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Mini-amp for Output Tube Distortion
Mini-amp for Output Tube Distortion
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Old Today, 04:34 AM   #81
Gnobuddy is offline Gnobuddy  Canada
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Join Date: Mar 2016
Quote:
Originally Posted by Tubelab_com View Post
IMD almost creates dissonance because the mixing products rarely fall on musical intervals. This is where "power chords" come in handy.
This is one of the reasons why I like my guitar amps pretty clean most of the time. I enjoy sus2, sus4, minor 7th, major 7th, half-diminished, and other similar "colourful" chords, but these don't sound good at all if there is appreciable IMD.

The 6JW8 mini-amp is clean enough to work well for these chords if I turn the volume knob well down. But if I turn up, I'm quickly into "classic rock" territory, and anything more complex than a two-note power chord sounds really nasty.

I'm not at all a good rock guitarist. I get bored very quickly with power chords and pentatonic scales and very limited playing dynamics. But this amp might help me get a bit better at this sort of thing - it's the first time I've had an amp that can be fully overdriven without becoming too loud for me to tolerate!
Quote:
Originally Posted by Tubelab_com View Post
I was getting similar "stretched" sine waves by playing with the screen voltage in a pentode gain stage.
Exactly! In his valve preamp book, Merlin Blencowe talks about pentode load-lines, and how altering them can change the distortion and timbre. Combining his information with what I see on an oscilloscope, it seems that either lowering the anode load resistor, or lowering the screen voltage, tends to produce these asymmetrically stretched signals with lots of second harmonic distortion. (The load-line moves "above the corner" of the pentode curves.)

Going the other way, either raising the anode load, or raising the screen voltage, tends to make the signal flat-top on both positive and negative half-cycles, and the timbre is definitely different. (The load-line moves "below the corner".)

Trying to figure out what this little mini-amp is actually doing is making it very obvious that there are at least two gaping holes in my test-bench instrumentation capabilities at the moment. I can't really do FFTs (except the crude one built into my 'scope), and I can only measure frequency responses by the very tedious, very slow, entirely manual process.

There is a third thing I wish I had, the ability to produce a tone-burst signal, so I can see what signal dynamics do to the amp (bias shifts, etc.)

I think I see some DIY audio instrumentation in my future!


-Gnobuddy
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Old Today, 08:35 AM   #82
Kay Pirinha is offline Kay Pirinha  Germany
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Join Date: Oct 2011
Location: Somewhere in Germany
Quote:
Originally Posted by Tubelab_com View Post
IMD almost creates dissonance because the mixing products rarely fall on musical intervals. This is where "power chords" come in handy. They are mathematically created such that the primary IMD components are less dissonant, and are usually limited to 2 or 3 notes.

Yes. Due to the fact that power chords usually are two notes a fourth or a fifth apart, which means frequency relationships in small fractions of 4/3 or 3/2, respectively, IMD's don't sound too disharmonic.
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Old Today, 03:19 PM   #83
Printer2 is offline Printer2  Canada
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Join Date: Apr 2010
Quote:
Originally Posted by Gnobuddy View Post
I believe you are missing the point by a country mile (or a country 1.60934 km, since we are in Canada.)
I must confess, I used the dB relationship to take your analysis one step further. I did not have the time to do it myself (me bad).

Quote:
Trying to figure out what this little mini-amp is actually doing is making it very obvious that there are at least two gaping holes in my test-bench instrumentation capabilities at the moment. I can't really do FFTs (except the crude one built into my 'scope), and I can only measure frequency responses by the very tedious, very slow, entirely manual process.

There is a third thing I wish I had, the ability to produce a tone-burst signal, so I can see what signal dynamics do to the amp (bias shifts, etc.)

I think I see some DIY audio instrumentation in my future!
If you have a computer running XP, Visual Analyzer might be something to look at.


Visual Analyser
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