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Mini-amp for Output Tube Distortion
Mini-amp for Output Tube Distortion
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Old 20th December 2018, 03:27 AM   #91
PRR is offline PRR  United States
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The doorbell on your house un-screws. Just saying.
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Old 20th December 2018, 06:16 AM   #92
Gnobuddy is online now Gnobuddy  Canada
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I think a good solution may have finally dawned on me. I have a couple of guitar pedals that have built in loopers. Instead of a tone burst generator, why not make a loop of a repeatedly picked guitar note, and just play that back as the test signal? I'll have to try that out one of these days.

-Gnobuddy
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Old 20th December 2018, 03:21 PM   #93
Tubelab_com is offline Tubelab_com  United States
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It should be easy to make a simple circuit with one of those CMOS switches to simply short across the top resistor in a voltage divider that's wired in series with the audio generator. Even if switched in the middle of a cycle, the resulting transient should be smaller than the original signal as long as no DC offsets are involved.

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The doorbell on your house un-screws. Just saying.
Chances are at least one of your guitar's has this function built in.

Have you ever seen a video Mr. Metal Shredder ripping into a screaming feedback laden lead rip then he makes the sound pulsate on and off? It's easy on a Les Paul, simply turn one pickup all the way up, and the other all the way down, strike a note or chord, then slam the pickup switch back and forth. Many other guitars with two volume pots can do the same thing....it's a little harder on a Strat or other guitar with a multi position switch since you can't just blindly slam it back and forth.

Then capture a piece of this pulse tone and loop it with whatever audio tools you have.
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Old 21st December 2018, 02:45 AM   #94
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by Printer2 View Post
Click the image to open in full size.

If anyone sees an issue now is a good time to bring it up.
I'm wondering if there is sufficient current to charge up the input capacitance of the IRF820 gates in the available time, and if the 470k bias resistors can discharge them sufficiently fast.

I looked at the IRF820 datasheet, and it says there can be up to 360 pF input capacitance. Also up to 24 nano-Coulombs of stored input charge at the gate. With 1 mA of drive from the LND150, I estimate that translates to a slew-rate of 2.5 V/uS at the gate (one-fifth the slew rate of the often despised TL072). Alternatively, it will take 24 uS for 1 mA to stuff 24 nC of charge into that gate.

So charging (positive swings to the gates) are a little iffy, but discharging through those 470k resistors worries me considerably more. 360 pF input capacitance and a 470k resistance translates to a cutoff frequency that's a bit less than 1 kHz. I think that's too slow even for a guitar amp.

I have little experience with power MOSFETs, so I may be wrong. Let's hope George chimes in. (Or you can steal his MOSFET driver schematic, which he's posted on diyAudio a few times.)

-Gnobuddy
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Old 21st December 2018, 04:29 PM   #95
Tubelab_com is offline Tubelab_com  United States
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In the normal common source sense "input capacitance" refers to the gate to source capacitance which is large.......but we don't care!

We are wiring the fet as a source follower. The source follows the gate, IE stays at the same relative potential offset. This effectively "bootstraps" the input capacitance and removes it from the picture. In the real world, no follower is perfect, but a high Gm fet comes close, so it is wise to avoid those 5,000 pF fets, but 350 something is a don't care.

Next up is the output capacitance (drain to source). Since we tie the drain to a well bypassed positive power supply, and the source can drive 2.5 amps with 3 ohms of ESR, again we don't care.

The reverse transfer capacitance, Crss IS important to us. This is gate to drain capacitance, and since the drain is bypassed to ground, Crss becomes the input capacitance of the stage. My (old) data sheet says 2.7 pF, on par with many tubes!

When searching for a suitable tube friendly fet, look at the capacitance VS voltage curves. Silicon life forms always have more capacitance at low voltages, so look for those that have a low and relatively flat Crss in the range of voltages where it will be used. This one goes well over 100 pF at 1 volt, but drops to about 3.5 pF at 15 volts then gently decreases to 1.5 pF at 500 volts. Feed this guy enough voltage where it always has more than 15 volts across it, and all will be well.
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Old 11th January 2019, 05:50 PM   #96
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by Tubelab_com View Post
Try a small pF cap, or series cap and resistor pair from the output tube grid to ground.
I spent a few minutes tinkering with LTSpice last night, and came up with the attached one-knob tone control idea. The frequency response tilts gently downwards over part of the tone knob's control range, which might be just what this somewhat bright little amp wants.

As a bonus, there is a slight mid-scoop at about the right frequency (roughly 500-800 Hz) for flatter "tone" settings. This might be good when dialing in clean tones.

Insertion loss is low, too, at around 6 dB.


-Gnobuddy
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