GK rails in a pedal?

[boomertech]

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

I looked at a 1982 400RB power amp schematic and came up with this low power version that could possibly be used as an effect. It needs dual rail power supplies... +/-9V min to +/-15V max. The only changes from the original is the circuitry to the right of Q4 simulating the output power transistors.

You obviously would need to put another preamp before this circuit. If anyone tries this circuit and finds an error, please let me know so I can correct it.

EDIT: New image because D2 and D3 were backwards.
EDIT 2: Added D4, D5 and C6 to better match the original power output section. Changed emitter resistor values and R3 load resistor value to keep the gain the same.
EDIT 3: Added missing bias resistor R17.

Good luck!
-Frank

 

[ryansalmond]

Mind. Blown.

 

[friendlybass]

I looked at a 1982 400RB power amp schematic and came up with this low power version that could possibly be used as an effect. It needs dual rail power supplies... +/-9V min to +/-15V max. The only changes from the original is the circuitry to the right of Q4 simulating the output power transistors.


You obviously would need to put another preamp before this circuit. If anyone tries this circuit and finds an error, please let me know so I can correct it.

EDIT: New image because D2 and D3 were backwards.
EDIT 2: Added D4, D5 and C6 to better match the original power output section. Changed emitter resistor values and R3 load resistor value to keep the gain the same.

Good luck!
-Frank

Daaaaaang playa

 

[friendlybass]

Or THIS. I would be so into that. Just a low-watt RB power amp however small they could make it.

shoot could you imagine a high powered one? Mix and match pres with this thing? Got am swr preamp thatd sound great with that gk growl.
But throw that pre pedal di guts onto a little low power guy a la boomertech and I give you one big.pedal for big growl!

 

[ryansalmond]

That schematic is "Shut Up and Take My Money" Moment of 2014.

 

[heavyfunkmachin]

Silent lurker clapping hard.

Please somebody build this.

 

[David Jayne]

The "Boost" is nothing more than a clean boost. "Hitting the rails" refers to the preamp section overdriving the power amp.

How does Boost change the tone the way it (very definitely) does? Is quite apparent that the Boost circuit itself adds a certain amount of distortion and harmonic content on its own, separate from the high - volume 'Rails' thing.

 

[sprag]

I looked at a 1982 400RB power amp schematic and came up with this low power version that could possibly be used as an effect. It needs dual rail power supplies... +/-9V min to +/-15V max. The only changes from the original is the circuitry to the right of Q4 simulating the output power transistors.


You obviously would need to put another preamp before this circuit. If anyone tries this circuit and finds an error, please let me know so I can correct it.

EDIT: New image because D2 and D3 were backwards.
EDIT 2: Added D4, D5 and C6 to better match the original power output section. Changed emitter resistor values and R3 load resistor value to keep the gain the same.

Good luck!
-Frank

Thanks for taking the time to put that together Frank.
If one were putting together a shopping list, what wattage should the resisters be rated to?

 

[Veldar]

I'm working on building the 800RB preamp into a pedal the size of a SansAmp. I've designed a circuit board small enough to fit, but it'll be a while before I have the funds to prototype it.

Kickstarter.

 

[bongomania]

How does Boost change the tone the way it (very definitely) does? Is quite apparent that the Boost circuit itself adds a certain amount of distortion and harmonic content on its own, separate from the high - volume 'Rails' thing.

Three answers:
1) Any boost can be designed in a way that colors the tone. Most "clean" boosts are not perfectly transparent.
2) The boost on the old RB's probably colors the tone AND pushes the power section into its own overdrive.
3) The boost button on the newer MB heads includes GK's own carefully-designed emulation of the sound of the old RB's boost and moderate clipping. It sounds really good, though I don't think it ever gets to fully hard clipping distortion in the same way as cranking and torturing an old RB could do.

 

[rolandm]

You know, Positive Grid makes the Bias and Bias Desktop apps. I've worked up a few GK-modeled rigs in there, and with the ability to throw in either tube, or heavily pushed power amp sections at low volumes, it's very similar to what you guys are asking for. It's just not in a pedal.

 

[tallboybass]

3) The boost button on the newer MB heads includes GK's own carefully-designed emulation of the sound of the old RB's boost and moderate clipping.

What head has a boost button?

 

[bongomania]

The MB 500, MB 800, and related models.

 

[karter2000]

I looked at a 1982 400RB power amp schematic and came up with this low power version that could possibly be used as an effect. It needs dual rail power supplies... +/-9V min to +/-15V max. The only changes from the original is the circuitry to the right of Q4 simulating the output power transistors.


You obviously would need to put another preamp before this circuit. If anyone tries this circuit and finds an error, please let me know so I can correct it.

EDIT: New image because D2 and D3 were backwards.
EDIT 2: Added D4, D5 and C6 to better match the original power output section. Changed emitter resistor values and R3 load resistor value to keep the gain the same.

Good luck!
-Frank

Thanks Frank, time to do some breadboarding over the Holidays.

 

[ambissonnett]

This might be an odd question, but I've always wondered if the volume coming from the bass itself has an impact on hitting the rails? I have a PJ configuration and have the neck pickup maxed and the bridge pickup at about 6. I have to keep my 700rb-II at about 9 oclock, so I'm not blowing ear drums. If I turn the input volume down from my bass and crank up the master volume, would this help?

 

[Stevorebob]

My experience is that hitting the rails is all about driving the RB power amp and not driving the preamp.

 

[tallboybass]

The MB 500, MB 800, and related models.

My MB800 doesn't have a boost button...do you mean Channel B?

 

[bongomania]

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".

 

[johnk_10]

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.

 

[boomertech]

Thanks for taking the time to put that together Frank.
If one were putting together a shopping list, what wattage should the resisters be rated to?

I made the R3 load resistor light enough so that a 1044 charge pump could easily supply enough current for the negative rail. So, you can use ¼ watt metal film resistors everywhere in this circuit.

I have not built this circuit, but it should simulate the original power amp characteristics under normal operating conditions. In the original amplifier, the power section is a typical AB unity gain follower with current foldback protection. I don’t know how much the foldback protection adds to that GK ‘rails’ sound, because nearly all amplifiers have this same protection circuit and I don’t recall anyone claiming that their brand X amp has that same sound. But I will try to incorporate that characteristic into the circuit and give the details on how to increase or decrease that foldback protection distortion so people can tailor it to taste. I should be able to do this within the next few hours.

I will also try to get some simulation results of this circuit later on so that we can see what the signals look like under various conditions at different points in the circuit.

BTW, this amp can easily be modified to drive headphones, but the power supply and driver transistors would need to be sized to handle the load.

-Frank