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BTW what signal levels should I use?

5/9/2000 5:23 AM
Steve A.
BTW what signal levels should I use?
They're everywhere, they're everywhere!!  
    I had tried the resource link feature from the main page here and came up with nothing. I'll print yours out, too, and figure out which one will work with the parts I already have around.  
    So what output level should I use from the signal generator to simulate a:) a vintage single coil pickup and b:) a vintage style humbucker?  
Steve Ahola  
P.S. I used to use my old Casio CZ101 keyboard as a signal generator for adjusting the heads on my tape decks. I'd find a patch close to a sine wave and loop the cable around so that it would hold down one of the keys...
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5/9/2000 7:11 AM
Joe Fuzz

Steve: why 1kHz? I'm pretty sure that on the guitar that works out to approximately the 19th fret on the high E string. In other words, about B5.  
When I was just starting to build effects, I sat down and measured the output of my electric with a storage scope: single notes, power chords, etc. My *very* informal results gave me anywhere from 300mVpp (single note high E, one pickup on) to 2.4Vpp (power chord, all pickups on). Your string height & gauge, pickup height & strength, picking technique, etc. obviously will all play into it. Granted, that 2.4Vpp, I was really trying to crank it out...  
At that particular frequency of 1kHz, my notes say I got somewhere between 500mVpp to 900mVpp with all the pickups on.  
FWIW, when I was fine tuning one of my distortion pedals for symmetry, I used 300mVpp @ 100Hz. Of course, on this particular pedal I was actually looking for symmetrical distortion. With a tube amp, you may not be...
5/10/2000 4:22 PM
Ed Rembold

I like 400hz, .1v to .2v input. Voltage range  
adjustable with a pot, Works for me, Ed R.
5/11/2000 12:10 AM
just to show how tastes differ, I like 200Hz because it is in the middle of the guitar's fundamental range. Also, I like a lower signal level, say 50mv so i can see what happens as the signal fades out. I also like a triangle wave instead of a sine.  
To each his own.  
5/11/2000 3:36 PM

There are a lot of variables  
Using test tones nearer the middle of the complete range of a 22 fret guitar at concert pitch  
Maybe you primarly use less than the complete range, and would rather have test tones to better simulate your personal habits  
200Hz sounds low to me if you were only going to use one tone, but I know it is quite valid since its in an important working guitar range  
I like my Tek signal generator since it is a precision instrument, and I like to sweep the frequency ranges with various wave shapes. With a good unit like this, as you change Hz, the amplitude stays the same so you get a very good idea of the overall frequency response of the whole amp or just the individual stage you are interested in. This is still an entry level unit, but from a good instrument company like Tek. For this I hook up a BNC *Y* to the Tek output and run one cable to the upper trace input on the scope to watch the signal actually being sent to the amp, with the other reference signal output cable going to the input of the amp stage to be measured. Then using the scope probe into the lower trace scope input on a higher voltage setting, test the amplified signal for similarity to the smaller input. Then increase amplitude of the test signal and you can see where your output begins to clip & whether you think its ugly or not. Sometimes whats ugly at one frequency is just fine in other ranges.  
Very worthwhile to do, but still nothing beats using guitars which amount to a more expensive source than the Tek was, and the priceless human ear instead of an ordinary $400 to $4000 scope.  
While you're at it might as well try every note on the fretboard of every guitar you can, with every pickup combination, at least once with all guitar knobs maxed.  
When I want to roll off more bass than average for small speakers like 8inches or less, usually there is still full amplitude at 200Hz even if it begins to drop a db or two by 100Hz.  
My choice of a single test tone would be between 600 & 800Hz, close enough to the *standard* 1000Hz. According to RDH4, increasing amounts of distortion are less unpleasant as the frequency range of the amp is reduced, and I am completely in agreement from my experiments from a hifi point of view. For guitar amps a serious lack of fidelity to the input signal at certain settings can be ideal so that needs to be considered. For hifi amps or similar guitar amps, regardless of how wide or narrow the frequency response is, it is usually centered near 1000Hz so that is probably why it is a common test tone which would not be subject to attenuation by any circuit being compared. It could also produce a more *flattering* test result without being too near any more difficultly reproduced extreme.  
For any experimental input tube stage, if I want to visually see with test gear the potential clipping of my hottest (still good) sounding metallic blue SG played aggressively, I use a 400mv RMS amplitude test tone. For direct input for later stages in the amp to be tested individually, you will need suitably higher voltage test tones to simulate actual working conditions. If your power tubes tend to draw grid current (who knows, you *might* crank it all the way up ;-) then you might want to have a low impedance signal source when driving them directly; OTOH, in case the impedance of your source is too low to be realistic compared to the amp stage that would normally be driving them at least you could then increase it with an adaptor circuit.  
My even hotter test guitar is a pawnshop flying V copy whose body seems like excellent executive conference table lumber. I restored it & installed the Gibson 496R & 500T pickups discarded from a new Les Paul. These are about 500mv compared to the 400mv from the early '80s metallic SG, and 300mv or less from the early '70's SG.  
The older SG always sounds good, the *heavy metal* one you have to realize will never be clean with some amps unless you reduce the guitar volume knobs. The flying V I don't even expect to be clean hardly ever.  
At the other extreme, with a vintage Strat playing softer jazz styles, you may be lucky to get 50mv into the first tube. This is a good level for making sure your amp is loud enough without too much background noise.  
When I got the spec sheet for my *vintage* Peavey MACE VT, they had rated it for the full 160Watt output with a 50mv input, I can only assume it was a similar RMS rating like you would measure for a hifi amp.  
Hope this helps,  
5/12/2000 1:48 PM
Fast, easy, cheap, good function generator
Download this .PDF:  
This describes a one IC function generator using an XR-2206 IC, you can get the IC from Jameco Electronics for $3.25. The circuit listed here outputs sine, triangle and square waves from 1Hz to 100kHz. Sine wave distortion is between 1 and 3% over the frequency range.  
I have used this chip in several apps, from a synchronous motor voltage source to test fixtures, very easy to use and works great.
5/31/2000 5:56 AM
John Fisher

I used to use a lot of different cicuits for signal generators but for the last while I usually use computer generated tones from a computer. The main program I use is "Sound Forge" which generates a wide variety of tones which are very acurate.  
With some of the pluggins you can also have a variety of far out ociloscope applications for audio. One such program is Spectralab.  
John Fisher

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