Marshall Major and Axe FX 3

[oacikgoz]

I know many of you are tube amp gurus and might have a particular dislike of digital units like Axe FX 3, but I still think you have all the answers for what I'm looking for.

I'm trying to get Marshall Major tone out of my Axe FX 3 unit, which has all Marshall models under the sun modeled except for Marshall Major. The unit has a TON of advanced parameters in the power amp and pre amp section which allow you to simulate the subtle effects of several components in a tube amp, starting from a baseline model. Obviously impact of these parameters would be familiar to tube amp experts, but not necessarily us, the regular guitar-playing folk. I thought I could pick your brains on this matter.

My understanding is that marshall major has the same preamp section as the garden-variety early 1970s Plexis, but the following critical components that distinguish it from other plexis of that era (let me know if I'm missing anything):

1) Ultralinear power amp section
2) KT88s
3) No cathode follower
4) Tone stack inside negative feedback loop
5) Concertina/cathodyne PI driven by a 12AU7 (as opposed to 12AX7)

To give you some idea, attached the default "advanced settings" for 1959 SLP as a starting point, I'm trying to figure out which ones would get me closest to the Major tone. What's more difficult is that I do not have anything to compare against... So far I figured out 2, 3 and 4. 1 and 5 proved to be a lot more difficult, some settings are not exposed to the user, but hopefully it can be achieved with these attached settings.

Any ideas on which ones to tweak and in what way? Btw, feel free to suggest some other amp as a starting point if you think 1959 is a bad one to start with. There are 300+ amps to play around with

https://wiki.fractalaudio.com/wiki/inde ... odels_list

[Stevem]

The Majors use a 12AU7 for the PI.

As in any tube amp the sonic nature and the Gm of the tube used in the first gain stage will have a large impact on how the amp sounds and how it feels.

How do you get these variations out of a model?

I have 32 vintage tube amps and digital modeling gear and all I can say is you will never get every great detail and nuance out of a digital model that you get out of the real thing , never never!

Play around all you want with that digital rig and I am sure you will get some great sounds, but you will never get the overall feel of the original, and certainly not the in between tones and feel of the original as you play it louder and louder, and especially if you play a vintage tube amp that has feedback right at the tipping point of running out of that feedback voltage.

Also digital models of speaker cabinets respond with no dynamics to them which I find to be a real issue / turn off.

[oacikgoz]

Appreciate the feedback. Well it's a digital unit, "never" will be the same is a correct, but tautological argument. It's aimed at getting arbitrarily close to analog, like every other digital equipment, amp or not.

In any case, my question was around what tweaks are relevant...

[Roe]

marshall major has the same preamp section as the garden-variety early 1970s Plexis, but the following critical components that distinguish it from other plexis of that era (let me know if I'm missing anything):

1) Ultralinear power amp section
2) KT88s
3) No cathode follower
4) Tone stack inside negative feedback loop
5) Concertina/cathodyne PI driven by a 12AT7 (as opposed to 12AX7)

5) isn't it ECC82 (12AU7)?

[cdemike]

These are pretty simple amps compared to later Marshalls, so even within “garden variety plexi” you will find significant variations between years and similar-appearing models. Those differences, while subtle on a schematic, will result in very different sounding amplifiers. For example, Cream-era Clapton used a model that is pretty similar to the Super Bass that Page used, but differences like the bright boost capacitor, negative feedback levels, power supply filtering, and operating voltages contribute to very different sounding amps (aside from the big differences between them as players).

You are correct about points 1-4, and 5 is partly correct. There is no 12AT7 in a Major, and all preamp tubes and the phase inverter are 12AX7. Forgive me if you already know this, but because 12AX7 has two triodes in one envelope (think of it as two tubes in one bottle), that provides 4 total available triodes. Two are input gain stages which are in parallel and amplify each channel separately (identical to other 4-hole Marshalls). Those signals go through their channels’ volume controls before getting mixed together into one signal which then is amplified by a second gain stage (also identical to other 4-hole Marshalls). This is where the differences start. As you noted, most Marshalls from this period will then pass the signal through a cathode follower at this point, whereas the Major passes the signal directly onto the tone stack. Additionally, as you also noted, this gain stage had negative feedback inserted here, which will increase this stage’s bandwidth (comparatively more highs and lows relative to mids) and reduce its gain. This stage also has a (nearly) fully bypassed cathode, which will increase the bass response compared to a lead model. Compared to a bass model, that bypass capacitor would reduce the amount of local negative feedback relative to the bass preamp’s un-bypassed cathode. However, the insertion of global negative feedback here complicates that and will sound different than global local negative feedback.

Driving the tone stack from the plate will reduce the amount of signal that makes it through the tone stack. As you also noted, the Major uses a concertina/cathodyne/self-split phase inverter which does not amplify the signal, unlike in more common Marshalls where the long-tail phase inverter will amplify the signal by about 25x. Cathodynes and LTPIs distort very differently also, and a large part of the “Marshall sound” that people tend to associate with Marshalls from this period is overdriving the LTPI. Cathodynes can have comparatively coarser distortion (think dimed Fender Princeton vs Marshall).

The phase inverter uses the last available 12AX7 triode, and the signal is now split into two signals out of phase with each other. Those signals are then independently amplified by a driver stage made up by a 12AU7 dual-triode, which is a much lower gain tube than 12AX7 and generally is biased so that it has higher input headroom; this is the case with how the Major’s driver stage is set up. The signal finally reaches the output stage at this point, which as you noted is set up for ultra linear operation, which sounds kind of like a middle ground between triode and pentode output sections. Some later Marshalls have a “half power” switch that switches the power tubes operation from pentode operation (full output) to triode operation (half output).

Like 50w and 100w Marshall’s, Majors also came in bass or lead spec configurations which impacts the input gain stage. These will be identical to late 1967-spec lead and bass spec input gain stages when they first introduced split cathode gain stages on lead spec preamps but before they started putting bright caps on the bright channel’s volume.

The sum of these differences, along with the very high voltages in the Major, make it much cleaner than other period Marshalls. It will also break up differently and respond to pick attack differently.

Off the top of my head, I can’t think of amps with similar topologies that may be a better starting point than a 70s Marshall, but I’m sure one exists. Long story short, though, it may make sense to use a different amp as a starting point to model it.

[Roe]

along with the very high voltages in the Major, make it much cleaner than other period Marshalls. It will also break up differently and respond to pick attack differently.

The major does not feel much louder than a superlead necessarily but it has a different feel and tone. It offers more power and punch that affects bass and mids a bit

[cdemike]

The major does not feel much louder than a superlead necessarily but it has a different feel and tone. It offers more power and punch that affects bass and mids a bit

Makes sense in terms of the 3db difference as power doubles. I haven't looked through too many Major gut shots, but at least on the schematics the filtering is pretty tight, which I imagine also contributes to the differences in attack you're describing.

oacikgoz, one thing I didn't mention in my prior post regarding the bass and lead-spec Majors is that they also share the tone stack differences with their 100w and 50w counterparts: 33K/500pF for lead and 56K/250pF for bass. That will make a significant difference in how the amp responds, so it may be worth experimenting with some tone stack swaps if the model isn't getting you there.

[Roe]

The major does not feel much louder than a superlead necessarily but it has a different feel and tone. It offers more power and punch that affects bass and mids a bit

Makes sense in terms of the 3db difference as power doubles. I haven't looked through too many Major gut shots, but at least on the schematics the filtering is pretty tight, which I imagine also contributes to the differences in attack you're describing.

oacikgoz, one thing I didn't mention in my prior post regarding the bass and lead-spec Majors is that they also share the tone stack differences with their 100w and 50w counterparts: 33K/500pF for lead and 56K/250pF for bass. That will make a significant difference in how the amp responds, so it may be worth experimenting with some tone stack swaps if the model isn't getting you there.

Yes, some even had the 2k7/.68uf cathode, which sounds a bit cold and brutal on bigger amps unless you have hot pickups/humbuckers