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GK rails in a pedal?

#141
johnk_10 said:
I agree that she ain't quite done yet. ;)
i'm also the process of drawing up an 800RB preamp PCB to be used with it (which will take some time), but when this GK poweramp sim is completed, i could also see it as a stand-alone pedal by itself to be used with any other preamp pedal.

once we get this one where we're completely happy with it, i'll probably post a DIY pcb layout for those that want to try their hand at building one.
Click to expand...

ryansalmond said:
I think that would be ideal- or to have a transparent/clean boost built-in to drive the input stage.
Click to expand...

How about a distortion detection circuit that lights an LED when clipping starts. So it would be easy to see exactly when the the rails are hit?

-Frank
 
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#145
BTW, the two red leds already in the circuit do, in fact, shine a little brighter when the circuit is clipping.
I socketed R16 on my proto pcb and tried a 10K for it and it clips A LOT less. the thing is, it clips the same amount wherever the output level on it is set, so it has a fixed amount of grit. right now, the only way to completely clean it up is to reduce the input level (bass's volume control).
 
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#146
johnk_10 said:
BTW, the two red leds already in the circuit do, in fact, shine a little brighter when the circuit is clipping.
I socketed R16 on my proto pcb and tried a 10K for it and it clips A LOT less. the thing is, it clips the same amount wherever the output level on it is set, so it has a fixed amount of grit. right now, the only way to completely clean it up is to reduce the input level (bass's volume control).
Click to expand...

This amp runs full bore, so we need to attenuate the input signal to control the amount of clipping. For test purposes, use a pot as a level control (50-100K) with the wiper feeding the signal input cap C3 and then connect the input source signal to the pot lug 1 and pot lug 3 to ground. You should use an input buffer before the input attenuation pot… a Boss pedal in bypass would work well. This is just for testing for now. BTW, that 10K R16 now scales this amp's AC gain to better match the original with the HV rails.

Also change the emitter resistors (R1 and R2) to 10ohm. I had the 220ohm in there to provide a little bit of gain for U1b, but let’s set that stage to close to unity gain now. This will also raise the clipping voltages seen at the output.

Since I understand how you guys want to use this pedal now, I will draw up a V4 for a standalone pedal with input attenuation and a high impedance buffer, after we get this one up to par. Plus connection points for an optional distortion detection circuit.

You could use those rail LED's as a "rails status" indicator on the outside of the box if you want to keep this 'rails' theme. Or as glowing eyes in an evil locomotive riding the rails box etching.:)

-Frank
 
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#151
just to be sure, Y and Z connect to where in the V3 schematic?

and here's a pic of my non transformer prototype pcb that i'm using for testing for now (but I've since drawn up a new corrected, nicer & slightly smaller one that I haven't etched or built up yet).
BTW, right now I seem to like it best with a 2k2 for R16, and a 4k7 for R7.

GKAmpSIM-ProtoPCB-01.jpg
 
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#153
johnk_10 said:
just to be sure, Y and Z connect to where in the V3 schematic?

and here's a pic of my non transformer prototype pcb that i'm using for testing for now (but I've since drawn up a new corrected, nicer & slightly smaller one that I haven't etched or built up yet).
BTW, right now I seem to like it best with a 2k2 for R16, and a 4k7 for R7.

GKAmpSIM-ProtoPCB-01.jpg
Click to expand...

Nice job, John!

Z from the distortion detector connects to the collector of Q4 and Y connects to pin 6 on U1. If the detector is too sensitive then increase R1 and R2. R1 and R2 need to be kept at equal values because of the diff amp. Also use a very efficient LED, something that will be bright with 2-4mA of current.

((Why did you change R7 to 4K7? That throws the balance of the output transistors Q1 and Q2 off and causes the output junction at the Q1 and Q2 emitter resistors to be at an approx -1V offset. Every amp I can recall, has the base bias resistors for the Q1/Q2 drivers equal in value... maybe change R5 to 4K7?)) <-- Ignore all of this... Frank, had a brain fart! Also, did you change the Q1/Q2 emitter resistors to 10ohm to keep the gain of U1b at unity?

johnk_10 said:
I also did receive the TY250 transformers so if I replace the socketed 1K resistor with a link I can also use it for the transformer version.
Click to expand...

I am going to run some tests today on the TY250 to see if the shunt attenuator (VR2) is going to have a significant impact on the frequency response. I will post plots when I get them.

BTW, with R1 and R2 at 10ohms we will be able to have R3 and the primary of the tranny in parallel. It's one of the changes for V4.

-Frank
 
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#158
just tried it with the 10R's instead of the 220's and it seems to be a bit better (smoother).
for me, 2k2 still seems to be the best value for R17. for me, a 10K is too clean and a 1k is too dirty.

i'm going to try it next with an TY250 output transformer next.

then i'm going to build a small Jfet input amp with an output level control to feed the V3 amp sim so I can vary its input level to dial in how hard that I want to push it.
 
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#160
johnk_10 said:
just tried it with the 10R's instead of the 220's and it seems to be a bit better (smoother).
for me, 2k2 still seems to be the best value for R17. for me, a 10K is too clean and a 1k is too dirty.

i'm going to try it next with an TY250 output transformer next.

then i'm going to build a small Jfet input amp with an output level control to feed the V3 amp sim so I can vary its input level to dial in how hard that I want to push it.
Click to expand...

Do you have the foldback off for these listening tests?

-Frank
 

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