High Gain? Why use it for an Amp?

[guitician]

Is it for sustain? Is it for Distortion? Both? Compression? I built an amp that was a little noisy and harsh in the treble. The power section is a classic circuit and drives celestion G12h-30 speakers, so I looked to the pre-amp. I sub'd some "Low Gain" tubes for the 12ax7's and the amp sounded much better, with dead silence at full volume and a perfect balance of tone, distortion and sustain. So, why does anyone use a high gain circuit design, if it just raises other issues with noise, and lack of a good tone. It's not like the guitar pickup has a low output like a magnetic phono cartridge. Both distortion and compression can be had without high gain, and sustain is what the guitar supposed to do in the first place. Voltage clamps on the output section should be the alternative to "overly gainy" preamps to get the amp to the "sweet spot" and maintain good volume.

[GibsonLover]

'Good sound' is in the ear of the beholder. You're absolutely correct, especially if you look at schematics of the high gain-ers. Much of the gain is dialed out between stages before hitting the next grid. What you suggest is one design approach that many have taken. Just remember when substituting low gain tubes in high gain sockets that it is biased for a low gain tube, typically ~1mA for a 12AX7. Put a 12AU7 in there and it's barely running at its 'knee' of turn-on. It'll handle 20mA, typically operated at 5-10 mA. The new 1mA bias is a little cold, but as you pointed out, may sound good to you.

[guitician]

Less gain, but still maybe just weak 12ax7's ?????

Actually, I dont know that the tubes are not 12ax7 because they aren't marked. But they look like old "tall plate" Euro made types. The pin out was the same and they work great. I wish they were etched with some codes so I would know for sure what they are.

[dodona]

interesting note. I was a long time high-gain maniac finally coming to the conclusion that high-gain and a good clean tone don't work. Therefore I sold my high-gain amp and bought an blackface fender. For distortion I use my EH english muffn and I am very happy with that combination.

[daz]

High gain preamps can sound anywhere from utterly horrible to heavenly depending on the design. If yours sounded bad till you dropped the gain, then the design isn't good. I spent almost a year trying to find the perfect high gain preamp design and in that time it sounded the gamut... horrible to heavenly where it is now. Often you can be ONE component change or design change away from great high gain tone. Its all in the design. I have literally had the amp sounding like garbage and made one single change that took it from garbage to very nice. of course at that point you have to then re-examine the entire preamp because on change will make all other component values suspect because they may no longer be the best choice for your "new" sound. in any case, it took me forever but mine sounds better than any marshall i ever owned at this point. So to answer your question, high gain sound great when done right, horrible when not, and then taking the gain down will usually sound better as you found out.

[guitician]

OK, the design is an exact copy of a '59 Fender Bassman, or early Marshall JTM. Its only the different tubes that changed. And it still has a very heavy metal gainy overdrive when dimed, just better sounding. I could now get the same output distortion I was before just by replacing my pickups with HI-Output ones.

[Steve Conner]

Everyone's idea of what constitutes "high gain", and "good tone" for that matter, is different. Beauty is in the eye of the beer holder.

An amount of gain that is usable in your home with the master volume turned down can just turn on you and drown everything in noise and feedback, when it's cranked at a gig.

A good guitarist needs less gain to sound good.

What are "Low gain" tubes, and how much gain do they have compared to regular tubes? The difference between a "High gain circuit design", which I take to mean a Boogie, Soldano or the like, and an old Fender, is at least one extra tube stage, which is about a factor of 50 more gain. And I don't know of any tube that has the 50 times less gain it would take to undo that.

I reduced the gain of all my home-made amps as I got experience with gigging and my chops improved.

[daz]

A good guitarist needs less gain to sound good.

Quite true, and in fact it's easier to play tastefully with less gain mainly because of the dynamics you can play with. However, in my case i find high gain amps more desirable because i find, and this is just MY taste, but i find that a less gainy sound that is had by setting the amp with a good deal of drive then turning the guitar down till you have just a little drive with dynamics, is for me at least a more desirable low gain sound. In other word, an amp with less gain where i am getting that same amount of drive but with my guitar on 10 to my ear and fingers isn't as juicy. But it really depends on the amp, and thats why i decided to build my own.....so i could design a pre that would do what i want better than the marshalls i've owned. took me a year but even not being a tech i did it, tho with a lot of help from all you guys here ! (Thank you)

So while i like high gain amps, (hi G pre that is) i just happen to like the way they sound better when using less gain by dropping it at the guitar. Different strokes...

[guitician]

Yeah, I know what you mean by running the guitars volume lower. You like playing in the non-linear transfer characteristic of the tube just before it clips. Nice warm tone there for sure.

[guitician]

What are "Low gain" tubes, and how much gain do they have compared to regular tubes? The difference between a "High gain circuit design", which I take to mean a Boogie, Soldano or the like, and an old Fender, is at least one extra tube stage, which is about a factor of 50 more gain. And I don't know of any tube that has the 50 times less gain it would take to undo that.

Three 12AX7's stages mu=100 100x100x100= 1,000,000 total
Three 12AU7's stages mu=20 20x20x20 = 8000 total
50x8000 = 400,000 total

The real gain is dependent on other factors like circuit componants, but the low gain tubes like 12au7 really lower a three stage amps overall gain that much.

[Enzo]

You do realize three 12AX7 triodes won;t amplify a signal a million times.

Why put lobster on the menu when there is perfectly good steak already there?

In many amps, more gain stages are used than "necessary" because they use each stage as an opportunity for tone shaping.

[guitician]

Yeah I do, but for comparison, it shows the difference between Low and High gain. I've seen a actual circuit voltage gain of 47 for 12ax7 in some data sheet. And at that gain 3 stages would get you over 100,000! I've only seen them wired like that on the one input stage of microphone inputs.

"Why put lobster on the menu when there is perfectly good steak already there?"
I'm not sure what you mean to say here. I just don't see a point in running a guitar through what amounts to a magnetic phono cartridge input. There are a million valid guitar tones and a market for all of them.

[chipaudette]

I've never been a fan of discussing "gain" (in the rock guitar sense) by evaluating the total voltage multiplier of each tube stage (ie, "gain" in the electrical engineer sense). It's just a mixture of languages that, I believe, helps no one.

I believe that the goal of a high gain (in the guitar sense) preamp is to get right gain (in the EE sense) at all the right points in the preamp so that you're riding the non-linear response of the tubes in just the right way. It's not total (EE) gain that matters, it's about getting enough (EE) gain to get the tube to color the sound.

If there's not enough (EE) gain, you'll be riding in the linear ("super clean") portion of a pre-amp tube's response. Clean is cool, if that's the sound that you're going for. A clean preamp can also be cool if you're shooting for all your distorted sound to come from the power amp. Just put a couple clean pre-amp stages in front of the power amp and BAM you've got some real rock 'n roll. Unfortunately, that approach is often ear-crushingly loud.

So, if you can't tolerate that volume, you've got to get your sustain/harmonic complexity/distortion from the pre-amp. In that case, you've got to hit the pre-amp tubes a little harder to push them into their non-linear (distorted) range of operation.

No, it's not about getting a total of 1,000,000x voltage gain. Instead, it's about building up enough gain to get some saturation in one tube stage somewhere in the preamp. But, you don't want all of your distortion coming from one stage. That tends to sound bad. It tends to sound better to get smaller amounts of distortion from several tube stages.

Therefore, all of the following tube stages are not about adding gain in the EE sense. Nope. They're about adding distortion in small amounts. Keep adding stages until you've got the amount of sustain/crunch/squeeling that you'd like. Plus, between each tube stage, you can add a little frequency shaping to tune the voicing of the distortion from Clapton to Santana to Metallica to Slipknot. It's quite an art to voice a high-gain pre-amp.

Let's see...the summary is that, no, it's not about total voltage gain...it's about getting just the right gain at the just right points in the pre-amp circuit to get just the right non-linear behavior out of each tube.

Or you could just turn it on, turn every knob to 12:00, and see if it rocks your behind. If so, then its got just the right amount of gain in the EE sense AND gain in the guitar sense.

Chip

[MWJB]

"I sub'd some "Low Gain" tubes for the 12ax7's and the amp sounded much better, with dead silence at full volume and a perfect balance of tone, distortion and sustain."

"OK, the design is an exact copy of a '59 Fender Bassman, or early Marshall JTM. Its only the different tubes that changed."

I'm not trying to rain on your parade but a well built 59 Bassman, or JTM45, should be quiet (just a little "blow" with all controls dimed and nothing plugged into the input, pretty well silent at moderate volume levels).

Unless you have huge output pickups then you shouldn't really be getting massive amounts of gain. As Steve says, most amps considered as "high gain" have at least an additional stage over your amp (as Daz's amp does).

If you use humbuckers then a 12AY/12AU might do the trick in V1 (or whatever configuration sounds best to you...there's no "right or wrong" as long as plate resistors can handle any extra current)? Most guys using single coils seem to prefer 3x12AX7.

Most manufacturers use 12AX7 principally because they are cheaper than other types (the widely fitted Sovtek 12AX7WA costs a big manufacturer pennies per tube), & because they usually give appropriate gain in most circuits (which, much of the time, have been developed around a 12AX7).

[guitician]

I'm not trying to rain on your parade but a well built 59 Bassman, or JTM45, should be quiet (just a little "blow" with all controls dimed and nothing plugged into the input, pretty well silent at moderate volume levels).

It was quiet with 12ax7, it just got DEAD quiet with the low gain tubes and the guitars volume down all the way.