Mesa MarkV / Saturation 'mod'

Prescence is cool on the big amp. On channel 2 crunch especially. It had next to no effect on the highs. Just the rawness and in your faceness of the mids. Loving the way it works now. Edge mode is killer now too.

Has anyone opened up their amp and run voltages on V4? While the max current on a 12at7 is greater than a 12ax7 the plate voltage max is about 10% less if I recall. The reason I ask is that on my amp after running the at7 in v4 for several hours channel 3 develops a faint noise similar to an ac current 60hz hum. Perhaps this is due to a really unusual coincidence or my amp just has an issue but it has happened on one new jan/phillips, one new jj, and one used groove tubes 12at7. Channel 3 is fine when I put any 12ax7 back in. I don't mean to throw a red flag because I see no reason this substitution would not work within specs.

sherrillsml said:

Has anyone opened up their amp and run voltages on V4? While the max current on a 12at7 is greater than a 12ax7 the plate voltage max is about 10% less if I recall. The reason I ask is that on my amp after running the at7 in v4 for several hours channel 3 develops a faint noise similar to an ac current 60hz hum. Perhaps this is due to a really unusual coincidence or my amp just has an issue but it has happened on one new jan/phillips, one new jj, and one used groove tubes 12at7. Channel 3 is fine when I put any 12ax7 back in. I don't mean to throw a red flag because I see no reason this substitution would not work within specs.

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Both 12AX7 and 12AT7 are rated for 300V at the plate.

IronSean said:

sherrillsml said:

Has anyone opened up their amp and run voltages on V4? While the max current on a 12at7 is greater than a 12ax7 the plate voltage max is about 10% less if I recall. The reason I ask is that on my amp after running the at7 in v4 for several hours channel 3 develops a faint noise similar to an ac current 60hz hum. Perhaps this is due to a really unusual coincidence or my amp just has an issue but it has happened on one new jan/phillips, one new jj, and one used groove tubes 12at7. Channel 3 is fine when I put any 12ax7 back in. I don't mean to throw a red flag because I see no reason this substitution would not work within specs.

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Both 12AX7 and 12AT7 are rated for 300V at the plate.

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Perhaps my old tube books are dated but they indicate 330v for ax7 and 300v for at7. It is probably a mute point anyway since it is unlikely Mesa is pushing v4 to the limits.

Sounds good!

That's strange. On my initial glance at the schematic I'm not sure:

The V4A in the second stage of the CH3 lead circuit goes into V3A next, before going to the Reverb and V6A triode before the EQ and then the Loop (and then V6B).

And V4B is the input to the reverb, so the output shouldn't be driving particularly hard into V6A either.

So nothing that seems to directly drive it different than usual.

kreatorkills said:

Update: I've been through hell, but i think i solved it. Very confusing.
Either bad 12AT7 or bad mod in V4 ruined 2 12AX7's in V6. V6 affects 6th gain stage channel 3 and effects return. Also, V6 has an effect on channels 1 and 3 because crackling and popping was on both channels.
I pulled 12AT7 from V4 and 12AX7 from V6 and replaced with new 12AX7's and it worked. Tried putting back 12AT7 and it ruined the new 12AX7 in V6. So put another new 12AX7 in V6 and put back 12AX7 in V4 and good again. Is it really possible for a bad tube or a bad mod to affect the V6 tube, and why? Because that seems to be the case. I have another 12AT7 on the way but I'm now scared to try it again!

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I've never heard of that before, especially in preamps. A bad power tube will pull it's partner down with it for sure but not heard if that in preamp valves. Not to doubt your findings but are you sure it ruined an ax7 in V6 the second time you tried it? Not just sounded bad because the at7 is a dud? Just seems strange because it shouldn't have any way of destroying another valve after it in a circuit. Even more so not with another triode between them. V4a can only drive v3a upto full saturation, which any ax7 would be able to do as well if not more so being higher gain. Any more and v3a would only go into cut-off because it can't produce anymore voltage swing. Therefore it shouldn't be able to overload the grid of V6, not to the point of damaging the valve anyway. I believe that's how it works anyway given my limited knowledge of the subject. This is very strange indeed.

Sounds good.

sherrillsml said:

Has anyone opened up their amp and run voltages on V4? While the max current on a 12at7 is greater than a 12ax7 the plate voltage max is about 10% less if I recall. The reason I ask is that on my amp after running the at7 in v4 for several hours channel 3 develops a faint noise similar to an ac current 60hz hum. Perhaps this is due to a really unusual coincidence or my amp just has an issue but it has happened on one new jan/phillips, one new jj, and one used groove tubes 12at7. Channel 3 is fine when I put any 12ax7 back in. I don't mean to throw a red flag because I see no reason this substitution would not work within specs.

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Have you tried the at7 in V6 ?

I usually have the AT7 in V6 but just for change I have it in V4 for the last couple of days. I am really intrigued by your finding because I faintly remember this hum as well. It doesn't have the hum when the amp is just started up but after an hour or so, there seems to be a slight hum. I have to also note that the Mark V now loves to have some volume with the tube swap, so maybe its the volume, or something else.. Will need further testing. I ll keep the 12At7 in V4 for a few more days just to be sure.

If you are hearing noise on any other channel than CH3, you can pull the V4 tube (this will also remove the reverb from the signal). If the noise goes away on CH1 or CH2 you found the culprit. However if it is still present, it is more likely V3 contributing, V1 is usually the culprit. If it is V6, put the FX loop into hard bypass and remove V6 (not really necessary as in hard bypass and using CH1 or CH2 V6 will not be in the signal path). Note this is for finding the affected tube causing crack and pop at the output. You would know if it is in the reverb circuit, just turning it off would be easy. Still the V4 reverb tube does not get eliminated by relay contacts if turned off. You can also remove V5 to eliminate the reverb circuit from CH1 and CH2. Note that a noisy V5 would affect all channels as well (if the reverb is used or off). You could always set the reverb signal level to off on the back panel.

Bankim said:

sherrillsml said:

Has anyone opened up their amp and run voltages on V4? While the max current on a 12at7 is greater than a 12ax7 the plate voltage max is about 10% less if I recall. The reason I ask is that on my amp after running the at7 in v4 for several hours channel 3 develops a faint noise similar to an ac current 60hz hum. Perhaps this is due to a really unusual coincidence or my amp just has an issue but it has happened on one new jan/phillips, one new jj, and one used groove tubes 12at7. Channel 3 is fine when I put any 12ax7 back in. I don't mean to throw a red flag because I see no reason this substitution would not work within specs.

Click to expand...

Have you tried the at7 in V6 ?

I usually have the AT7 in V6 but just for change I have it in V4 for the last couple of days. I am really intrigued by your finding because I faintly remember this hum as well. It doesn't have the hum when the amp is just started up but after an hour or so, there seems to be a slight hum. I have to also note that the Mark V now loves to have some volume with the tube swap, so maybe its the volume, or something else.. Will need further testing. I ll keep the 12At7 in V4 for a few more days just to be sure.

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Thanks and let us know how it goes for you. To answer your question, my preference is running at7s in v4 and v6 at the same time.

In my case the hum will start in a new at7 in v4 after a couple of hours (in channel 3 only). After that if I continue to leave that at7 in v4 the onset of the hum will occur sooner with every use and eventually will happen just as soon as the amp warms up. The hum is present only in channel 3.

What I am really concerned about is after this happens the tube is no good. I can put it into my original Super Reverb or Twin Reverb in the phase inverter slot and it is too noisy.

This mod may be Mark V production lot dependent. Not really sure what changes have taken place over the years since 2009. So far I have only used the one JAN/Phillips 12AT7 and have not had any problems with it. My Mark V was made in 2012 if that matters. So far all is good (knock on wood). I would say I have at least and estimate of 50+ hours on mine with the 12AT7.

I would personally be weary of using a non military grade 12AT7 as the JAN version will take some abuse as it was designed and selected for military service under hash conditions.

sherrillsml, did you actually put the 12AT7 in the phase inverter in either of your Fender amps or are you indicating it will sound terrible if you do as you can do it but have yet to do so? Perhaps I am reading into things too much.

What tube type of 12AT7 are you using? It could be related to the tube, when it was manufactured and brand as many tubes were made in many locations over the decades of not longer.... I would prefer the unused versions of the NOS JAN/Phillips vs one that was tested good but had been used. I have seen many places selling used tubes but tests good. If you are getting a hum, it could be the heater element in the tube. Non-coiled heater elements are better than the coiled versions as tubes with the coiled heaters had given me grief with that issue (12AX7 tubes with coiled heaters Tung Sol, EHX, Mullard). I found I could not use them in V3 in the Mark V as the hum would creep up after a week of use. CH1 was always noisy. Once the hum set in the tubes were useless in other amps as well.
Also what other tubes are loaded in the preamp if you do not mind me asking?

It is what it is, not all mods work in all amps, especially if there were changes made during the production years that may seem transparent to the end user until something like this thread comes along. Have you tried a 5751? I also have a preference for the JAN/GE 5751 but they are a bit expensive compared to current production 12AX7 and 5751.

Bandit, really good points. With at least 2 different mark V versions that we are aware of , the behaviour of the 12AT7 could change based on which version its applied to. With my Mark V, I didnt notice that ice pick tone since I got mine( Mine's probably a 2010, need to recheck), however, there has been a distinct improvement in tone with the 12AT7 in V4/V6. I prefer mine in V6 but I like the V4 vibe too. Hard me to pick but if this hum continues, I ll stick to using the 12AT7 in V6 only.

I did fire up my Mark V today, albeit, just for 30 minutes. I changed V4 back to stock and played some. Not too bad but there was a distinctive volume jump. 12AT7 went back to V4 and all went well. No hum yet. Then I couldn't help myself but put the 12AT7 to V6 and V4 back to stock. I love this mod Played some more , shut the amp down, put 12AT7 back to V4 , V6 stock. The next few days should help me settle on which slot the 12at7 would sit.. more playtime for me, so it's all good 8)

I've noticed that the physical/ very definitely percieved volume jump between IIC+/Mark IV mode and Extreme also isnt a thing anymore.
(On the 90w head) Especially in 10w mode.

Woohoo! \m/

This is starting to worry me a little bit. Not had any issues at all with my Mark V though so far touch wood.

Kreatorkills is worrying, although sounds like that particular amp kills tubes anyway from the list of tubes its gotten through so far. Might consider having a tech look into that as no amp should eat tubes the way this one does.

I've not noticed any hum yet either. However, haven't had my Mark V on for much more than a couple hours at a time and then only 2 or 3 times over the last month or so. Really struggling to find the time to play around work and family life.

Bandits had a lot of miles on his mark v with the mod and apeman too with no side effects so I'm hoping it's not a problem with using this type of valve.

no problem.

That is too bad on the limited use you get with a 12AT7. I wonder if the PS is running on the hot side as it seems according to the 2009 schematic, the hot side of the reverb transformer is at 445Vdc where as on the other circuits it is 418Vdc. This is just a print value and may not be actual if measured. I would have the DC blocking cap checked that is between V5A and feeds into the grid of V4B. If it is damages or leaking dc voltage that would definitely cause some issues. It is either that or the choke is shorted which would raise the vB rail to 448V as well as the rest of the supply rails. I hope what ever is the bug in the amp gets repaired, assuming you need it repaired.

I thought my amp was a Lemmon as it would kill power tubes anywhere from 2 seconds to 2 weeks. As it seemed the Mesa STR 440 6L6 tube was a sure kill. Had to use SED =C= 6L6 and they performed quite well and no red plating.... Since the Svetlana company in St. Petersburg shut down production of consumer grade electronics I had no other choice but to mod my amp on the bias circuit. (Changed one fixed resistor 82k with a 91K) and now I can run Mesa power tubes as well as others. I prefer the fixed bias actually since trying to adjust bias for the center pair may result in too cold on the outer. Thinking about changing it to a 87K to raise it just a bit. I always thought the Ice pick was due to that, in part it is. The Mark V was nothing but a headache for a few years but I stuck with it for some dumb reason. Was ready to let it go since I have the JP-2C and then Apeman commented on a different thread I posted on and well......

If you amp is not blisteringly bright that sounds like broken glass in your ears when you get to gig level or just about there, you probably do not need to change tubes.

kreatorkills said:

Yea, i might of got a lemon because it's been in the shop twice plus all the tubes. But i was running EL34's for a long time, and I'm constantly switching modes, wattage's, Variac, etc. I know it's made for that but i think it comes with a price. This is why i think I'm done with the 12AT7 experiment which is unfortunate because one of the main reasons i bought the amp was the 2C+ mode, but I've always thought it was the worst mode because of the the shrill ice pick effect. The 12AT7 solved it for a minute and i was loving it, but unfortunately it was too good to be true i guess.

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As Thaymz indicated in one of this post, running the Mark V in 10 watt mode with stock tubes does have a similar taste to the 12AT7 mod. Its almost as good as the 12AT7 mod and IIC+ sounds quite good too. The key here is to have the channel 3 volume halfway with your preferred settings and adjust stage volume with the overall Master Volume. That additional halo or overtones you get with the 12AT7 in V4/V6 is achievable with stock tubes with the only caveat being that you need to use the 10 watt mode. The addition of the rectifier tube in this circuit (probably?) creates the pleasing overtones that we all love.

Edit: However, the 10 watt settings is not a direct replacement of the 12AT7 mod. This magic tube just brings out the pleasing overtones so much into the fore. I am skeptical that my dialled settings would translate well to a gig. I guess we'll never know since we are on an indefinite haitus at the moment.

I always use to wonder when people say that a particular tone is close but not 'There' yet. I understand this now with the 12AT7 vs stock tubes :lol:

With my RP1000 multi effects processor, the tones were harsh and lackluster. I made that work for years and it panned out quite well in my band but when I got the Mark V, there was no comparison. My RP1000's simulation is just not up to scratch. Granted, i have not had a Axe Fx or Kemper, both of which I was contemplating while getting the Mark V (they were cheaper than the MV as well), but I can honestly say that I don't regret buying the Mark V even though its bloody costly in this country.

Another variable to consider is that my amp is the export version, so it has a different Power Transformer. Not sure if it should make a difference but just putting this out there..

What is the line frequency in your country Bankim? Is it 50Hz or 60Hz? I am assuming you are running at 220 to 240 VAC. Just curious as the lower line frequency will reduce the effective output of the power supply just a bit and probably has a bit more sag on power cording. Makes me wonder if the large capacitors are the same as the non export version (assuming they are the same). Power supply will have a bit more ripple in it but doubt it would be noticeable.

We are running 220v, 50 Hz. The prescribed voltage is 220-240ac but I have never seen 240. Have seen below 220 sometimes though.. Guess that'll introduce a variac power on its own..lol..