Mesa MarkV / Saturation 'mod'

[bandit2013]

Now I am confused even more..... perplexed or just dumbfounded. I ran some simulations at work using Altium Designer simulation tool and the SPice models for a 12AX7. I used the cascaded circuit without the feedback loop just to see if there was a difference between the simulation with the grid to cathode capacitor on V4 and without. The frequency analysis revealed some change in the output of V4 (used a 500k resistor as a dummy load to measure voltage). With the capacitor in the simulation it seems to attenuate the upper frequency and provides more roll off on frequencies above 5khz. The plot compared to the simulation without the capacitor actually had an increase in amplitude and a slight change in slope on the roll off at the same point. Perhaps the model is not accurate or the analysis just did not work out (I generally do not use the simulation part of the program as I found it does not always work out properly). Time to take home the Oscilloscope (to bad it does not do signal analysis but just about everything else ) actually I will be using it for a different purpose to observe what is going on with the bias circuit as the values do not measure up to print. (way off), May just take the amp into work as that may be easier to do.

 

[bandit2013]

Hmmm. Another example what cannot be captured in a recording..... I did manage to solder on a ceramic disc cap that had the same value and ratings as the original component. I did attempt to record but this time I used a pre-recorded track that I captured using a TC Electronics Ditto. I let the recording repeat and on the repeat changed from one voice to the next (IIC+, IV, Extreme) then swapped cabs. Mark V combo with Redback, Mesa Vert 212, I left the mics on the Vertical 212 and started with that after I removed the capacitor. Followed up with the Redback, repeated the Redback since I realized I did not have the bright switch turned on (it was off for the first part anyway). Then followed up with the JP-2C though the Redback (CH2 and CH3). The bummer is this.... I cannot tell the difference in the recording between with the cap or without. In person I it is in the feel from the amp (attack) as well as a slight change in the midrange. Sorry if you sit though this thinking you will notice a change. At least on my deaf ears I will not.... I have listed too many times to try to hear if there is or not....I am beginning to hate my song now (or this part of it anyway but it has combination of muted and open playing). This one may have ended my career in music ( when did I have a career in music? no worries). Such is life, there is a change but cannot be heard. I should have done this without a pre-recorded track as I think it may have hidden the character I wanted to hear. Oh well. (last two repeats were done on the JP-2C....)

https://soundcloud.com/user-353100000/mkv90cap1wav

 

[bandit2013]

I also was running the channel master and global master a bit lower so I can record without using the power attenuator. That would have been a better solution. I can always put the capacitor back in and do it all over again.... which I may just do.

 

[bandit2013]

I did a small change on the bias circuit and so far I have not red plated any power tubes. (long story but I changed bias a while back so I could run the Mesa tubes..... now the amps is much closer to what it should be and the bottom end is tight but also a bit brighter in tone). Since I removed the capacitor on V4 I was left with more response on pick attack. Also the minor change in bias voltage regained some of the top end along with the bottom. Had to try the JAN/Phillips 12AT7 in V6 as well. Hmmm... where to start.... It is F****** unbelievable. The S&&& hit the fan on that one. Where was this tone before? Now I am a double mod believer I can see how some love the 12AT7 in both V4 and V6. Before I could not stand the effect with the 12AT7 in V6 but now it just makes more sense. Perhaps the 12AT7 in V4 was one small step to perfection, removing the grid to cathode cap on V4 and adding the 12AT7 to V6 was one giant leap towards the JP-2C. Now I am completely satisfied with the end result.

Note (I changed bias due to an issue, all I needed was a small change to prevent red plating, it was not just Mesa tubes that would fail as that included most tubes with the exception of the =C=. Have a set of Mesa 6L6GC STR440 (reds) that I have been using for some time now, also did the sure kill test and did not even notice a change in color on the plate seams (Awesome). To top it off with the 12AT7 in two positions now I am happy. Also have a speaker on order and will see how that turns out in the combo. Actually I am digging the Redback speaker at the moment since it is now broken in. Note: I was using the Mesa Vert 212 during my short tone quest. Also the amp gained some volume on the way so all good for now. 8)

 

[bandit2013]

All I can find on the grid to cathode capacitor is the Miller effect and intrinsic parasitic capacitance that is apparent but of no concern except for high frequency signal. Mille effect is the grid to anode capacitance being affected by gain of the circuit where the gain actually acts like a multiplier to the intrinsic capacitance. Based on small signal models, the intrinsic capacitors just form a low pass filter. In essence it is not doing much of anything but reducing the gain of the frequency response. What is more critical is the cathode bypass capacitor. I found some interesting articles on tube circuits specifically for guitar and it is not loaded with too many equations and other things that would put many to sleep.

http://www.valvewizard.co.uk/gridstopper.html not a bad explanation on Miller effect. I can reference something far worse on this (too much weeds for me to dig into after work, I need a break from that) but I will not as it becomes tiring, also more related to solid state components such as BJT, MOSFETS, JFETS, and other similar devices.

http://www.valvewizard.co.uk/gainstage.html The download article is worth the read if anyone is interested in this sort of thing. This got me thinking about making a few calculations and perhaps see what a different cap will do on V4 cathode bypass.

Also: ever hear the pop corn or bacon frying noise? I am sure that some have had that experience. I never knew it was due to cathode leakage current to the heater circuit. Good sign tube is on the way out or just bad (obvious) but could also be related to design if the voltage differential between heater and cathode is too large. Yes the article does have some equations but they are not too complex and are quite simple actually. I think the next time I pull up the schematics on the V I will have to do some math and see what is what now that my curiosity has been awakened. So far the amp sounds great an no need to do anything else except try out a new speaker in the combo when ever it gets here. Organic Timbre Rhapsody G12F. Will have to do a speaker comparison between the EVM12L BL, and the Celestion Redback just for fun. I am loving the Redback in the Mark V combo as it is broken in finally.

 

[Markageddon]

I got a Red Stripe Mark III yesterday.
Forearmed with a bit of research knowledge from along this and other trails, I took the AT7 out of the Mark V:25 and lobbed it in the PI slot for the III. Same effect. Smooth creamy vintage liquidy heaven. Massively increased playability.
Heavenly.

So down to the store next week for new AT7s...

 

[Bankim]

I got a Red Stripe Mark III yesterday.
Forearmed with a bit of research knowledge from along this and other trails, I took the AT7 out of the Mark V:25 and lobbed it in the PI slot for the III. Same effect. Smooth creamy vintage liquidy heaven. Massively increased playability.
Heavenly.

So down to the store next week for new AT7s...

Forgive me for being an ignoramus, too lazy to dig around , but is the red stripe one of the raw'er sounding compared to the blue and stripe et al ? Just curious..

On the subject of this thread, I have noticed that my pre-amp tubes seem to have a shorter life with the 12AT7 in V6.. I changed V1 just a couple of months back n it crapped out on me again today. At first, I didnt think it was the problem and changed V4 V5 V6 (since only Ch3 was cracking/popping) but it didnt help. Changed the V1 and all was well.... :|

To change things a bit, 12At7 went to V4 this time. Did I mention that the 12At7 in V6 crapped out as well.. .. Changed that.. Moved the new 12At7 to V4, didnt like the sound at all.... spent 20 mins with MTBP + EQ and finally landed on the 12AT7+V6 tone that I loved.. Whew !! Nyway, I have run through more tubes in the last few months than in the last few years of owning this amp.. Is this the price I have to pay to get that massive glorious 3D sound that I am used to now ? We will have to see..

... More later...

 

[Markageddon]

Its really aggressive, big and smooth. And yes raw in the 'How in your face is it?' department. It reminds me of Post-Mod V25 on crunch maybe with a bit of midboost and a TS9 in the front end.. Its the 60w only version. Fully loaded other than that though. Very IIc+ Especially with the Pull Deep/Bright options etc. The third revision apparently, the Red Stripe. IIC+ Preamp, regular Mark III Power transformer. Definitely more balls to it than the Purple I tried. I understand that the Blue and Green stripe after it were more aggressive in the highs, And the Greenstripe was Pentode. But this has aggressive highs all day if you want em. So I dont know what to say about that!??!? lol It can be set to total hooligan savage, depth marauder, leviathan mode. Red stripe is a really pissed off IIC+ and when you get the AT7 in it, all that aggression becomes smooth, playable and instant. The only other Mark I wanted other than the V. This one has the EV speaker too.

 

[bandit2013]

Awesome find, I did miss my Mark III but the JP-2C seems to cater to that need.

As for the life span of the AT7 in V6, interesting to understand what is up with that. I am using one in V4 and V6. May try using an AX7 in V4 and see what happens. My amp was supper brittle with stock tubes, could not even set the gain above 10oclock otherwise it just killed my ears, it got worse through any cabinet with V30 speakers. Even the EVs were brutal. at7 in V4 cured 80% of the nasal tone and ice pick. removing the one cap helped with the nasal tone but increased the brightness a bit but not brittle or ice. AT7 in V6 shoed in the mojo even more! Sounded very thing before the cap removal but now it is full and just how I think it should be. Will find out soon enough how long the AT7 in V6 lasts in mine.

 

[Markageddon]

Thanks Bandit.... and oh yes I bet it does. :twisted:

 

[Bankim]

Its really aggressive, big and smooth. And yes raw in the 'How in your face is it?' department. It reminds me of Post-Mod V25 on crunch maybe with a bit of midboost and a TS9 in the front end.. Its the 60w only version. Fully loaded other than that though. Very IIc+ Especially with the Pull Deep/Bright options etc. The third revision apparently, the Red Stripe. IIC+ Preamp, regular Mark III Power transformer. Definitely more balls to it than the Purple I tried. I understand that the Blue and Green stripe after it were more aggressive in the highs, And the Greenstripe was Pentode. But this has aggressive highs all day if you want em. So I dont know what to say about that!??!? lol It can be set to total hooligan savage, depth marauder, leviathan mode. Red stripe is a really pissed off IIC+ and when you get the AT7 in it, all that aggression becomes smooth, playable and instant. The only other Mark I wanted other than the V. This one has the EV speaker too.

Lucky You!! All the clips I have heard of the Mark III attest to what you are saying. Hopefully, I'll have one in my hands someday.

 

[Markageddon]

Its really aggressive, big and smooth. And yes raw in the 'How in your face is it?' department. It reminds me of Post-Mod V25 on crunch maybe with a bit of midboost and a TS9 in the front end.. Its the 60w only version. Fully loaded other than that though. Very IIc+ Especially with the Pull Deep/Bright options etc. The third revision apparently, the Red Stripe. IIC+ Preamp, regular Mark III Power transformer. Definitely more balls to it than the Purple I tried. I understand that the Blue and Green stripe after it were more aggressive in the highs, And the Greenstripe was Pentode. But this has aggressive highs all day if you want em. So I dont know what to say about that!??!? lol It can be set to total hooligan savage, depth marauder, leviathan mode. Red stripe is a really pissed off IIC+ and when you get the AT7 in it, all that aggression becomes smooth, playable and instant. The only other Mark I wanted other than the V. This one has the EV speaker too.

Lucky You!! All the clips I have heard of the Mark III attest to what you are saying. Hopefully, I'll have one in my hands someday.

You wont regret it. Between your V with all the other modes and the III, you'll have it all covered.
I really do think the mod took the V further in that more vintage direction. The III's input vol dial and other attributes just allow extra stuff to be possible. But they are definitely kin.

 

[bandit2013]

I had thoughts about adding this and that, well basically to model what is on the Mark III, IV and Mark IIC+. Adding a plate load resistor bypass capacitor to reduce high end frequencies. I tried it and the amp sounded terrible. Sure it cut the high frequency as well as gain (perceived), keep in mind there is a feed back network that couples the output to the input of the driver cascade circuit, actually it is the same as that found on the older Mark Models which is basically a 3.3M with a 20pf bypass cap (nothing special there). I am happy with the tone of the amp as is (with C39 out ) but no more mods for me. I am done with making any other attempts to mod this amp, I like the natural tone with the V4 and V6 change. Amp sounds as good as it is going to get. I have better things to do (play the guitar.... and now I am enjoying the minor change with a little help of two glass mates and some smoking mojo (everything is alright as long as the tone is smoking and not the amp)

 

[sherrillsml]

I am happy with the tone of the amp as is (with C39 out )

I have been modding amps for decades and may open my V and try this mod. I bought my amp new in May of 2010 and have thought it always had an annoying higher frequency response to all of the distorted channels. It has been my experience that running an AT7 in both V4 and V6 has made the amp desirable but I am always interested in getting a little more out of the V. Would you expand on the improvement you experienced with the C39 mod for me as it sounds like our versions are similar. By the way my amp blew a couple of AT7s in V6 when I first tried the swap but except for a slight hum in channel 3, the 3rd AT7 in V6 is hanging in there so far.

 

[bandit2013]

I have done some deep digging on what the grid to cathode capacitor is really intended to do. All I could find is the aid to reduce parasitic oscillations that may become apparent at higher frequencies. You may see this used in many RF amplifiers and square wave generators (you will not get a good square wave from a triode as there will be some distortion either due to cut-off or clip (depends on where the load line is). I ran a few PSPICE models with and without the capacitor in the circuit. With it in the circuit I see a reduction in signal strength by a few dB vs taking it out. In reality, with the cap in place I was still getting that honky tone or at least to describe it better, squashing of the mid range. Small signal modeling of the circuit suggests it acts as a low pass filter that will be parallel to the Miller effect capacitance which is a multiplier of the parasitic capacitance between grid and anode (generally in the magnitude of 100pf). Parallel capacitance is additive so you wind up with 220pf associated with the input impedance and output impedance forming a low pass filter. With the cap removed the signal strength increased to some extent. CH3 did gain some brightness as well as a bit of boldness added to the bottom end and not as much compression on the midrange. However the slight change in the output characteristics of the cascaded driver circuit made sense to change the 12AX7 in V6 to the 12AT7 to increase the warmth of the channel. It sounded good with the AX7 but the AT7 in V6 just has more depth. Also, before the removal of the cap the amp was not as responsive. The first thing I noticed without the cap was the attack response. Mark V has become more forward and reactive to the guitar. In other words I felt more connected to the amp thought the guitar which is about the same feeling I get thought the JP-2C and the TC-50 (RA100 included in that relationship). When I use the Mesa Vertical 212 cabinet I am rewarded with tone I could not get before the change.

Suggestion, if you are uncertain of the side effect, only cut one leg on the cap and pull it away from the PCB (leg farthest away from the small signal relay). If you do not like the change you can always solder the leg back into place. Since I had cut the silicon goop that holds the capacitors together I felt it better to remove it completely so I could always remove the solder and remains of the capacitor leads from the plated holes and install another one. It is very tight in there. If and when you decide to open up the chassis, look at the tone control circuit as well as there was a change to the tone control circuit sometime in 2010. I think that information is somewhere in this thread. Do a search on APEMAN as it may be in another thread.

As for short life on the 12AT7 in V6, that is a puzzle. Did you buy the amp new? If so the parts should be all stock around all of the triode circuits unless they were changed or modified. You may have a bad heater connection on V6 Side A uses pin 4, Side B used pin 5 and the common pin for the heaters is pin 9. Voltage measured across pin 9 and pin 4 should be the same as voltage measured across pin 9 and pin 5. You can also check the socket for loose terminals or contamination. V6 is the last gain stage on CH3, also where the the Extreme mode and Mark IV circuit is tailored with the bypass capacitors.
If you look at the drive circuit in its entirety it is path is V5A -> V4B -> V3A -> V6A. V3A has been overlooked here but does have two switchable cathode bypass capacitors switched with J175 FETS. 100uF on one, and 2.2uF on the other. My guess is that the 100uF cap is active for the clean channel, but not so sure what channel is pulling in the 2.2uF as there is no detail on switch logic for the FETS in relation to channel or channel voice switch. Could be related to any of the three channels as V3A is used by all of them.

I would assume that the coupling capacitor may be passing DC between V3A to V6A. (C33 = 0.047 uF) If it was you should also be having issues with CH1 and CH2. The tubes you have failing on you, can you determine if it is side A (CH3 master pins 1,2,3) or side B (FX loop return pins 6,7,8). V6 uses a 6.2VAC heater circuit. I would look for any solder defects (solder balls or other contamination round V6 such as gray looking flux residue). You can clean the areas around components with acetone (best to use the weak stuff = nail polish remover but with out the oils) and a Q-tip. If it is taking time to cause issues, you may have flux residue that may be activating due to heat that may contribute to conductivity causing issues between the cathode and the heater. If you are getting a frying bacon sound, that is the issue due to leakage current between cathode and heater element. It is either that or you have a few bad 12AT7. What tube are you using and where did you get them?

 

[bandit2013]

First time to try the FX loop with AT7 in V6. I did not notice any tone or volume change with the FX loop on or off while using the FX loop active. That is good. Still the amp requires compatible FX processors (primarily the input buffer needs to have a wide span or high headroom) since the send level has not been effected as that is generated with transistors from the 5BEQ circuit.

Was thinking of getting some more gear so why not get the Eebtech LLS-2 and see how that works in the Mark V loop with instrument level pedals. I am down to two effects that will not work all that great in the Mark V loop since I replaced most of my gear with Strymon stuff (primarily delays of sorts). Line6 DL4 is one and the TC electric Flashback X3 is the other (it works and does not degrade as much as the L6 DL4 but it still compresses the signal thought the effect, I have tried it through my other amps with no issues and same as the L6 DL4). If the Ebtech works with the Mark V, I can use that in other ways such as slaving out to another amp using the FX loop return and an isolated signal splitter and or sending a stereo feed to another amp from the loop). I have a few pedals and would like to eventually build up a pedal board so I can use it with any of the amps. I had considered the G-System but would rather use pedals vs on line effects as I have not been all too pleased with the multi effect units I bought in the past, however since the G-System does not include overdrive or amp modeling it is tempting to get one.

Now that being said, considering the full drive circuit of CH3, what is left is V3. Changing V3 will also affect CH1 and CH3. V1 position has the most influence on everything. Long plate 12AX7 could be used to tailor characteristics as this drives the tone stack for CH1 and CH3, and V2 drives the tone stack for CH2. The circuit response of V1 still has a role on the tone of CH2 (Edge and Mark I voice makes tuning CH2 a challenge).

Finally there is the PI tube. This also effects overall tone and how it reacts with the feedback circuits used as the presence controls for each channel as well as the final triode set that is influenced by the preamp. I have installed 12AT7 in V4, V5 and V6 just to hear what would happen long before this thread came to the board. It made CH3 sound dry, I may have to revisit that again just for kicks.

 

[bandit2013]

Concerning the C39 capacitor: To this point I am uncertain to the purpose of the grid to cathode capacitor. It is there for a purpose as it is present on V5 and V6 in the CH3 circuits. Its behavior does act like a low pass filter on the small signal analysis and in reality it does have that effect. Without the cap there is a slight rise on the treble and a change in attack response. However, I would assume this is for stability for high gain settings to prevent oscillations resulting from feedback. It may be easy to cut out but to install a replacement would be a task best left for a master who knows all of the tricks of the trade also having the proper gear to do the task as well as the skills is paramount.

Yes, there was some degree of change but not much. I am going to the trouble to install another one so I can hear the difference again. I may take it out as I do like the subtle change that occurred in its absence. It worked great with the Red Back speaker in the combo as well as the Mesa Vertical 212. However I want to explore going back to a 12AX7 in V4. I can start without the capacitor and see what happens as that was the first tube I installed in V4 after the capacitor removal. When it comes to circuit mods, it is not always easy to restore back to original. A tube change is always a better solution than adding or removing components.

 

[Markageddon]

A bit more live 'At7 in 4' on the 90w malarkey.

More Sabbath for Wayne.

https://www.youtube.com/watch?v=_t-_m5UT-ds

 

[mace]

I had some difficulty getting the chassis out as the dog was trying to be as close as possible, storm was rolling in. Thunder.... I did manage to get a picture in focus and marked it up. Arrows are pointing to where the 120pF used to be. Note that it was glued to the other ceramic disc cap that is also glued to the large orange metalized film capacitor. It is a tight space between the V4 tube socket and relay. Had to cut the white silicone rubber goop to free it from the other parts. I will try to get a recording this weekend. For your reference...... For me something like this is an easy task, I would not do it if you are not familiar with working on this type of electronics as it is easy to make a mistake (including electric shock potential). If your amp only has the one ceramic capacitor you do not need to remove anything. Also my amp dates to July 16, 2012. Unknown if later models have this component or if the value of the part has changed. Also, if you like the way your amp sounds as is... do not remove it as it will be more difficult to put back. 8)


https://www.flickr.com/photos/159449305@N05/36542222983/in/album-72157686850818214/

Thanks much for the photo. I'll take a look at what my amp has next time I take it out of chassis. I suspect I also have the C39 cap as the amp was purchased new in February 2012.

 

[bandit2013]

You will gain a bit more in the upper end with the cap removed. I did not get into ice pick territory since the change but I actually prefer the 12AT7 in V4 anyway. At the same time I removed the capacitor I also added a parallel resistor to the bias network to bring the bias point closer to stock as I had changed one of the bias resistors to run the tubes a bit cooler than stock. ( I had reason to do so due to red plating most tubes except for SED =C= 6L6GC which are no longer in production). Yesterday I changed the bias point back to what I had worked as the chassis temperatures were very hot but not as hot as it would get with the stock resistors on the bias circuit. With the amp closer to original bias I found it necessary to curb the output of CH3 using a 12AT7 in V6. Now that the bias was restored to what I found that worked better the 12AT7 in V6 was not needed. Now I am back to Just one 12AT7 in V4. Also changed all of the other preamp tubes to Mesa 12AX7 (with the exception of V2 which is a Beijing tube). I may return the other Beijing tubes in the front end since they provided better hum cancelation on the clean channel than the Mesa tubes. I have thought it over about installing the C39 cap, actually I am going to leave it out as I found the amp far more enjoyable to play without it. Of course that is subjective. I would have to run the simulation again to determine if the output level also shifted as well as the frequency response. If it did, it is minor but the amp definitely seemed a bit louder. Perhaps I need to plot one curve over the other to see the relationship in better detail.

That last statement I made in the previous post was to make clear it is on you if you remove the cap. If you are curious, just cut one leg and try it out as you can always bend the leg back and solder together again (easier said than done as you may need to remove a few ribbon cables on one side of the connection to prevent melting any insulation with the iron. Also any work done to your amp will void the warranty if you still have one. Never work on the amp without measuring for stored voltage on the supply capacitors! I had worked on it recently and found the capacitors were at 120Vdc (I was operating the amp in open chassis format with speaker on the bench to verify effects on bias change also to determine if there was any AC in the bias voltage since it was not measuring up to the print, no AC as the bias voltage was super flat without any trace of ripple. It took a good 10 minutes to drop to a safe level shortly after it was powered down. Always check for stored energy before touching the internal circuitry even if you are just looking at it for S&G. DC current though the body hurts much more than getting shocked from an AC outlet and due to potential and possible current that can be supplied by the large supply capacitors it can be lethal. It is not necessarily the voltage that will kill you, it is the current !