[allthumbs]

Very simple clear answer I found on the Euro-tube site.

What is gain?"

Answer: Gain equates to, and is volume. In older amps there was volume or gain meaning exactly the same thing. When master volume amps entered the scene in the early 70's then the word "gain" gathered a new definition meaning distortion. In modern amps distortion is referred to as gain which is a product of overdriving and / or cascading the signal from your guitar thru several preamp tubes to compound the gain.

[fly135]

Yep, that pretty much nails it. In any technical speak except guitar lingo gain is amplification.

[coldethyl]

As clear as mud!

Nah just joking AT. I agree, that is a very good explanation of gain and probably the best one I've seen as well.

[P-90]

Yep, it's volume.

In the amps of the '50s and '60s, if you wanted distortion, you turned up the volume really high. Unless you had deaf neighbours, you could never get a dirty sound while practicing. Gain controls started showing up in the late '60s or early '70s and changed all that, people could turn up the gain and get distorted without playing loudly. It goes like this:

Distortion is primarily the product of "clipping," making your amp try to reproduce a signal that's too loud for it. If it has a 40 volt power supply, and it's being told to create a signal that's 45 volts, several volts of that wave are not going to be happening. Instead, the amp will hit the limits of its power supply, and just stop at that point, chopping off the top and/or bottom of the wave, making the wave form flat at the top and bottom, instead of rounded. This is what's called a "square wave," and it's the sound everyone associates with fuzzes. Here's an example square wave, courtesy of Marshall.


guitar--->gain control-->preamp-->master volume-->power amp

Play with the gain up and the master volume down, and you drive the preamp into clipping without the power amp working up a sweat, because the master volume has cut the (clipped) signal way down. Play with the gain down and the master volume up, and you get a clean signal right up to the point where you drive the power amp into clipping.

Gain controls don't do the whole job, though. When Dick Dale would drive his Fender Showman into clipping, the preamp tubes might clip some, but the output tubes and transformer would definitely be dimed as well. And preamp tubes (usually triodes) don't distort in the same way as output tubes (usually pentodes), which produce nastier sounding distortion.

So don't be surprised if your overdrive pedal never sounds exactly like a cranked tube amp. Gain can get you 90% of That Sound, but the last 10% will require a tube/transformer output stage which is pushed past its limits.

[Tekker]

Yep, in electronics gain is simply the output divided by the input. So it is a ratio of output to input or 'amplification'.

-tkr

[Tekker]

Originally Posted by P-90

So don't be surprised if your overdrive pedal never sounds exactly like a cranked tube amp. Gain can get you 90% of That Sound, but the last 10% will require a tube/transformer output stage which is pushed past its limits.

I would say the pedals can get you 3% of the way there. Even some of the nicer distortion pedals I've heard (SansAmp, Boss, etc) haven't even come close to a nice tube amp.

-tkr

[P-90]

Originally Posted by Tekker

I would say the pedals can get you 3% of the way there. Even some of the nicer distortion pedals I've heard (SansAmp, Boss, etc) haven't even come close to a nice tube amp.

-tkr

Well, I was assuming that you have at least one triode tube at your input, like a Valvetronix. If you have no tubes at all, your mileage may vary. FETs can be made to distort a lot like tubes, but the common sort of transistors (bipolar junction transistors) can't. All they can be made to do is participate in some sort of simulation. Sometimes that sounds OK, sometimes not.

[fly135]

I just picked up a Hafler T3 3 channel tube preamp. It has 1 clean and 2 distortion channels, plus a footswitch. I think it was made in the 90's. I really the way it sounds, so I've put all my distortion pedals on Craigslist.

I may keep a distortion pedal or two on my board but will need to put an loop switch on it. The T3 is too noisy if I put pedals in front of it, even when they are in bypass. Otherwise it's quiet. I put my delay, reverb, EQ, and modulation after the T3. Funny thing, if I put the delay/reverb pedals in the T3 FX loop they are noisy, but if I put them after the T3 they are fine.

[P-90]

The noise floor is set by the first gain stage, and bipolar transistors are inherently much noisier than tubes or FETs. The preamp will probably also preserve a little more of your guitar's highs, for similar reasons.

My FET stompbox project is growing into a complete combo amp, and, while it's a bit of a PITA to have to stick an effects loop into each channel, I don't see any good way around it.

[fly135]

I'm thinking that a high input impedance on the preamp may contribute to the noise when putting pedals in front of it.

[P-90]

That could be, and there's no easy answer for it. A lot of pedals don't have the current capabilities to drive low input impedances well, and sound pretty bad when made to. So some pedals will be unhappy whichever direction you go. Someone ought to make guitar cords out of shielded twisted pair.

I guess there is also some possibility of ground loops, if that turns out to be a problem you can put two plain old rectifier diodes (in parallel) between the jack and ground, one facing in each direction... that will let current flow if there's some fault condition, but not let through noise (which is too low amplitude to make the .7 volt drop across a diode).

[Noodler]

Originally Posted by P-90

Well, I was assuming that you have at least one triode tube at your input, like a Valvetronix. If you have no tubes at all, your mileage may vary. FETs can be made to distort a lot like tubes, but the common sort of transistors (bipolar junction transistors) can't. All they can be made to do is participate in some sort of simulation. Sometimes that sounds OK, sometimes not.

I think the Valvetronix uses an AX7 in the output stage. ie uses a pre-amp-type valve but in the power amp section. What do you think of that? Is it just a gimmick, so they can call it a "valve" amp?

What do you think about pedals that have valves in them, like some of the Vox Overdrives, used with a SS amp?

[P-90]

Originally Posted by Noodler

I think the Valvetronix uses an AX7 in the output stage. ie uses a pre-amp-type valve but in the power amp section. What do you think of that? Is it just a gimmick, so they can call it a "valve" amp?

What do you think about pedals that have valves in them, like some of the Vox Overdrives, used with a SS amp?

You're correct about the 12AX7, but it's not really in the output stage, it's in a sort of faux output stage. Let me bore you with some geekiness.

In a normal tube amp, you have something like a couple of 12AX7 triode tubes doing the preamplification, running the tone and gain controls, and so forth. These operate in Class A, meaning that they are operating in the most linear (undistorted) fashion possible. Class A, however, is inefficient for output stages, so most tube amp output stages use more power-friendly pentode tubes, in Class AB configuration. Unlike Class A, AB lets one tube cover the positive half of the wave, while a second tube covers the negative half. Tubes and power supplies being equal, Class AB will get much louder, and it will also introduce more distortion, and audibly different distortion. Class A and triodes are heavy on second harmonic distortion, which sounds pretty sweet and pleasant. (JFETs can be made to act exactly like triodes in this regard.) Normal bipolar transistors, and pentodes, tend to be heavier on 3rd harmonic distortion, which is grungier sounding. Class AB topology also cancels out second harmonic distortion, making the 3rd harmonic stuff more audible. You could kind of think of Class A triodes as a clean overdrive, and AB pentodes as being more like a mid-'60s fuzz tone.

So in a tube amp, what you're getting is a little sweet triode sound in the preamp stage, followed by nasty grunge from the AB pentodes in the output stage, and some bonus coloration added by the output transformer. If the tube amp has gain control, or is being driven by a booster, the triodes may be driven into clipping, but if you're just playing LOUD, the sound of the AB pentodes + hardworking output transformer will dominate.

Most attempts to get tube distortion with only one tube use triodes, with the 12A*7 family being by far the most common. They are usually operated in Class A, and are followed by a very routine sort of transistor power amp stage. This is fine if you want it to sound like you're overdriving the preamp stage of a tube amp. It's still undeniably tube distortion, and if it weren't for clipping Class A 12AX7s, nobody would ever have heard of Mesa Boogie (who introduced the "gain" knob on amplifiers). But it won't sound quite like a tube output stage: the use of triodes is different, as is the lack of an output transformer, and using Class A instead of AB. The designers are thinking that if you can really crank the overdrive, you won't miss the fuzz tone too much, and that's arguably a decent compromise.

The Valvetronix take it one closer, by putting the 12AX7 into Class AB. They don't use an output transformer, but do use some circuitry designed to act sort of similar. This drives the output stage, which is normal solid state. So now you've got something halfway between the overdrive and the fuzz, and most people seem to like it a lot. I don't consider it a gimmick at all, just a compromise, and a reasonable one.

I, myself, am prototyping a similar topology, in the hope of improving on that. I'm using JFETs for the front end, since they sound like clean triodes anyway, and using that to (over)drive a Class AB 12AU7 (very similar to Vox's Class AB 12AX7), and following it up with a real output transformer, albeit a very tiny, low-powered one. This, in turn, will feed the effects loop, and head back into what is a very conventional solid state output stage. So... kind of an audiophile Valvetronix. AFAIK, nobody has ever made a commercial amp based on a 12AU7 output stage, but a guy named Doug Hammond came up with a very nice DIY one, called the Firefly. You can hear one here, it doesn't get Marshall pentode grunge, but still sounds pretty tasty: YouTube - FaultRock Firefly homemade tube amp

Tube pedals vary a lot, a couple even use pentodes instead of triodes. Some run at 9v, others at 350. A few are very clean, others are incapable of being clean. So while I think it's a fine idea in theory, I wouldn't want to comment on tube pedals as if they were a homogenous category. They vary as much as transistor pedals do.

[allthumbs]

I just ran across this DIY tube pre amp pedal. The clips sound pretty good.
http://www.el34world.com/projects/tube_box.htm

[P-90]

Here's another, it's very minimalist -- one 12AU7 twin triode -- but it can run off a 9-12 volt power supply, which makes it both cheap and safe (nobody ever got electrocuted by 12 volts). The electronics, including tube, socket, resistors and capacitors runs about $10. Add another $10 for the power supply, and $20-$40 for enclosure, knobs, pots, plugs, etc., and you're set.

Tube boost + overdrive running off a 9 volt battery