Drive Channels: why two gain knobs?

jaymenon · August 2023

Many amps have two gain knobs on the drive channel:

Gain Control: sets the amount of signal for the first gain stage of the OD section

Drive Control: controls the level between the 2 gain stages of the OD section

In practical terms, does it make a difference if you have just one gain control?

jasonbone75 · August 2023

Two stages can be voiced differently with simple component selections to build a gain structure with different frequency response so that is definitely something that can make a difference between the two approaches.

jasonbone75 · August 2023

And the diehard axiom of course :-) More Gain = More Gooder lol!

darthed1981 · August 2023

Basically the same as having a gain on a pedal and on the amp, options-wise.

You can even do that on digital modelling amps, on the Boss Katana for example you can put a drive pedal on and still have amp gain.

jaymenon · August 2023

But these are (subtractive) potentiometers - not amplification stages.

please correct me if I’m wrong

darthed1981 · August 2023

jaymenon said:

But these are (subtractive) potentiometers - not amplification stages.

please correct me if I’m wrong

You want my learned friend @ICBM ;

LastMantra · August 2023

I'd imagine they effect different parts of the circuit and just give you the option rather than one or the other?

ecc83 · August 2023

Each "gain" control sets the level that dives the next stage. One common setup is a gain pot after the first triode stage which sets the level into quite often the next stage that dives the tone stack. There then can be a pot post stack. Distortion will be different whether you distort pre or post tone stack.

I put gain in " " because the pots are actually just volume controls. Proper gain control is effected by a negative feedback circuit. Often one or both pots will have a 'situp' resistor in its bottom end to prevent total loss of signal.

Dave.

StefB · August 2023

jaymenon said:

Many amps have two gain knobs on the drive channel:

Gain Control: sets the amount of signal for the first gain stage of the OD section

Drive Control: controls the level between the 2 gain stages of the OD section

In practical terms, does it make a difference if you have just one gain control?

What amps are you thinking of? I can’t call many to mind that have this feature.

In fact the only one off the top of my head is the drive channel from Fender’s SuperSonic range, which is utter crap in no small part thanks to having two gain pots.

jaymenon · August 2023

StefB said:

jaymenon said:

Many amps have two gain knobs on the drive channel:

Gain Control: sets the amount of signal for the first gain stage of the OD section

Drive Control: controls the level between the 2 gain stages of the OD section

In practical terms, does it make a difference if you have just one gain control?

What amps are you thinking of? I can’t call many to mind that have this feature.

In fact the only one off the top of my head is the drive channel from Fender’s SuperSonic range, which is utter crap in no small part thanks to having two gain pots.

Redplate BlackVerb

Image: https://tfb-user-images.eu-central-1.linodeobjects.com/bdb6b3da5892e278755bd8178a9bf4d2295a347a7ec915c109

Ceriatone / Dumble ODS

Image: https://tfb-user-images.eu-central-1.linodeobjects.com/dd637e5077cf75e3b04b5e3422bf28d9fa0793ab28f884f1b7

jaymenon · August 2023

Though I wouldn't know how to adjust them to taste...

One knob seems so much simpler (despite the trouble mine has gotten me into over the course of my life)

Sassafras · August 2023

LastMantra said:

I'd imagine they effect different parts of the circuit and just give you the option rather than one or the other?

Apparently, they "feel" different!

Neilybob · August 2023

jaymenon said:

StefB said:

jaymenon said:

Many amps have two gain knobs on the drive channel:

Gain Control: sets the amount of signal for the first gain stage of the OD section

Drive Control: controls the level between the 2 gain stages of the OD section

In practical terms, does it make a difference if you have just one gain control?

What amps are you thinking of? I can’t call many to mind that have this feature.

In fact the only one off the top of my head is the drive channel from Fender’s SuperSonic range, which is utter crap in no small part thanks to having two gain pots.

Redplate BlackVerb

Ceriatone / Dumble ODS

That's interesting. My Ceriatone amp doesn't have a OD Trim, instead it has this on the back.

jasonbone75 · August 2023

Ch3 on boogie mark IV (V too probably but have not looked at one)

randella · August 2023

From memory, the JCM900 has gain and “sensitivity”, one controlling a valve stage and the other a secondary diode clipping stage.

Probably a gross over-simplification

Modulus_Amps · August 2023

It gives the user much more control over the amount of drive available.

The trimmer control could just be two resistors inside the amp pre set by the amp builder.

The trimmer control is probably a set and forget type control, but if you have ever bought an amp and the drive channel has way to much gain or not enough, you would wish you could change the internal gain - with this set up you can.

dazzajl · August 2023

Surely you’d have two knobs so you can go up to 22?

jaymenon · August 2023

One knob has landed me in enough trouble…

exocet · August 2023

The Dumble Overdrive Special type amp has 4 "gain stages".
The standard input volume / gain sets the amount of attenuation between stage 1 and 2 (Valve 1).
Then there is a trimmer or rear volume pot to control attenuation between stage 2 and 3 (Valve 1 to Valve 2 )
Then there's the Overdrive control (front panel) that attenuates between stage 3 and 4 which is followed by the (Ratio / level control) that acts as a Master Volume for the Overdrive signal only. It's simply a volume pot at the output of stage 4.
There's a lot of signal gain available when you cascade 4 ECC83 triodes together. The various controls allow more control at various stages in the signal chain preventing too much clipping from taking place in one part of the chain.

Ddigger · August 2023

exocet said:

There's a lot of signal gain available when you cascade 4 ECC83 triodes together.

TBH, I don't know much about amps.

When you say 4 triodes, does that mean two ECC,83 valves and you are using both sides of each valve?

exocet · August 2023

Ddigger said:

exocet said:

There's a lot of signal gain available when you cascade 4 ECC83 triodes together.

TBH, I don't know much about amps.

When you say 4 triodes, does that mean two ECC,83 valves and you are using both sides of each valve?

Yes. A single ECC83 / 12AX7 contains two triodes (2 gain stages).

AntonHunter · August 2023

I've also never seen this on an amp (mostly playing clean Fender types anyway) but the more I read this thread, the more useful it sounds! Having a gain pre and post tone stack sounds really useful.

ecc83 · August 2023

AntonHunter said:

I've also never seen this on an amp (mostly playing clean Fender types anyway) but the more I read this thread, the more useful it sounds! Having a gain pre and post tone stack sounds really useful.

You might like to read Merlin Blencowe's book on valve pre amps? There is SOME maths but you don't really need it and in any case it is mostly Ohm's Law which is really just Time Distance Speed.He shows many variations of circuits that give varieties of distortions, many of which have never been explored by the main amp makers AFAIK. They are a (small c) conservative lot!

Boost then EQ a bit gives a different result from EQ then boost. Take that through a number of stages and vary the EQ frequencies and you get a very versatile tonal palette..

Dave.

AntonHunter · August 2023

Ah nice. Will investigate. I'm not afraid of a bit of maths...

Neilybob · August 2023

My ole Peavey Classic has a Post and Pre knobs for the lead channel. I think the Pre works as an master volume sort of and the Post is all out gain. Really hard to photograph a mirrored back plate.

steamabacus · August 2023

Back in the 80s, a mate of mine had the little Squier 15 practice amp which has this Volume-Gain-Master setup.

Image: https://i.pinimg.com/originals/9a/a7/e5/9aa7e523d4d6b2cd5e46fe0f8eed2861.jpg

I'm not sure of how this functions in the solid state circuit. My vague memories of it (and neither of us was very experienced at the time) was that balancing the volume and gain vaguely affected the tonality of the bee-in-a-jamjar fizz (of which there was oodles!)

ICBM · August 2023

Neilybob said:

My ole Peavey Classic has a Post and Pre knobs for the lead channel. I think the Pre works as an master volume sort of and the Post is all out gain.

Pre is simply Peavey's term for 'gain' and Post for 'volume' (channel), although some other makers call this Master, if there isn't an overall master, just to confuse things further.

Ddigger · August 2023

exocet said:

Ddigger said:

exocet said:

There's a lot of signal gain available when you cascade 4 ECC83 triodes together.

TBH, I don't know much about amps.

When you say 4 triodes, does that mean two ECC,83 valves and you are using both sides of each valve?

Yes. A single ECC83 / 12AX7 contains two triodes (2 gain stages).

Thanks Exocet.

HerbieTheRadDorklift · August 2023

My Boogie Mark IV had this.

See pages 5 & 6 in the manual https://mesa-boogie.imgix.net/media/User Manuals/Mark 4.pdf

PS, reading that manual reminded me why the Boogie Mark IV is the greatest amp ever.

p90fool · August 2023

I used a Burman in the early 80s which I'm pretty sure had three gain knobs on one channel.

It sounded like arse whatever i did with them, but that might have been user error given that I almost exclusively played 4-hole Marshalls at the time.

ICBM · August 2023

p90fool said:

I used a Burman in the early 80s which I'm pretty sure had three gain knobs on one channel.

It sounded like arse whatever i did with them

It did. They did. I couldn't get anything usable out of the overdrive channel either. The clean channel was OK.

They weigh as much as a medium-sized galaxy as well...

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Drive Channels: why two gain knobs?

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