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Old 16th May 2014, 01:23 AM   #1
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Default Harmonic Generator / guitar amp

Hello,
Is there a simple way to build an effect/pedal that will mimic a harmonic generator ?
I have tried in vain to obtain a schematic for a PRS HG-70 guitar amp to try and use a part of the circuit that causes the effect. They are no longer in production and obtaining one is out of the question.
Contacting Pritchard and PRS came up as a bust. So has anyone tried to duplicate this effect ?
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Old 17th May 2014, 07:58 AM   #2
JMFahey is offline JMFahey  Argentina
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Don't know what it *actually* does, got any explanation/analysis/comment somewhere?
Even if only verbal, without schematics.
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Old 17th May 2014, 09:41 AM   #3
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Send a message via Skype™ to JonSnell Electronic
Try here;
Chorus Boss CE1[pict] - Chorus - guitar effect pedals gitarowe Guitar Pedals - schemes drawings schematics - schemes drawings schematics | GuitarElectric.eu

Hope that helps
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Old 17th May 2014, 09:56 AM   #4
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Default Harmonic generator

Hello,
Here is a response I received from Mr Pritchard. It is as follows, and is in very
vague terms. He also no longer has a schematic and repairs them from memory.
Two harmonic generator circuits in amp, one to produce even harmonics and one to produce odd. Resistor/diode arrays. The resistors are printed inc and the diodes are surface mount.
The nature of a sort of current mirror circuit, except that the drop two diodes is fed into a transistor that has only one diode drop. The net effect is a mathematical squaring which leads to a second harmonics. And since the pitch is not perfect it also produces thirds. The ratio of seconds to thirds happens to mimic a triode.
OK now after all of that I am starting to think I have achieved the same sort of a
result by just changing one of the diodes in the clipping circuit in my Fender Squier 15
The result of that was the sound being richer and sounding somewhat more like a
tube amp that I think could be a result of more harmonics being introduced into the
final out put.
I also think I have fallen prey to chasing a pipe dream as there were only about
300 of the amps made. The schematics are basically nonexistent.
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Old 18th May 2014, 04:07 PM   #5
teemuk is offline teemuk  Finland
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Google Patent search his patents.

They are somewhat ambigous, take a while to decipher and do not go into little details but circuits are still circuits and after you study a bunch of those patents you get a pretty good idea what he is doing circuit wise.

The Harmonic Generator amp's circuitry covers only a small portion of his patents, one or two of the early ones. In his later career he developed gain stages that mimic triode characteristic curves and overall behavior and solid-state power amps that mimic many characteristics of push-pull -type tube amps. Fascinating stuff, most of it based around same basic ideas (e.g. the resistor diode/transistor array with triode-like "three halves power" transfer function acting like a diode) so once you start to get hang of it a lot of that ambigous stuff starts to open up.

The US patent relevant to Harmonic Generator amps is # 4,809,336.

In addition to Pritchard's explanation the ideas presented in the relevant patent focus on limiting effective bandwidth according to level of overdrive. Two methods are presented, one based on a negative feedback loop enclosing several stages with hi -and low-pass filtering, another based on envelope follower driving vactrols of which's resistive parts are used in lo -and hi-pass filters.

I can't unfortunately help with exact schematics of his amps either.

Last edited by teemuk; 18th May 2014 at 04:23 PM.
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Old 18th May 2014, 06:06 PM   #6
JMFahey is offline JMFahey  Argentina
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Thanks Teemu for leading us to the source.

And yes, narrowing passband when overdriven is one of guitar OT characteristics which in part explains smoother distortion.

For those interested in advancing in the SS realm, I suggest reading:
http://www.thatraymond.com/downloads...ttala_v1.0.pdf

The best SS MI amps data available anywhere.
An incredible resource.
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Old 18th May 2014, 07:21 PM   #7
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Quote:
Originally Posted by JMFahey View Post
Thanks Teemu for leading us to the source.

And yes, narrowing passband when overdriven is one of guitar OT characteristics which in part explains smoother distortion.

For those interested in advancing in the SS realm, I suggest reading:
http://www.thatraymond.com/downloads...ttala_v1.0.pdf

The best SS MI amps data available anywhere.
An incredible resource.
I totally agree!
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Old 18th May 2014, 08:35 PM   #8
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Hello,
Thank you to all the people who replied. It seems this thing is way to complicated for me to even understand. I did not want to research and construct something as complicated as a NASA
computer. I just thought since I couldn't find the original schematic, I could somehow find a way to construct a pedal, or add something to to an amp that may mimic the sound of tubes.
While reading some of the posts about all of this it seems even getting the schematics there
are some parts of the circuit blocked out as proprietary.
So I guess I will give up on it at this point. Once again thanks to everyone who replied
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Old 18th May 2014, 08:46 PM   #9
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Visiting pritchards homepage I stumbled over his "tunnel-back" speaker cabinets.

referring to his patent

US Patent # 6,411,720. Speaker systems with lower frequency of resonance - Patents.com

excerpt:

"THE OBJECTS OF THIS INVENTION

The object of this invention is a speaker system having a driver or drivers in a smaller than prior art cabinet, said cabinet having an air passage, without fiber fill, although might have fiber lining, with a cross-sectional area approximately equal, ranging from 1/2 to 3/2, to the total active area of said driver or drivers, wherein the front sound waves of said drivers exit immediately to the exterior of the cabinet, and wherein said passage receives the rear sound waves of said driver, transmits them to the exterior of the cabinet, and loads the driver or drivers with an acoustic mass that reduces the frequency of driver resonance by more than 20 percent.

To object of this invention is a speaker system having drivers with free-air resonance within the frequency range of interest for said speaker system and having an air passge which loads said drivers to reduce the resonance below the range of interest.

The object of this invention is a speaker system similar to a transmission line type, ie. with approximate constant cross-sectional area per driver, but with a shorter path from the driver or speaker to the exit.

Another object of this invention is a shelter for an electro-mechanical spring reverberation device which partly defines the air passage.

Another object of this invention is a cabinet and speaker combination for a musical instrument which reduces the speaker resonant frequency so that it is below the frequency range of said musical instrument."

This looks interesting to me. Pritchards cites a Bose patent as well and suddenly their "acoustimass" came into my mind...

can anybody contribute more information about this concept?
any hearing experience?
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Old 19th May 2014, 12:05 PM   #10
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Detuned Demystified.
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