AMT Legend R2 pedal or schematic?

[Paul.Ruby]

Since some interest was shown in PCBs with only the drive channel, I've made a project at PCBWay... These are stand-alone, so include everything needed to make a drive-only pedal.

The PCB project is shared on PCBWay here:

AMT Legend Series II Drive Channel ONLY - Share Project - PCBWay

This PCB can implement any of the AMT Legend Series II drive channels. Does not include the clean channel or cab sim. This will fit easily in a 125B case.I also have a PCB that includes the clean chan...

www.pcbway.com

There are links to interactive BOMs on that page as well as schematics for each version I've drawn up. Since it's generic, it can be used for any of the AMT Legend Series II drive channels. But, I've only drawn up the C2, E2, K2, M2, O2, P2 and R2.

 

[rossbalch]

I'd love to discuss the theory behind these pedals a bit more. Did we ever get to the bottom of the purpose of the J177?

Also, why do the FETs not need to be biased individually, is it something about the design, or is the parameter spread of the 2sk280-R just really tight?

 

[Blackboarcult]

The way I understand it (someone correct me if I'm wrong), J177 (or BAT diodes gate-to-ground on the Series 1) are there to simulate valve cutoff and subsequent dynamic shifts. What puzzles me a bit is that some jfets already exhibit gate cutoff even at 9v, so, unsure about the need for added parts to sim that behaviour... Comparing the same circuit with or without J177s, there seems to be some more going on when they are present, but at really hi-gain levels, not sure it brings anything else than a bit of added noise.
There are a couple of threads on FSB about the 1st and 2nd series of these pedals, each several pages long. Some useful info can be gleaned there I also recall seeing some russian website with the theory behind this design, but can't for the life of me find the link again.
SMD 2SK208 are pretty tightly specc'ed indeed. I've breadboarded two or three of these and never had to change drain resistor to fall within acceptable parameters.

Edit: pls also see post #18 on this thread

 

[temol]

or is the parameter spread of the 2sk280-R just really tight?

This.

 

[Paul.Ruby]

I have an F1 on the bench right now and tracing the schematic. These do have a bias adjust on the input jfet but it's set to 2k, which is exactly what would be picked for just about any mmbf5458 today. There's another bias adjust pot but haven't gotten that far yet. I've made lots of series 2 versions and no need for bias adjust. 2n5458 input fet and 2sk208r along the rest of the path. I have a bunch of J201 but I will likely never use them because they are such a PITA.

The project right now is to make a really good sounding fender preamp using fets and something similar to the AMT F1 to combine with a TPA3118, cabsim, balanced out and send/return after the preamp all in a 125B. I have a prototype built using jfets to match, as best as possible, the gain structure and frequency response of the Particle Accelerator tube based fender preamp. At the moment the Particle accelerator, the F1 and this prototype all sound about the same. But, the F1 and my prototype each have an extra gain stage, so the gain range will get to some dirt beyond about 10 o'clock on the knob. The Particle Accelerator has no break up but has enormous signal level using over 200V. So, it's clean output but giant. I didn't use a source bypass cap on my fet input stage to keep the gain low ahead of the tone stack and stay clean even with large input signal (like when using for a bass guitar). The F1 has a bass shelf on the input stage using a 330nf source bypass cap on the 2K source resistor, so it's brighter and aimed more at guitar wanting to get a bit of dirt from the pedal. Once I have this dialed in, I'll share it... My prototype is using 2n5458 for the input stage and J113 for the others. I may end up J113 for the input stage, too. These bias nicely with 1k5 source resistor.

 

[rossbalch]

It's interesting how the J201 kind of just became the standard for these types of circuits. Was the J113 just not really around back then?

 

[Paul.Ruby]

It's interesting how the J201 kind of just became the standard for these types of circuits. Was the J113 just not really around back then?

I have no idea. I was looking at Vgs-off for various jfets at mouser and bought about 10 different types. The goal was to land between 1k and 2k for source resistor, about 4k to 15k drain resistor and have about 4V drop on the drain resistor. J113 biased well so made it into my prototype.

 

[Feral Feline]

It's interesting how the J201 kind of just became the standard for these types of circuits. Was the J113 just not really around back then?

Way back when, the closest thing to a J113 was the MPF102.

 

[rossbalch]

I have an F1 on the bench right now and tracing the schematic. These do have a bias adjust on the input jfet but it's set to 2k, which is exactly what would be picked for just about any mmbf5458 today. There's another bias adjust pot but haven't gotten that far yet. I've made lots of series 2 versions and no need for bias adjust. 2n5458 input fet and 2sk208r along the rest of the path. I have a bunch of J201 but I will likely never use them because they are such a PITA.

The project right now is to make a really good sounding fender preamp using fets and something similar to the AMT F1 to combine with a TPA3118, cabsim, balanced out and send/return after the preamp all in a 125B. I have a prototype built using jfets to match, as best as possible, the gain structure and frequency response of the Particle Accelerator tube based fender preamp. At the moment the Particle accelerator, the F1 and this prototype all sound about the same. But, the F1 and my prototype each have an extra gain stage, so the gain range will get to some dirt beyond about 10 o'clock on the knob. The Particle Accelerator has no break up but has enormous signal level using over 200V. So, it's clean output but giant. I didn't use a source bypass cap on my fet input stage to keep the gain low ahead of the tone stack and stay clean even with large input signal (like when using for a bass guitar). The F1 has a bass shelf on the input stage using a 330nf source bypass cap on the 2K source resistor, so it's brighter and aimed more at guitar wanting to get a bit of dirt from the pedal. Once I have this dialed in, I'll share it... My prototype is using 2n5458 for the input stage and J113 for the others. I may end up J113 for the input stage, too. These bias nicely with 1k5 source resistor.

I have the F1 and I really like it. It does that ever so slightly crunchy clean super well and is a great pedal platform pre-amp.

 

[Bumfluff101]

Since some interest was shown in PCBs with only the drive channel, I've made a project at PCBWay... These are stand-alone, so include everything needed to make a drive-only pedal.

The PCB project is shared on PCBWay here:

AMT Legend Series II Drive Channel ONLY - Share Project - PCBWay

This PCB can implement any of the AMT Legend Series II drive channels. Does not include the clean channel or cab sim. This will fit easily in a 125B case.I also have a PCB that includes the clean chan...

www.pcbway.com

There are links to interactive BOMs on that page as well as schematics for each version I've drawn up. Since it's generic, it can be used for any of the AMT Legend Series II drive channels. But, I've only drawn up the C2, E2, K2, M2, O2, P2 and R2.

Hi, just building the R2 variant and I think R22 and C14 are missing on the pcb (they are shown on the BOM) so I assume it's just a case of bodging them from pin 2 of RVP61?

 

[Paul.Ruby]

O

Hi, just building the R2 variant and I think R22 and C14 are missing on the pcb (they are shown on the BOM) so I assume it's just a case of bodging them from pin 2 of RVP61?

Oh shoot. Yes. I thought I updated the gerber file in the shared project but forgot. I just updated it at PCBWay. But, unfortunately, you got the old one. Here's where those go on the new PCB.

The easiest bodge is to first solder R22 and C14 in series, add some heat shrink to that so it's one new component and then bodge that to the outer two pins of the volume pot.

And the schematic of those two components...

 

[rossbalch]

O

Oh shoot. Yes. I thought I updated the gerber file in the shared project but forgot. I just updated it at PCBWay. But, unfortunately, you got the old one. Here's where those go on the new PCB.
View attachment 96707
The easiest bodge is to first solder R22 and C14 in series, add some heat shrink to that so it's one new component and then bodge that to the outer two pins of the volume pot.

And the schematic of those two components...
View attachment 96708

Ha ha guess I'll have to do that on mine too.

 

[rossbalch]

I have an F1 on the bench right now and tracing the schematic. These do have a bias adjust on the input jfet but it's set to 2k, which is exactly what would be picked for just about any mmbf5458 today. There's another bias adjust pot but haven't gotten that far yet. I've made lots of series 2 versions and no need for bias adjust. 2n5458 input fet and 2sk208r along the rest of the path. I have a bunch of J201 but I will likely never use them because they are such a PITA.

The project right now is to make a really good sounding fender preamp using fets and something similar to the AMT F1 to combine with a TPA3118, cabsim, balanced out and send/return after the preamp all in a 125B. I have a prototype built using jfets to match, as best as possible, the gain structure and frequency response of the Particle Accelerator tube based fender preamp. At the moment the Particle accelerator, the F1 and this prototype all sound about the same. But, the F1 and my prototype each have an extra gain stage, so the gain range will get to some dirt beyond about 10 o'clock on the knob. The Particle Accelerator has no break up but has enormous signal level using over 200V. So, it's clean output but giant. I didn't use a source bypass cap on my fet input stage to keep the gain low ahead of the tone stack and stay clean even with large input signal (like when using for a bass guitar). The F1 has a bass shelf on the input stage using a 330nf source bypass cap on the 2K source resistor, so it's brighter and aimed more at guitar wanting to get a bit of dirt from the pedal. Once I have this dialed in, I'll share it... My prototype is using 2n5458 for the input stage and J113 for the others. I may end up J113 for the input stage, too. These bias nicely with 1k5 source resistor.

Is the bias resistor 1.5k at 9v specifically? I'm still trying to get my head around biasing FETs. I'm keen to design some circuits around the J113 as well as it seems much easier to bias and it cheap as dirt buy like 100 at a time.

 

[Blackboarcult]

Personally, for jfet stages, I focus on 3 things:
- Bias point (asymmetry)
- Clipping point (gain)
- Source capacitor if present (freq response)
When converting from 9 to 18v (or more), leaving all source components the same and just changing the drain resistor to bias the jfet will get you in the original frequency response ballpark, but bias and clipping point will most likely change. If you want to preserve the exact gain characteristics of the original circuit, the best thing to do is to run a spice sim and adjust the values to get the same waveshape.

 

[Paul.Ruby]

Is the bias resistor 1.5k at 9v specifically? I'm still trying to get my head around biasing FETs. I'm keen to design some circuits around the J113 as well as it seems much easier to bias and it cheap as dirt buy like 100 at a time.

Its very similar to bias of a triode tube... The resistor between source and ground sets the bias current and is (nearly) independent of supply voltage. And, it can be bypassed with a cap to increase gain at higher frequencies depending on the value of the cap. The drain resistor will determine voltage gain, primarily, and output impedance, secondarily. The supply voltage determines how much headroom the stage has to an incoming high point, outgoing low point of the signal. The jfet can only pull the output down to the source voltage, usually around a volt or so. The jfet gate, being a diode junction, will forward bias on incoming high voltage as well, so can also limit the low-going output signal. The gate can only reach a few hundred mV above the source voltage.

So, IMO, perhaps the most important spec of a jfet is the cut-off voltage because it determines where the source voltage will land. With a low cutoff voltage, which can be as low as 0V on a J201, you get a source voltage around 500 or 600mV and that limits the high-going input signal and it will clip there. For more headroom, need higher cut-off voltage. That's why I'm using J113, which are up around 1V, so much more headroom to incoming high-going signal... Unless chosen to draw grid current with a clipping diode from the gate to ground or to some portion of the source voltage, which AMT does a lot. J201 are all over the place on cutoff voltage and bias adjust resistors can only do so much to deal with that. I won't use the damn things anymore. 2sk208R are much more controlled and cut-off is around 600mV. J113 also seem well controlled with a higher cut-off, so capable of more headroom when desired.