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Old 14th March 2013, 04:26 PM   #21
akis is offline akis  United Kingdom
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Hi Christian

On your first paragraph, we set up the op-amp to have a fixed Av, then we mess with the feedback circuit so that the Av varies depending on conditions. In no way are we pushing the the op-amp into clipping - and I am thoroughly testing that there is no "accidental" clipping at all.

Secondly we talked about headroom somewhere above - why does the valve amp multiply by 50 at the input with no questions asked? What if you want to play in a quiet room and do not need 50 ? Would it not be better to multiply by whatever you actually need based on a pot or on some switch? Obviously the answer is no. You need Av=50 and in one respect, I believe because you want to get the +/-50V swing that goes together maybe with 1% distortion and other artefacts that make this "valve" sound.

The power amp is a very good point. Much of the valve amp sound comes from the way we (over) drive the 6L6s and how they drive the speakers. But when you play quietly most of the sound "hue" comes from the pre-amp and its tone controls.

I am not so impressed with Yahama or Roland since their very expensive "acoustic" piano sounds are awful. They have failed to capture this most fundamental musical instrument - and they do it by sampling I read somewhere. I own and play a Yamaha U3 and there is no similarity in the sound with any of the Yamaha clavinovas. I would so much prefer to have a silent clavinova but the U3 is so much better.

The first guitar amp I built for my son had no distortion and bright LEDs when it was clipping to warn the user. "If you want distortion or other effects, get the necessary pedals" I had said to my son, and he did get the POD XT plus another dozen or so effects. Most of them are really not very good although they purport to emulate exotic things, like the position and distance of the mic in respect to the cabinet.

For reverb I imported two huge strings from the US - superb sound. For distortion I also want to do it right if possible, else I will have to fork out 2,500 pounds for just a head let alone the speakers cabinet...
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Old 14th March 2013, 04:30 PM   #22
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Quote:
Originally Posted by akis View Post
Hi Christian
. For distortion I also want to do it right if possible, else I will have to fork out 2,500 pounds for just a head let alone the speakers cabinet...
A lot of bands just buy a small valve amp and put it in an enclosure and mic it.
High power valve amps cost a fortune.
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Old 14th March 2013, 06:43 PM   #23
Fenris is offline Fenris  United States
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Two simple changes:
Change R3 to a voltage divider to reduce the gain. Try changing R3 to 4.7K and putting a 1K from the right side of R3 to ground.
Make U1B a buffer rather than an amplifier (get rid of R6 and R14 and connect the - input to the out).

These changes won't affect the clipping array, so the sound should stay the same, just at lower levels. The output should be at low enough levels to properly feed a power amp. An LM3886 is a good choice for a power amp, or even a discrete type of amp could be used.
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Old 14th March 2013, 11:27 PM   #24
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Why you so agressive?

Forget it. I don't want a military war. Ignore my advice.
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Old 14th March 2013, 11:50 PM   #25
benb is offline benb  United States
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If you want to read more about tube distortion and simulating it, these seven chapters are interesting:
Peavey.com
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Old 15th March 2013, 04:15 AM   #26
akis is offline akis  United Kingdom
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Originally Posted by benb View Post
If you want to read more about tube distortion and simulating it, these seven chapters are interesting:
Peavey.com
And what a lesson that was... Amazing.

PS: and having a quick think about it: when you play quietly but with overdrive, you do not engage the effects of the sagging PSU, or the output transformer saturation, or the speakers inability to respond and the back EMF issues. The only component involved is the nature of the sound produced by the pre-amp. This includes of course the tone controls, but also the nature of the tubes clipping.

It seems I need modifications to account for the asymmetrical clipping as well as inverting input amps to avoid the +1 which are both elements I had not considered before.

Last edited by akis; 15th March 2013 at 04:24 AM.
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Old 15th March 2013, 02:07 PM   #27
Fenris is offline Fenris  United States
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Guitar amps are very different from normal audio amps. The goal of an audio amp is to amplify signals with minimal distortion. Guitar amps have evolved to provide various levels and types of distortion (clipping, compression, non-symmetrical, delay, etc.). Really, the guitar amp isn't so much an amp as part of the instrument as a whole. Different circuits can cause different types of distortion, some pleasing and some not.

The circuit in post 10 could best be considered an effects stage for an amp. You need the higher voltage to make the diode array be effective, but then you need to lower it back down again to levels that are good for an input to a power amp. Reducing the gain is what's needed so get rid of the gain in the last stage and maybe add a voltage divider (or alternately make the last stage an inverting amp with a gain of less than 1). What type of power amp are you going to use to drive the speaker? These higher voltage levels might be just the thing to drive a final tube amp stage.

If you're looking for an inexpensive tube amp, look in the "Musical instruments" forum for the "100 buck challenge" thread, there are lots of ideas there. Also there is ax84.com which is all about building guitar amps based on the common EL-84 tube.
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