Plexi input modeling question

skydog

skydog

Experienced
Offbeat question:
There are models of jumpered plexi inputs. All good.

What might I need to adjust, to simulate the effect on the first preamp tube-stage, using an instrument y-chord plugged into both high-gain inputs of the normal and bright channels on the plexis? Some factoring of the input trim setting?
 
Skydog, unless I've missed something huge, which is always possible, my understanding is that the jumped amps are already jumped with, for example, a treble drive and a normal drive. As far as I can see, you only need to adjust the two drives to taste to duplicate the y-chord effect you want, no input trim adjustment required.
 
I posted, because playing my actual vintage plexi 50W, using a y-cable from guitar to the top inputs of the Hight Treble and Normal channels definitely does not sound the same using a traditional jumper. The former is brighter and the latter is rounder (as if treble has been filtered off).
It might be in my ears or in my head, but it isn't my imagination.

I should have provided more information. I'm trying to duplicate the sound of my old Marshall Plexi to the my AxeFx III.
In this experiment, the AxeFX III is feeding a QSC RMX, which sends the signal to a duplicate Marshall slant cabinet with Cerwin Vega ER123 and Celestion G12H30 speakers.
The plexi is running to an identical cabinet.
I set both to identical volumes.

1. When use one or the other input for both systems, they match.
2. When jumper the plexi and use the jumped models, they match.
3. When the plexi is run with the y-cable, I haven't figured out how to make the AxeFX to sound the same. This is the way I like to run the plexi.

I've messed with EQ, but it's a brightness that I cannot nail.
Actually, today I started to get closer, slightly decreasing the Gain and slightly increasing the Input Trim. Don't know why.
 
Skydog, the Y-cable is affecting the impedance of your pickups. Ocnor, I think, is on the right track.
 
I'm thinking so, as well. I'm staring at the schematics a few hours from now, and will get back.

I have the photographs, confirming Allman y-cabled out of his guitar, and then into the hi-power inputs into each plexi channel. I can also hear it, when using two 50W plexi amps wired just like in the photos. Btw, I am trying the amps versus the AF3 in a huge (empty) hall, so these things are breathing. As I said, there is a tonal and a structural difference in the sound, versus using a jumper across the inputs.
 
Depends on what the actual construction of the Y-cable is. If it's two cables running from your guitar then you've got likely much more cable capacitance loading your pickups which shifts the resonant frequency.

If it's a single guitar cable and you've got a short splitter cable at the end then it shouldn't be much different than jumpering the channels.

However... even a small bit of capacitance difference at the inputs of these amps makes a difference. Our models assume a fairly short jumper cable (3-4 inches). In my tests just increasing the length of the jumper cable to 1 ft. changed the frequency response significantly. So if your Y-cable is more than a few inches long it will change things.
 
Thanks. I'm duplicating the photographs from the Fillmore East videos and others from that same time. The cables are stage-length.

These cats were from the early golden-years of amplification, and they obviously weren't always thinking clearly, so who knows if any of this makes sense, lol. He was using three y-cables to feed two amps.

The only other bit of first-hand information I have, is that spare parts in 1970 were an issue, with a 300-day touring schedule, and that the Marshalls were swapped with Fender 50W OTs and 6L6GCs. This is possible. Studio work was done with Supers, and Layla was through a Champ.

Thanks a ton for your response and the response of others. I'm not doing this because I'm a madman. I've been asked to participate in a Fillmore Concerts recreation. I'm going to use the AF3. Therefore, I figured that I would give this last bit of the chain a shot, what the hell. Yeah, maybe I am a madman, lol.

On a personal note, thanks for being the best.
 
Definitely not a madman, I noticed the same thing years ago with my '78 50 watt 4-prong Marshall. It was louder if I used a pair of cables from my WEM Copicat outputs than when I jumped from the bottom of one channel to the top of the other. I always put it down to the fact that the jumped channel signal had passed through 3 x 68 k.ohm resistors, whereas the direct plugged channel had only gone through one of them. (I know I'm over simplifying the voltage divider, but the signal into the second channel is effectively attenuated a little).

Reading Cliff's response, my thinking was maybe way too unsophisticated back then!

Liam
 
wait, so what is the preferred method to mimic cable capacitance in the Axe? Is there a modeled solution?
 
LiamH said:
Definitely not a madman, I noticed the same thing years ago with my '78 50 watt 4-prong Marshall. It was louder if I used a pair of cables from my WEM Copicat outputs than when I jumped from the bottom of one channel to the top of the other. I always put it down to the fact that the jumped channel signal had passed through 3 x 68 k.ohm resistors, whereas the direct plugged channel had only gone through one of them. (I know I'm over simplifying the voltage divider, but the signal into the second channel is effectively attenuated a little).

Reading Cliff's response, my thinking was maybe way too unsophisticated back then!

Liam
Click to expand...
I was talking to Bruce Egnater yesterday morning, and he said something similar, both about inputs and about capacitance. I’m still experimenting, now that I know a bit more. I have cloned a few amps completely from
scratch, but I do not claim to understand the complete picture. Not by a long shot.
 
johnnypig said:
wait, so what is the preferred method to mimic cable capacitance in the Axe? Is there a modeled solution?
Click to expand...
Well yes, the modelled solution is the "jumpered" models that Fractal have already given us, and they assume a short jumper cable from the first channel to the second. Not sure if the voltage divider effect is modelled, but it's moot, as it's just resistive rather than reactive. So turning the second gain knob up a little (3 dB as far as I can see) will yield exactly the same tone if it isn't. From the OP's posts, sounds like that method has been nailed in the models, either because the jumper cable capacitance from one channel to the next has been modelled, or because it is insignificant, so doesn't need to be modelled.

I'm pretty sure it would be an interesting challenge to model a Y-cable that splits any distance ahead of the amp inputs though. It would be a capacitive interaction with the inductance of the pickups, all in front of anything the amp input gets to see. I'm pretty sure it would be relatively easy to add a parameter that combines cable capacitance per inch with distance of Y-cable join from amp inputs. But its interaction with pickup impedance would probably be non-trivial, and ultimately, is it worth the effort... ?

I'd suggest we ask @skydog to let us know what happens if he turns the normal volume up a bit in the jumped Axe-FX model and see if that gets closer to the real amp tone/feel. (Assuming that's the second channel he plugs into when "jumpering", this might also be an "order of channels" issue between real amp and model).

From my own experiments back in the day, and granted that my guitar output impedance was buffered by effects pedals and a WEM Copicat before going into the amp inputs, tweaking the gain controls on the Marshall amp turned out to get me back close enough that I didn't care anymore.

Liam
 
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"But its interaction with pickup impedance would probably be non-trivial, and ultimately, is it worth the effort... ?"

I understand the jumped models being an ideal case of 3 - 4 inches of cable from one input to the other. Rather, I'm interested if there's an electronic equivalent modeled circuit to mimic 50ft of cable in front of a Trainwreck, without using a simple digital LP filter.

Worth the effort? Yes, it would be really cool actually!
 
FractalAudio said:
Depends on what the actual construction of the Y-cable is. If it's two cables running from your guitar then you've got likely much more cable capacitance loading your pickups which shifts the resonant frequency.

If it's a single guitar cable and you've got a short splitter cable at the end then it shouldn't be much different than jumpering the channels.

However... even a small bit of capacitance difference at the inputs of these amps makes a difference. Our models assume a fairly short jumper cable (3-4 inches). In my tests just increasing the length of the jumper cable to 1 ft. changed the frequency response significantly. So if your Y-cable is more than a few inches long it will change things.
Click to expand...
sometimes I wonder if it is the products or the forum that I like best about fractal
 
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