GK rails in a pedal?

[Need Gigs]

You could but why would you want to?

You could mount an SVT to a pedalboard if you really wanted to.

I want to! Maybe I could put an 8x10 cab on my board too!

 

[wolfkeller]

Wouldn't that really be putting a pedal board on your 8x10 instead?

 

[boomertech]

THIS IMAGE HAS BEEN REMOVED. WE ARE WORKING ON A LATER VERSION NOW.

Here is another version of the GK amp simulator with current foldback protection like the original amp. I also added an output transformer so that the load has a reactance that will be more similar to a speaker voice-coil than a resistive load… obviously the tranny and speaker voice-coils are very different types of loads, but they both have reactance. The R3 load resistor will need different values if the tranny is installed or not. The tranny is shown with an optional ground lift switch if you are interested in DI applications.

The foldback current protection is set to approx 6mA peak load current, so it should be just about right for the tranny specified. If you want to increase the load current protection threshold you will need to decrease the resistor values for R19 and R20. The opposite is true if you want to decrease the max load current by increasing the value of R19 and R20. If you are not using the tranny, then you can just adjust the value of the R3 load resistor to change the foldback protection threshold. Also D6 and D7 are schottky diodes, because in this new circuit they need to have a low forward voltage drop.

R11 sets the bias for the Q4 driver. The gain of this driver is approx 15. Since this Q4 driver can clip asymmetrically with different bias currents, I suggest that it be made variable from 2K2 to 25K for tweaking to taste.

I will try to get some simulations done soon so we can view the waveforms…..

Edit: New image. Changed the values for R17, R18, R19 and R20.
EDIT 2: New image. Made some corrections and modified the output current foldback protection to work with low voltages.

-Frank

 

[boomertech]

cool that you drew that up Frank. thanks. i'll build one up later this week and try it out.
then i'll make a GK pre in a pedal form to see if if sounds anything like my actual. GK RB head.

That's great John! If you can feed the pre sig out from your RB head to this, you should be able to tweak and match the tone if the circuit works as I hope.

-Frank

 

[johnk_10]

cool! so I assume the LF353's in your circuits are powered with the +&- 15V at pins 4 & 8?

 

[johnk_10]

That's great John! If you can feed the pre sig out from your RB head to this, you should be able to tweak and match the tone if the circuit works as I hope.

-Frank

and yeah, that's what I'm planning to do.

 

[boomertech]

cool! so I assume the LF353's in your circuits are powered with the +&- 15V at pins 4 & 8?

Yes, they have the same pinout as a standard TL072. I don't know if the 353's have an impact on the tone, but that is what GK used in 1982 and I wanted to keep as much of the original circuit as possible in this circuit.

-Frank

 

[johnk_10]

yep. I noticed that they used the LF353's too.

 

[tallboybass]

Well yeah, but it's not a stereo or two-channel amp, it's a mono amp with a switchable boost, and they call that boost a "second channel".

Show me the switchable boost.

 

[boomertech]

yep. I noticed that they used the LF353's too.

If you want it to be more like the original, you can use the MPSA06 NPN in Q3 and MJE15031 PNP in Q4. They are all GP transistors so they may not make any difference at all... too tired to make calculations right now.

-Frank

 

[bongomania]

I guess it's my turn to throw my hands in the air. Push one button and you get one gain level. Push the other button and you get the second gain level, plus their "drive" emulation. Or use the footswitch, as I always did, and it's one button to switch between them.

 

[boomertech]

I don't think D4 and D5 are necessary in this circuit, but they are in the GK power amp circuit for protection.

-Frank

 

[friendlybass]

I want to! Maybe I could put an 8x10 cab on my board too!

Anyone else imagining the SVT stack on a pt nano?

 

[friendlybass]

Played through the gk fusion 550 thingamajig and it was great. Now I want a preamp of that one too!

 

[skulletwhip]

This thread made me break out my 800RB and crank the hell out of it.
Thank you.

 

[ryansalmond]

What I love about the RB power amp sound is this: any other kind of bass dirt I've ever played, be it mild, boutique analog overdrive or modern, digital fuzz, or anything in between, has made the breakup sound sort of separate from the rest of the note. I'm not sure if it's a distinction between harmonic and fundamental or what, but it's not a quality issue- I've had some fantastic dirt circuits and they've all had one thing in common, even if they sound great on their own- when I've played them in a mix, either the breakup cuts through the mix way easier than the note or the breakup gets totally drowned out by the mix. The GK growl is the one sound I've ever played where this doesn't happen- the way it distorts seems somehow perfectly integrated with the bass sound.

 

[ryansalmond]

Show me the switchable boost.

Even though there are two gain knobs, it is one gain circuit. Having two knobs just lets you switch from one setting to the other.

 

[johnk_10]

yo Frank. i'm going to first build the V2 without the transformer first since my TY-250P trannys won't be here for a few days, but i'm guessing that you use 2 & 4 for the primary, and 5 & 8 for the secondary with 6&7 tied together?

 

[boomertech]

yo Frank. i'm going to first build the V2 without the transformer first since my TY-250P trannys won't be here for a few days, but i'm guessing that you use 2 & 4 for the primary, and 5 & 8 for the secondary with 6&7 tied together?

Yes, set the tranny up for 1:1 exactly how you have it above.

I am running sims right now and there are going to be a few changes. I missed a bias resistor and I'm working on the current foldback circuit... so don't plan your layout just yet for V2.

I will add the bias resistor to the first version (because that one will work properly without the foldback) and update that schematic.

-Frank

 

[johnk_10]

okay. thanks for letting know. I just finished drawing up a PCB for V2 and was just about to start etching it, but i'm glad that I didn't now.