initial preamp stage clipping

[lowell]

Got a JCM800 in that the customer would like 1st stage to distort earlier. I suggested rebiasing the tube but he'd prefer to plug a different tube in instead. He's running a TS9 into the amp. I looked at a 12at7 datasheet and it seems that the grid lines are more bunched up if it were biased with your typical components. Rk=1.5k, Ra=100k. I'm gonna try it and see but figured if anyone has any input please chime in. The following stages are all going to see less signal and thus distort later too so maybe it's all for naught.

[JoeM]

Wouldn't be easier to run a booster into the 1st stage? Cant see how having less gain in the 1st stage is going to help.

[lowell]

Joe,
It's not so much "gain" as it is about overdriving that first stage. I guess a gainier tube would be more prone to overdrive since it might extend further into the non-linear regions though so there's that. Well in any case I just did a little experiment. With a JJ ecc83 I got some compression on my scope with 1.5vacRMS input signal at 1khz. Swapped in a ecc81 and it took 2.5vacRMS to achieve the compression. The ecc81 sits with a Vk of 3.7v. The ecc83 sits with a Vk of 2.2v. Btw there is a 2k7 Rk in the first stage.

So to answer my own question, a 12ax7/ecc83 distorts earlier with your typical triode preamp circuit and bias arrangement.

[JoeM]

I'm just thinking that if you bias the stage so it's way into a non-linear region, does it ever clean up? Do you lose the effect that tubes amps do where rolling back your volume control cleans up the sound?

[g1]

Which particular model of JCM800 is it? Seems to me some of them daisy chain the channels so you get an extra gain stage depending on which input you use. If this is not one of those, it may be fairly easy to make it so.
However, I'm a bit surprised he can't get enough extra gain using the TS9. What exactly is he trying to achieve?

[lowell]

That's a good question... not really sure what he's trying to achieve. The amp is plenty gainy overall. He just made it sound like he wants the first preamp tube to overdrive earlier. I'll inquire further. It's the 2203 100watt master volume.

[Gaz]

That's a good question... not really sure what he's trying to achieve. The amp is plenty gainy overall. He just made it sound like he wants the first preamp tube to overdrive earlier. I'll inquire further. It's the 2203 100watt master volume.

You're the tech, so you're supposed to tell the customer's (politely) the how's and why's of their hair-brained idears. there's not much you can really do if he won't let you rebias the first stage, and even then the result would be subtle. Good luck!

[Merlinb]

Stick a couple of anti-parallel schottky diodes between grid and ground. Don't tell him.

[Steve Conner]

I smell internet magical thinking. Ralph Denyer's Guitar Handbook has a simplistic explanation of tube amps that puts forward "overdriving the first preamp tube" as a desirable goal, because it allows you to get a distorted sound at any volume.

Of course a high-gain master volume amp already takes care of that for you.

[Chuck H]

I mentioned (in another post, in another thread) that players today simply don't understand the gear like they use to. I blame information technologies. A little information can be a dangerous thing. Once someone thinks they have the lowdown on something it's hard to change their minds. As in. someone who is a better player and gets the "sound" they want says "You gotta overdrive the first stage man." Or they read that some tech for one of their favorite players said as much. Maybe they even heard or read that someone used a "higher gain tube". Now when you try to explain the reality to them, your percieved as the less informed one that's just not hip to this cool, great sound and how to get it.

We can discuss the technical reality of it for a hundred posts and still not solve for customer ignorance and arrogance.

I like the diode idea. When I was just a player I knew the reality of overdriving the first stage was a matter of sheer input voltage. No misconceptions. Just turn up the boost pedal more and turn down the master volume. That's the real answer. Now, getting it through to the customer is the tricky part. Back in the day before three channel, uber gain amps players seemed more saavy with the amps. Now you see guys on the gear forums discussing particular settings like they're not even sure how to turn a knob or what it does. Many times I've known experienced players that simply don't understand how to adjust their amps properly. Any amp with more than five knobs and a bright switch is confusing for them. I guess some modern amps have twenty knobs and/or a digital panel as well. Game over.

[R.G.]

... Any amp with more than five knobs and a bright switch is confusing for them. I guess some modern amps have twenty knobs and/or a digital panel as well. Game over.

This gets some discussion when we talk about our pedals. SOMEONE in one of these discussions suggested that we put in a knob labeled "More" that either was not electrically connected at all, or did a minor adjustment of LED brightness.

I can just picture the never-ending harangues about exactly where to set the "More" knob for mega-tone.

Of course we'd never really do that.

[NateS]

This gets some discussion when we talk about our pedals. SOMEONE in one of these discussions suggested that we put in a knob labeled "More" that either was not electrically connected at all, or did a minor adjustment of LED brightness.

I can just picture the never-ending harangues about exactly where to set the "More" knob for mega-tone.

Of course we'd never really do that.

Sometimes I go and purposefully back the gain, or other knob de jour that I've come to associate with magical tone (lately it's the mid knob) down a notch or two from where I think I want it. I have a theory which is the converse of RG's idea, that says that good stuff should always leave you wanting just a little bit more of it. Actually that's kinda sorta in this ancient jewish proverb "Honey is good, but if you eat too much you're gonna puke". Guitar players are a bit like goldfish in this regard.

This is more of a customer problem than a technical problem, since he's trying to tell you how to do your job, rather than what he actually is trying to accomplish.

[J M Fahey]

Funny thing is, Distortion pedals basically do exactly that: "put a couple diodes in parallel with the first grid".
Some literally (MXR Dist+, etc. ); others add some (basically unity gain) tone shaping after the diodes (Tube Screamer/RAT).
Oh well.

[lowell]

All brilliant responses. :-) haha. Yeah what I don't get is that he should be able to overdrive the first stage with his TS9. Even with the "gain" on the pedal down. Then just turn the preamp volume knob to taste for volume. The first gain stage is before the preamp vol so this is suitable. Chuck you're right a little info on the web is a dangerous thing. So many people are reading forums and buying the hype about stupid crap, and it's frustrating being a tech and trying to correct someone and say "no, actually that's not right, and THIS is how I can make that happen." And they're like, "no thanks I wanna try this other thing." Oh well. At least I'm honest with customers and I try to be as helpful as I can. I'm gonna talk with this customer and see if I can't figure out what he's after.

[lowell]

Just had a chat. So he wants the preamp tubes to breakup earlier. And he thought that there might be a tube that would do that with LESS signal applied, AND that this mod might "stress" the amp and tubes less. I told him there are not tubes that fit that profile, and that 12ax7's are the ones, and that they are not necessarily "stressed" anymore so than another tube type might be if they were to distort with less signal. phew.... Anyway it's all said and done.