GK rails in a pedal?

[boomertech]

thanks for posting V3 Frank.
i'm gonna redraw my layout now. an i think that it's a good idea on the led's feeding the trannys.
I have a feeling that power supply sag possibly does have an effect on the 'rails' growl but maybe we should at least start with your V3 and then go from there.

I agree 100% John. If power supply sag is needed, we can always make that as a separate power supply circuit that feeds this one.

-Frank

 

[johnk_10]

I agree 100% John. If power supply sag is needed, we can always make that as a separate power supply circuit that feeds this one.

-Frank

that's exactly what I was thinking.
BTW,Merry Christmas!

 

[boomertech]

that's exactly what I was thinking.
BTW,Merry Christmas!

Happy holidays to you too!

-Frank

 

[johnk_10]

it took me a while to keep updating the V1 to the latest V3 schematic for etching but it's in the etching solution right now. hopefully, I drew it up correctly as I intend to build it up for testing later.
if this one works properly & sounds good, I'll probably end up drawing up a whole new one since I just kept modifying the V1 pcb until it matched the V3 schematic.
I have feeling if I drew the V3 up from scratch I could make it smaller and have less top links on it, but this one should work for now and it'll still fit in a 1590B.

UPDATE: etched. now I just need to drill it and mount the components.

 

[boomertech]

it took me a while to keep updating the V1 to the latest V3 schematic for etching but it's in the etching solution right now. hopefully, I drew it up correctly as I intend to build it up for testing later.
if this one works properly & sounds good, I'll probably end up drawing up a whole new one since I just kept modifying the V1 pcb until it matched the V3 schematic.
I have feeling if I drew the V3 up from scratch I could make it smaller and have less top links on it, but this one should work for now and it'll still fit in a 1590B.

UPDATE: etched. now I just need to drill it and mount the components.

Nice job, John!

I was just looking at the TY-250P tranny’s that I ordered to play around with… they are really tiny and cute. They have less mass than I expected, so PCB mounting should be a breeze. I can see these finding their way into several effects pedals or DI circuits. Having the dual secondary coils opens up even more possibilities.

***Here are a few tweaks that this circuit may need (if it works properly):***

If you find yourself needing more level from output1, then reduce the value of R7.

If you want a similar sensitivity as the 100W GK, then increase the value of R16 to 10-12K.

***If the circuit works, please take a few measurements for me if possible:***

Ground the input signal and take DC readings from TP1, 2 and 3. TP1 should have an offset voltage of a few volts negative. TP2 and 3 should be very close to 0Vdc.

Ground the input signal and look for AC stability (no oscillations) at the three test points.

Total current draw from the 9V supply with the input signal grounded, so we have a measurement for idle current. If you can get each rail current measurement… that would even be better.

Input a 200Hz signal and adjust the amplitude so that the output has a heavily clipped signal... Then take the supply current measurements again. If you are feeling ambitious, place a 1ohm resistor in series with the supply... You can then take a differential reading across the resistor with your scope to measure peak currents.

Positive and negative clipping voltages, with and without the foldback.

Thanks for prototyping this circuit!

-Frank

 

[johnk_10]

Hi Frank,
i'll try to take as many measurements as I can after I listen to it for a little bit.
thanks,

john

 

[boomertech]

Great John! When we get this circuit up to par I will also add test point information to the schematic for anyone else who builds this thing.

I was just simulating the foldback circuit and observed some really nice round top clipping with a 4Vp output signal. I adjusted the input to have a nice clean 4Vp (8Vp-p) output signal, then switched on the foldback and got some nice round top clipping.

I have been looking thru several power amp schematics to try and figure out what makes this GK 'rails' tone. I have a feeling that it has to do with the multiple positive and negative feedback paths with the Q3/Q4 current driver and the input LF353 when the amp clips. When the output transistors clip, the feedback (or lack of feedback control) causes the input opamp and the Q3/Q4 current driver to clip and distort in interesting ways. If the PS is saggy, then that will also add to this tone.

Looking forward to your results.
-Frank

 

[76jazzbass]

I love reading this stuff but man I wish I understood what the hell all this means, hahaha... Thank you for all you add to the 'world of tone'!

 

[boomertech]

I love reading this stuff but man I wish I understood what the hell all this means, hahaha... Thank you for all you add to the 'world of tone'!

Thanks! John is doing the hard part. I've only made it harder for him by finding my mistakes and making modifications. Every time the poor guy was ready to etch his PCB, I would change something else. But this isn't an original design at all... it's basically the GK power amp with the heavy output section replaced with a very low power version that should (in theory) operate the same way. If we can not get this to work properly then I have some other ideas for another circuit.

-Frank

 

[johnk_10]

well, I tried it but there must be something wrong with my PCB layout since it's super distorted and the voltages don't look right to me. it's also pulling more current at idle than I think that it should.
i'll have to go over my layout from front to back to see if I made any errors and then i'll report back.

 

[shinybeast]

Great thread! At the risk of wandering off topic a bit, could anyone point to some good examples of recordings featuring the "hitting the rails" sound?

 

[boomertech]

well, I tried it but there must be something wrong with my PCB layout since it's super distorted and the voltages don't look right to me. it's also pulling more current at idle than I think that it should.
i'll have to go over my layout from front to back to see if I made any errors and then i'll report back.

Out of all of the PCB's I have designed, I think I can count on both hands the ones that have been 100% correct from the start.

-Frank

 

[dannster]

Great thread! At the risk of wandering off topic a bit, could anyone point to some good examples of recordings featuring the "hitting the rails" sound?

Appetite for Destruction.

 

[ryansalmond]

Great thread! At the risk of wandering off topic a bit, could anyone point to some good examples of recordings featuring the "hitting the rails" sound?

Blood Sugar Sex Magik

and +1 to Appetite for Destruction

 

[johnk_10]

Out of all of the PCB's I have designed, I think I can count on both hands the ones that have been 100% correct from the start.

-Frank

well, I found a few errors and fixed them. fortunately I was able to use the PCB with errors for now by making a few corrections on it. I also drew up a corrected PCB but i'm going to experiment with the modified first one for now.
it's now reading a lot more normal voltages but it's clipping the input with a p bass plugged straight into it. if I roll the volume knob on the bass down, it cleans up nicely but IMO, it's still not quite right.
i'll post some voltage readings and possibly some scope pictures of its waveshape tomorrow. BTW, i'm running it with a bipolar 9V supply using an LT1054 to create the -9V.

 

[sprag]

Frank and John, You guys are amazing. I ask a question a week ago and you two, bouncing of one another are already testing prototypes. Now I just need to decide if I'm going to build one when you guys are happy with the circuit, as my first project. Or try and get my hands on 001 if it makes it to the market place.

 

[boomertech]

well, I found a few errors and fixed them. fortunately I was able to use the PCB with errors for now by making a few corrections on it. I also drew up a corrected PCB but i'm going to experiment with the modified first one for now.
it's now reading a lot more normal voltages but it's clipping the input with a p bass plugged straight into it. if I roll the volume knob on the bass down, it cleans up nicely but IMO, it's still not quite right.
i'll post some voltage readings and possibly some scope pictures of its waveshape tomorrow. BTW, i'm running it with a bipolar 9V supply using an LT1054 to create the -9V.

Nice, that LT1054 provides more than enough current for this circuit.

This circuit has an AC gain of approx 34dB. In spice, the negative part of the waveform starts to clip the output at -4.75V with a 100mVp input (200mVp-p). If I increase the input amplitude by 10mV, the positive parts of the waveform start to clip at 5V. Keep in mind that this circuit is designed to have full output with a 1.1V input signal with voltage rails of 60-85V. Try putting a 10K pot in series with R16 to adjust the AC sensitivity, that might be all you need.

Also the input impedance is only 100K. Which is a heavy load for passive instruments. If you want to have an instrument plug directly into this circuit you will need to change R12 to 1M.

-Frank

 

[boomertech]

Frank and John, You guys are amazing. I ask a question a week ago and you two, bouncing of one another are already testing prototypes. Now I just need to decide if I'm going to build one when you guys are happy with the circuit, as my first project. Or try and get my hands on 001 if it makes it to the market place.

I really don't plan on building these, but John has been making a lot of unique and rare pedals for people. If he isn't going to build them either, then there are many other builders out there that will do it for you. But don't rush to have one done just yet because at this stage it is a fairly raw circuit that needs more refinement... possibly a preamp with EQ in front of this.

-Frank

 

[johnk_10]

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.

 

[ryansalmond]

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.

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