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Anyone with a *real understanding* of power tube distortion?

7/15/2000 6:31 PM
Kurt, K.
Anyone with a *real understanding* of power tube distortion?
Hi, I have read many things about tube amps, but never seen any paper that fully describes (with enough technical detail so that the effect could be replicated correctly with newly designed tube amps, and.. even maybe simulated digitally) what happens in a tube amp's output stage when the amp is cranked?  
Does anyone know or would be able to write such a paper?  
7/15/2000 6:48 PM
Matt W.

That's like trying to write a book about what good sex is like. You have to experience it! All the eloquent words in the world won't do it justice. But it is a combination of tubes run at full tilt, transformer saturation, and very important, speakers running at near their limits.  
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7/15/2000 7:25 PM
Steve Slick

There is some stuff over at Harmony-Central, FAQs. There are a bunch of related articles. (  
There was an outstanding aritcle on distortion listed there a couple of years back. But I tried to go there a year back (it was the Jet Propulsion Labs, I think), it wouldn't allow access. Probably a government thing. That paper got into the effects of harmonics caused by distortion and how SS vs. vacuum tubes compare.  
As to THE ONE SOURCE, unfortunately, it doesn't exist. I've expressed this frustration here before. But when I think of it, I think I sounded kinda spoiled. SERVE ME, NOW!  
Unfortunately, you'll have to get it in bits and pieces like the rest of us.  
Steve Slick
7/16/2000 3:36 PM
One good place to look would be Radiotron Designer's Handbook edited by F. Langford-Smith. The listing for distortion in the index is longer than most peoples descriptions. This text is truly the tube bible, not Mr Pitmans book which was a nice attempt in a very dark time to spark some interest in tube amps :^O  
It's not that people don't know how to make a tube amp today just that it's a lot easier and more profitable to make a lame one. You don't have to know all the science behind it to make a chocolate cake either. The proper ingredients in the right proportions will produce good results even without knowing all the theory behind it. Most tube amps makers today are really no different than the people who make chocolate cake, they tell you that thier product is wonderful, most people beleive it while a few others who question it often find out that the sugar has been replaced with corn syrup, the vanilla has become vanillin and the chocolate used isn't produced quite the same way it used to be. "Would you like some ice cream with that?"  
7/16/2000 6:15 PM
Ken Gilbert

there's nothing really magic about power tube distortion. what makes it a bit different is the inductive nature of the OPT primary. there's a lot of factors at play.  
i have a beef with most (like 99%) amps out there... they use pentode output configurations, then wrap NFB around the whole lot to lower the output Z. screw that! when the output stage starts to clip your NFB goes out the window--now all it's doing is making you sound like ass and pushing the PI tube with way more error voltage than it's used to.  
take it from me (and dr.z too for that matter), get RID of the NFB and wire up your finals in UL or triode operation. that's the first step. then use a low impedance grid drive on your outputs (i.e. cathode or source follower) direct coupled so that you can push into grid current without farting. last step, wail away. revel as the screaming power tubes beg for mercy.  
simulating it? yeah, right. there's waay too much bullshit going on there... speakers/OPT core saturation or hysterisis/OPT winding flyback/clipping tubes--it's all there and it's all good.  
7/16/2000 7:57 PM
While removing the negative feedback will certainly get you more power and drive it is certainly not the only way to get good power tube distortion. Marshall which is the most well known amp for distortion uses NFB as you call it as well as fixed bias and standard tetrode or pentode connection of its power tubes.  
Good output tube distortion is easily attained with no modifications to your amp, it's a simple matter of how you adjust things and how your amp is set up.  
If you have a good non master volume amp (NMVA) you need to turn the volume above half maybe even more. There you go! output tube distortion (OTD) "Wait" you say "There's not enough distortion" Easy fix again, so don't go warming up the soldering iron quite yet, get yourself a nice overdrive unit (don't be frightened by a transistor or two) A Rangemaster, Klon, Z VEX SHO, or an MXR Micro amp if your budget is only $50 will push the preamp harder which will in turn do the same to the power tubes causing... OTD! The only thing to be careful of is getting a good mix between how high the overdrive is set vs the volume on the amp so you don't end up with a sound that's so compressed that is unusable.  
Master volume amps can present some difficulty because they allow you to push the preamp and at the same time starve the output tubes, if it's a decent amp simply not setting it that way solves the problem, if it's not then your in trouble.  
The biggest problem in attaining OTD is that most guitarists can not come to grips with the fact that their amp has too much power. The result is that they end up with a 50 or 100 watt amp that can not be run at the level needed to sound good without having neighbors, club owners, and soundmen screaming to turn it down. Even an amp as small as 5 watts puts out some serious volume when pushed really hard.  
7/16/2000 9:45 PM
Steve Slick

You've hit on a pet theory I've had based on my limited experience. I can't prove it because it's just a gut feel. But I think the most pleasing distortion is when the whole SYSTEM (preamp, poweramp, OT, and speakers) kind of eases into saturation simultaneously.  
I can't prove it because it'd take a lot of equipment, time, money, and knowledge that I don't have. But that's what seems to me to be the key to me.  
Having said that, it's hard to get that from a run-of-the-mill amp. That requires a lot of tweaking and thus, the reason only a few amps have it.  
Just my opinion, I could be wrong.  
Steve Slick
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