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Can someone explain the Gallien Krueger "Boost" knob?

prater

prater

Aug 4, 2011
211
24
#1
Not to sound like a total noob, I've been around the block on bass amps and get a lot of requests from other guys in my studio to tweak their heads for them. I understand a thing or two about EQing bass but the "boost" knob's exact function has me confused.

I just picked up the 400 RB IV and I've got a pretty good sound dialed in finally. For the first few days I had the boost set at zero because I wanted a clean tone and I couldn't get it to sound very good, it would begin to overdrive anytime I turned the volume much past 10 o'clock. I finally did some searching and turned the boost up to just before 12 o'clock and... problem solved, great clean tone without sounded overdriven and incredible volume.

I love this amp and it sounds fantastic but the boost knob has got me thinking and I just can't figure out exactly why I couldn't get any sound I liked, or decent volume with it at zero.

Is it the equivalent of a gain knob on other amps? If so, is there a reason it is strictly on the output instead of on the preamp?

I know the book says it adds "growl" but that is just a layman's term. What exactly does it do and why?
 
#3
When I had 400RB amps I always ran the volume at 12 o'clock, boost at ten o'clock and used the master volume as just that. I always found the boost more affected by the first volume knob, as in the more it was turned up, the grittier the boost tone. The way I had it set with my 2x15 cabs it added a nice mild overdrive that always seemed pretty touch responsive. The harder I played, the grittier it got. But that said, I prefer a slightly dirty "clean" tone.
 
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#6
What exactly does it do and why?
Click to expand...

As said, it's a post EQ gain stage. The "boost" works like a gain knob and with enough gain the "boost" stage overdrives and introduces distortion to the signal.

The volume knob also controls the gain (naturally) and with enough gain things again get overdriven. However, this overdrive is likely directed to gain and (active) tone control stages that are not designed to overdrive as "musically" as the "boost" stage.

At least in this particular amp the "boost" feature shouldn't have any major impact on the overall response of the amp but the distortion it introduces will introduce harmonics that are likely perceived as change in tone's "brightness".
 
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#7
prater said:
Not to sound like a total noob, I've been around the block on bass amps and get a lot of requests from other guys in my studio to tweak their heads for them. I understand a thing or two about EQing bass but the "boost" knob's exact function has me confused.

I just picked up the 400 RB IV and I've got a pretty good sound dialed in finally. For the first few days I had the boost set at zero because I wanted a clean tone and I couldn't get it to sound very good, it would begin to overdrive anytime I turned the volume much past 10 o'clock. I finally did some searching and turned the boost up to just before 12 o'clock and... problem solved, great clean tone without sounded overdriven and incredible volume.

I love this amp and it sounds fantastic but the boost knob has got me thinking and I just can't figure out exactly why I couldn't get any sound I liked, or decent volume with it at zero.

Is it the equivalent of a gain knob on other amps? If so, is there a reason it is strictly on the output instead of on the preamp?

I know the book says it adds "growl" but that is just a layman's term. What exactly does it do and why?
Click to expand...

The Boost knob adds a post E.Q. gain stage using G.I.V.E. technology (Gate Induced Valve Effect) adding “growl” as it is turned up.

In 1972, Bob Gallien developed what he calls G.I.V.E. Technology (Gate Induced Valve Effect). This 'valve effect' is integrated into the design of all RB Series heads and is part of what gives them their distinct sound. The RB Series heads deliver a warm, punchy tone with a fast transient response that not only sounds better, it feels better. GK's renowned voicing filters and intuitive four band active EQ make it simple to dial in a great sound. Everyday thousands of players around the world in arenas, clubs and basements plug into GK heads because they love that sound. The Boost G.I.V.E. Technology (Gate Induced Valve Effect) is also now on the new MB amps and etc. BTW, I get great tone from all my GK amps with the Boost off.

EDIT: PS - see this thread http://www.talkbass.com/forum/f15/how-utilize-gain-master-controls-g-k-mb210-853656/
 
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#9
Technically I know what it is--a post-EQ gain stage. But what it actually does for me, I've never quite been able to appreciate. It doesn't seem to add anything like the overdrive one might get from a pedal effect. Maybe it's time for me to experiment with it again. I haven't turned that knob since the first week I've owned my MB500, about a year ago.
 
#10
will33 said:
It adds gain as well as growl. You have to turn it a little above "off" to make the amp loud. It stays pretty clean in the lower ranges of the Boost dial. Gets dirtier the more boost you add.
Click to expand...

If you are referring to the signature GK growl, the REAL growl comes when pushing the power section as well, as opposed to just cranking the preamp and boost.

Apparently they designed some special method of soft clipping into the power section so when it clips it doesn't sound like SS barf, it adds a delicious growl.
 
#11
jerrymcdougal said:
If you are referring to the signature GK growl, the REAL growl comes when pushing the power section as well, as opposed to just cranking the preamp and boost.

Apparently they designed some special method of soft clipping into the power section so when it clips it doesn't sound like SS barf, it adds a delicious growl.
Click to expand...

They all do kinda work together/affect each other. Running the input gain higher will get you more growl out of the Boost gain stage. And yes, GK's are one of a very few SS amps that sound musical pushed towards their limits, including the power sections. I have a 400 and an 800 RB's and they are among my favorites. It's kinda nuts how loud the little 400 can get, and the 800 can hang with an SVT. They basically "feel" about as strong as other stuff with published power ratings twice their size.
 
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#12
Finally got my first GK last week (GK 1001-II) and been playing with it here and there and I really like it. I kinda was holding out for a 2001 but there are hardly any out there F/S used, but this is freaking plenty so far. Still working out some bits w/ the EQ as well but I've got a pretty good grasp.
 
#13
The key to getting the boost to work is having the input to the amp as high as possible without clipping.

Use something between your bass and the amp input jack to boost your level as high as possible, until the clip light flashes on the amp. Then back off a touch so the light is not on. I use the output gain knob on a compressor to do this. If you have a really hot or active bass you may not need this.

Next, have the "boost" control set at least to 12 o'clock, more=more grrrr. It also affects overall loudness.

Set the "volume" in the 10 to 11 o'clock range, depending on how much overdriven sound you want.

Adjust the "woofer" level to the volume you need.

I found the whole key was getting the signal level as high as possible going into the amp, just below where the clip light flashes. The volume knob does not control this level - it controls the level AFTER the clip light.

If your input signal is not very hot, like mine before I started doing this, the boost knob does not seem to do much to the tone other than make things a little louder. But once you get that level up, it gives you some nice growl.
 
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#14
Cristo said:
The key to getting the boost to work is having the input to the amp as high as possible without clipping.

Use something between your bass and the amp input jack to boost your level as high as possible, until the clip light flashes on the amp. Then back off a touch so the light is not on.
Click to expand...

ORLY. I may have to try this myself!
 
#15
I just looked at a GK schematic (RB100RB) and the boost knob is simply a volume control, right after the tone controls (right after the effects return). There is a pair of diodes from signal to ground at the point of the input of that boost knob, so turning the boost knob, affects the signal clipping at those diodes (apparently...but I could be wrong.) The output of the boost pot goes to the crossover section. Turning down the boost knob apparently mutes the effect of the diodes. There is nothing that should affect EQ with this circuitry. It's simple, and does work as GK describes, but it's a simple circuit...just a pot, and a couple of diodes.
 
#18
There is a pair of diodes from signal to ground at the point of the input of that boost knob, so turning the boost knob, affects the signal clipping at those diodes (apparently...but I could be wrong.)
Click to expand...

And wrong you are. The diodes have a static threshold, uneffected by the knob. The clipping threshold is also very high (12 volts or so) so the diodes merely act as limiters for any static spikes in the signal that can exceed the clipping threshold. This rarely happens. The "boost" knob controls amplitude of signal hitting the next stage (which is the aforementioned JFET common source amp).

This at least in 400RB IV. The way Gallien-Krueger decided to implement "Boost" feature in other amplifiers may differ so it's best to treat every amplifier as a case of its own. Even their concept of the "G.I.V.E." differs a lot from amp to amp: In some amps "G.I.V.E" means using JFET input to opamp stages, in some amps using a plain JFET amp stages and in some amps they use two back-to-back sourge-gate junctions as current limiters. I think the latter one was the original idea from which the name came from.
 
#19
jerrymcdougal said:
If you are referring to the signature GK growl, the REAL growl comes when pushing the power section as well, as opposed to just cranking the preamp and boost.

Apparently they designed some special method of soft clipping into the power section so when it clips it doesn't sound like SS barf, it adds a delicious growl.
Click to expand...

Robert Gallien has posted here that you get the GK growl when you push the amp rails. He has suggested that if you're not exactly playing Carnegie hall, its better to go smaller on the amps wattage and push it a little. So what you say makes sense.

It is my understanding that you can still get this growl at lower volumes by turning the gain down and the boost up and that you would not need the boost control as much if you were pushing the amp.
 
#20
Apparently they designed some special method of soft clipping into the power section so when it clips it doesn't sound like SS barf, it adds a delicious growl.
Click to expand...

Some earlier (as in 1970's) G&K amplifiers did have soft clipping schemes but they have been abandoned. The amps will clip with "hard edges" but on a contrary to popular belief it alone doesn't sound all that bad. Most tube amps do it too.

Generic SS power amps may have other kinds of problems while clipping such as "ringing" at the edges of the clipping, "rail sticking" (where transistors have a delay in which they remain saturated even after overdriving them has stopped), and "hysteresis" (which is delayed start or stop of the clipping, which on scope looks like pieces of the wave have been "bitten" off). These will introduce overall instability and first and foremost, very nasty sounding high order harmonics.

G&K power amps counteract these effects with few simple tricks such as:
- Use of completely push-pull voltage amplifiers instead of the usual scheme with single-ended voltage amp loaded by a constant current source (including bootstrap). The push-pull arrangement handles clipping much better than constant current sources, which usually go haywire during clipping.
- "Baker clamp" diodes, which prevent deep saturation of the voltage amplifiers.

On that note, none of these are unique to G&K amps. Fender's high power amps have employed same methods since early nineties and both of them were practically pioneered in PA amplifiers.
 

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