Mesa MarkV / Saturation 'mod'

[bandit2013]

Changed back to 12AX7 for a while, now back to just the one 12AT7. So far that is the best for me.

 

[user 40871]

This is awesome! Thanks to you guys (APEMAN and bandit2013) my Mark V is cured from the ice picking highs!

I'm using a JAN GE 12AT7 (all other tubes are the original Mesa ones) on V4, CH3, Extreme Mode, Triodes, and man... It's so **** close to the sounds that I heard online of the JP2C!

Thanks again!!!!

 

[bandit2013]

You are welcome. It is why we are here in the first place. Share what we learn and learn what others share.

 

[Wayno]

Dropped amp off at Essex Amp Repairs yesterday, fingers crossed!!!

Really nice bloke, really did seem genuinely happy to have a Boogie to work on. Gave me a very good impression/feeling so i'm happy. Makes a change from the usual, "boogies are notoriously difficult to work on" disclaimer you hear from tech's so often. Also said he has a really good relationship with Westside, the UK distributor so thats good too if he needs parts. He says he's never been beaten by a broken Boogie yet!

Thanks for the recommendation Markageddon, Westside themselves recommended them too when they eventually replied to my email.

 

[bandit2013]

Good luck with that Wayno. Definitely would like to know what the culprit of your Mark V woes turns out to be. Aside from the obvious, it may turn out to be something completely different.

 

[domct203]

Changed back to 12AX7 for a while, now back to just the one 12AT7. So far that is the best for me.

In V6?

Dom

 

[bandit2013]

I had tried the 12AT7 in V6 a few times. That seems to take too much from the amp. I prefer the 12AX7 tube in V6 as that is much louder. I found the sweet spot was change in V4 to 12AT7. Speaker choice may be the thing for my choice. If I was using a V30 speaker then V6 would make more sense. I am running the Organic Timbre Rhapsody in the Mark V combo and that speaker fits the amp tone and characteristics quite well (physically it was not an easy drop in, neither was the EV before it as I have it front baffle mounted in the combo, extension cab, standard rear mount is fine). Also sounds great with all 12AX7 tubes too.

 

[Markageddon]

Dropped amp off at Essex Amp Repairs yesterday, fingers crossed!!!

Really nice bloke, really did seem genuinely happy to have a Boogie to work on. Gave me a very good impression/feeling so i'm happy. Makes a change from the usual, "boogies are notoriously difficult to work on" disclaimer you hear from tech's so often. Also said he has a really good relationship with Westside, the UK distributor so thats good too if he needs parts. He says he's never been beaten by a broken Boogie yet!

Thanks for the recommendation Markageddon, Westside themselves recommended them too when they eventually replied to my email.

Fantastic news Wayno...!!! You'll be going at it again in no time. Top work from those guys. Ah yes that is very good news indeed.
8)

 

[user 40871]

Just performed the C39 removal... Wow... the amp sounds even better! Now I can achieve very good tones from all the CH3 modes!

 

[bandit2013]

I still would not recommend it if your amp sounds awesome from the start and may not be needed. However for those with the ice pick woes and lack of feel or missing tone I would recommend it. I found the amp response to appear more forward than it was along with improved upper midrange that I felt the amp was missing. It will make things a bit brighter but the improved midrange response makes up for it. With that, the 12AT7 in V4 sounds more impressive such that I actually like the amp far more than I did before taking the gamble of the C39 Removal. A similar capacitor is on V5 and V6 but I would leave those alone. From the initial design starting with the IIC+ and following that design with the Mark III, and then Mark IV, the grid to cathode capacitor was not present in the drive circuit but did appear in the Mark V. However, the change in tone is not extremely dramatic but it does change the attitude of the amp in all modes of CH3. Even the bright switch seems to make things more aggressive. So far the ice has become less of a burden even with the 12AX7 in V4 but I still prefer the 12AT7 as this deepens the grind characteristic and improves the midrange more so than the 12AX7. I have found the similar capacitors in other Mesa amp designs, some with switches (marked as dry) and others with resistors or direct. It is, however, not what I had originally thought it would be (feedback circuit since it is driven from the cathode which is basically at or near the grid by a few volts and will not contribute as a local feedback loop) If you find your amp is way too bright there is another trick one can do with a small capacitor value (10pF up to 30pF) rated for 1kV if mounted across the anode resistor. I have not experimented with it yet with smaller capacitance values but have tried the 500pF as this was in the IIC+ schematic but omitted for simul-class. That did not have a favorable result. Actually I do not see the need to make any further changes but may try the 47pF I have just for fun. Aside from that I am playing though the Mark V more than ever as I can get the tone I want at lower volume levels compared to the JP-2C which is just a beast (not bad at reduced volume and there are ways to peak up the treble at lower volume as the bass becomes dominant, the shred switch does that trick quite nicely for bedroom level using the 100W power). Still the Mark V is not a JP-2C killer as it has its own tone and characteristic. Actually the response and harmonic twist you get with pinch or tap is just much better. Overall tone for CH3 is also more pleasing to the ear than it was prior to the C39 removal as it seemed to have lifted the blanket a bit farther off the speaker than the stock form.

 

[domct203]

Does anyone have a picture of where C39 is located on the PCB?

 

[bandit2013]

Before removing any components on the amp, confirm it is the correct part. Use a DVM to check continuity of the part in question. One leg of the cab should be connected to the grid, and the other must be connected to the cathode. Keep in mind there may have been some changes to the design over the years. Before I removed C39 (assumed, reference designation is difficult to read and is not completely visible as the tube socket material covers part of it) C38 that is right next to it is readable. Since I was uncertain if this part was actually in the circuit (schematics are of the initial design and are slightly different than the 2012 model year I did confirm that the part in question was connected to grid and cathode. C28 also is connected to the grid of V4B but the other leg is not connected to the cathode. the image shows the location where C39 was as it is no longer on the PCB of my amp. Keep in mind that its removal will make the amp a bit brighter but at the same time adds more upper midrange and improves pick attack (perceived). I did not notice the ice pick issue I had even with the 12AX7 tube. Also note if you remove it completely the task to restore it would be very complicated due to limited space. Important notice: if you need to disconnect the ribbon cable for more room, remove the cable with care, do not pull on the wires to remove the plug from the header. Also, most important, do not power up the amp without installing the ribbon cable as you may end up with no sound or possible other issues due to transients that may cause harm to other parts on the rear PCB due to open circuit. Do not do this if you are not familiar with working on circuit boards or products that may store lethal voltages. Do this on your own initiative as I am not recommending this task even if I think it makes the amp a bit better in tone and character.


Here is the image of where the suspect part is located. You could clip one leg but leave room so you can solder it back together if the end result is not what you expected. Component value is a 120pf cap. Should be marked on its side with 121 with the voltage of 1KV.

 

[domct203]

Thanks for the very detailed reply bandit.

Dom

 

[Wayno]

Also note if you remove it completely the task to restore it would be very complicated due to limited space. Important notice: if you need to disconnect the ribbon cable for more room, remove the cable with care, do not pull on the wires to remove the plug from the header. Also, most important, do not power up the amp without installing the ribbon cable as you may end up with no sound or possible other issues due to transients that may cause harm to other parts on the rear PCB due to open circuit. Do not do this if you are not familiar with working on circuit boards or products that may store lethal voltages. Do this on your own initiative as I am not recommending this task even if I think it makes the amp a bit better in tone and character.


Here is the image of where the suspect part is located. You could clip one leg but leave room so you can solder it back together if the end result is not what you expected. Component value is a 120pf cap. Should be marked on its side with 121 with the voltage of 1KV.

Please bare all this in mind, this is a very tricky amp to troubleshoot if something goes wrong. I fully stand behind this mod, but, be carefull. If your not 100% confident, don't do it, get a tech to do it for you. I got my fingers burnt, and I mean no body's fault but my own, I forgot to reconnect the ribbon cable and ended up with a broken amp. I don't mean to sound negative, I understand the positive effect the mod will have, just be carefull and pay attention. But upmost, hopefully you enjoy the result.

 

[bandit2013]

Wayno,
Any news on your V? I hope you get it back soon as I can understand it has been a non-rewarding experience. I feel your pain on this.



So far I have been lucky not to have the same issue as I too had forgotten to reconnect the ribbon cable when I removed the cap. I took the initiative on this and if the amp failed on my account I would not be totally upset with it as I have other amps to feed the need. Why I am so compelled to improve on this amp is beyond me as I have the JP-2C. The Ice pick was one major drawback but that does seem to be cured with the 12AT7, however, the fundamental tone of CH3 was lacking tone, sounded better but still was tiring to listen too when playing though it. It was not possible to regain the midrange with the 5BEQ. It just sounded chocked off. C39 removal does help in this regard and improves performance. Still does not make it perfect but dang close to it as close as the Mark V can get. I have also noticed an improved bass response too. I will have to try the Mark V though the Vertical 212 again. The 12AT7 helps with tone when using a V30 loaded cab. Also using one in V6 also improves the V30 experience but at a sacrifice to output levels. That is more notable when driving a different speaker like the EV or OTR. With C39 removal on V4B triode circuit, the gain ladder of the Mark V becomes the same as the IIC+ (if the FX loop is in hard bypass). I looked again at the III which does not have any of the grid to cathode caps (schematics do not indicate color of stripe so it may be based on the first one) On the Mark IV, the grid to cathode cap is only located on the equivalent tube to V5A. In short, the Mark V is closer to the IIC+ than the III or IV (on paper) with the exception it has one additional cap on V4B. All of the other grid to cathode caps should remain where they are.

Ice pick woes may be more related to the tone stack for CH3 and does not appear to be a factor of the input buffer to the 5BEQ which is always in the circuit path. Uncertain if there is any clipping to the emitter follower Q1 but doubt this is an issue with the circuit. I was also looking at CH2 as this circuit uses bypass caps to add brightness to the signal in Edge mode which tends to make it brittle almost worse than the CH3 ice pick. that does not bother me as much as I never use Edge mode anyway but may explore a minor change on that circuit someday (all depends on how accessible the circuit mod would be and what that mod is needs to be determined.)

I did have a short time to play though the Mark V last night before I had to go. With the 12AT7 in V4 and no C39 on Mark IV voice selected, moderate volume level along with a set neck mahogany guitar with maple top, I was nailing the Led Zeppelin sound quite well (or I think I was anyway, still was fun even if not exact).

 

[Wayno]

Wayno,
Any news on your V? I hope you get it back soon as I can understand it has been a non-rewarding experience. I feel your pain on this.

Was working just down the road from Essex amp repairs yesterday so popped in to see how they're doing. No joy with my Mark V yet, he did have it in bits on the bench and is pretty sure he's narrowed the problem down to the mute circuit. Just a waiting game now for me. Sure they'll have it fixed soon though. Getting withdrawal symptoms now!

Was watching the "guitologist" on YouTube last night and he fixed a very similar problem on a roadster. Ended up replacing a few of the Jfets, very interesting video. Good channel as well with some great repair vids, worth checking out if your interested in valve amps.

 

[Markageddon]

At least you can be assured that it will be done well.
Glad to hear they've gotten further along the trail.

Did you ask them to do the mod for you, or just 'restore to stock'...?

 

[sherrillsml]

Is anybody using or have tried EL34s with the AT7 mod or do you prefer 6L6s?

 

[Wayno]

At least you can be assured that it will be done well.
Glad to hear they've gotten further along the trail.

Did you ask them to do the mod for you, or just 'restore to stock'...?

Told them I'd tried to do the mod, didn't think to ask them to do it. Ampguy up in Norwich reversed it when he had it. Dunno, will ask their opinion on it and maybe ask them then. Hope to hear something soon, getting really itchy now.

 

[Wayno]

Is anybody using or have tried EL34s with the AT7 mod or do you prefer 6L6s?

I have tried it with both and it works just as well with either type. EL34's still favour the tweed and channel 2 modes and 6l6's still enhance the clean modes and CH3 likes the thump they provide as before, but, I found the clean and CH3 modes with EL34's much better with the at7 mod than without. If I do try the C39 mod again I'm very intrigued as to how that will sound with EL34's.