed light onto your confusion about one tube, a 12ax7 (and their families) are dual triode tubes. Which means that they actually have two halves in one tube. Pins 1,2 and 3 is one half and 6,7and 8 is the other. 4,5 and 9 are for the 6volt heating elements. Glad you got er running! Jim
tommyhawk13
Well-known member
Has it been a while since you played through it?
Tube amps are happy when they are played often. If they sit for a wile, and played only short periods, the crud builds up on the tube sockets and connections.
Try cleaning it with DeOxit before you take it to a shop. There are plenty of threads here about cleaning the sockets and jacks.
Tube amps are happy when they are played often. If they sit for a wile, and played only short periods, the crud builds up on the tube sockets and connections.
Try cleaning it with DeOxit before you take it to a shop. There are plenty of threads here about cleaning the sockets and jacks.
Hey thanks, it's been played a good amount recently. I was thinking it could have been crud in the sockets and my changing tubes and wiggling them was really was fixed it rather than a new tube. Because.....I put the suspect tube into my old .50 caliber, and it's working fine. Also, I noticed that you too have a .50 which is where I scavanged the good tubes. It's a great solid Mesa amp, workhorse for me for decades. Clean is signature, dirt is a bit lacking but the clean is so good. I used to run both the .50 clean and the IIc+ dirt in tandam. Can't do that much lifting anymore though!
I have some DeOxit so if it flares up again I'll look into that. Thanks for the tip.
Well I'm back with bad news. All new tubes, Deoxit on all the sockets, still having problems - volume increase/decrease and with crackling noise. The volume changes seem less severe than at first, but still not ok. I'm seeing that the first Volume knob (far left looking at front of amp) has crackling as you turn it. I've wiggled it around a lot but still crackling. Mid tone control also crackled a bit but after wiggling it seems to have cleared (probably because it was dirty from never being used). Volume1 gets used all the time. No noise from the other controls. I'm thinking the problem with Volume1 is related to the rest of the amp problems. So now what? Not happy...
user 41014
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Use that de-oxit on the openings of the back side of your pots. Run the pots between your hands or fingers back and forth a bunch and for the switchable pots open and close them too as you are working the de-oxit throughout the pots. If that doesn't take care of the crackle your pot is likely bad.
At least this will tell you if your pots are good or not and then you can take the next step. It shouldn't help volume swells but once cleaned you may better determine if the pots have any dead spots or going dead.
I work small pieces of tissue under the string of pots and drape more over the adjacent parts of the pcb to make clean up a breeze with a pair of tweezers
Boogiebabies
Well-known member
Is the volume swelling? Make sure the effects loop jacks are clean and tight.
If you start cleaning the pots you better have some lubricant or they will feel like sandpaper.
If you start cleaning the pots you better have some lubricant or they will feel like sandpaper.
Greetings Mr. Morgan! Where have you been? I hope all is well with you. I've been a fan, follower for years. Blessings. ,Jim
