Tube Amps / Music Electronics
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|12/22/1999 12:52 AM|
||Re: impedence of guitar?|
Sooo... a lower value volume control pot on the guitar, like J Fletcher's suggestion of 100k, would give less output impedance change and more even "tone" (less loss of highs) across the volume control range?
Somewhere, this would start loading the pickup...
|12/22/1999 2:37 AM|
Actually, J Fletcher was proposing a 100k grid resistor for the input stage of the amp in lieu of the usual 1Meg grid resistor. Resistor noise is directly proportional to the square root of the resistance value (multiplied by a few other things), so reducing the value of the input resistor will reduce the noise in the amplifier (this goes for all stages, not just the input stage). Unfortunately, because the output impedance of the guitar varies and can be rather high, if you go too low in input impedance on the amp, it will load the signal down and screw up the taper on the volume pot.
As for pickup loading, I was assuming the pickup impedance was 10K, but I don't really know, having never measured one. If it is indeed that low, a lower value volume control could be used, but as you suggest, it would start loading the pickup at some point. A good "rule of thumb" is to make the load impedance at least ten times the source impedance.
I would suspect the magnitude of the pickup impedance is different, perhaps larger than the DC resistance, otherwise, guitars would come with lower value pots.
I'll have to get out the signal generator and measure the impedance of a pickup when I get a chance. If anyone wants to try it, just connect a trimpot in series with the output of a signal generator set for 500Hz or so (make sure the generator is set for a very low signal level to avoid destroying the pickup - 50mV to 100mV is probably okay), and adjust the trimpot until the signal read on the other side of the trimpot at the pickup is half the input signal. The measured value of the trimpot is then equal to the pickup impedance magnitude at 500Hz (or whatever frequency the generator is set for). You can also plot the impedance curve by setting the generator for different frequencies and measuring the magnitude at each frequency.
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|12/22/1999 4:45 PM|
So is the Z of the pickup the same when it's acting as an AC generator, as compared to the Z when it's acting as a load for an AC generator. I was wondering about that...Jerry
|12/22/1999 5:02 PM|
I was trying to be careful to mention that there is a distinction between impedance, a frequency-dependent complex quantity, and simple DC resistance, which can be very close to the effective impedance value at some mid-band frequency where inductive and capacitive reactances have minimal contribution. I also kept the visualization simple by only allowing the volume control to be wide open.
Guitar pickups are inductors which happen to have some finite DC resistance due to the copper wire size & length, and capacitance due to the winding geometry. And the rest of the guitar wiring scheme includes some stray inductance and capacitance. Due to these frequency dependent parameters, the guitar doesn't have a linear frequency response. The shape of the output vs. frequency curve looks like the profile of a hill or inverted cereal bowl. The greater the pickup's inductance, the lower it's maximum frequency capability will be.
The effect of lowering the volume control resistance (in shunt with the coil) depends on the pickup's characteristics. I notice that lowering the value of the volume control mainly affects the pickup's extreme brilliance, but not the bass. The effect going from 1meg to 500k, and from 500k to 250k on a low-wind single coil, such as a typical strat or tele bridge, is easily heard. Typical humbucking pickups already have a lower HF rolloff point. I think the guitar manufacturers fit 500k volume pots (instead of a lower value) to leave in as much treble clarity as possible with these darker sounding pickups (which have both larger DCR and inductance than a SC pickup).
Adding a resistance in parallel with a signal voltage generator lowers the output voltage.
Lower impedance pickups can have lower resistance values in parallel with them (volume control and/or amplifier grid leak resistor) without affecting the frequency response. Lower impedance pickups also are less prone to hum & noise pickup, but they have fewer wire turns and thus produce lower signal voltages. You need more preamplifier circuit gain to compensate. (Les Paul uses low impedance pickups & preamps for the low noise and less colored frequency response.) Guitar cable shunt capacitance, usually on the order of 10's of pf's, which can be a concern in high impedance schemes, altering the perceived "brilliance" of the guitar, are of no practical concern in low impedance circuits. You can easily demonstrate this cable effect by trying different cables with a typical tube amp and a strat, then compare with a SS amp using the same input devices. You'll most likely hear no difference in treble clarity trying various cables with the SS amp, which has an input impedance at least one order of magnitude lower than the tube amp. ("Acoustic" amps excluded, they usually have 5meg Zin.)
I guess what I'm implying is that the whole thing works as a system, and that the guitar volume pot and amplifier's input impedance vs. pickup loading and sound coloration is dependent very much on the pickup's electrical characteristics.
Has anyone tried a cathode follower stage ahead of the first positive gain stage? It would alter the complex touch response we all like about tube amps. Actually, a wireless unit does the same thing.
|12/22/1999 5:43 PM|
You can easily demonstrate this cable effect by trying different cables with a typical tube amp and a strat, then compare with a SS amp using the same input devices. You'll most likely hear no difference in treble clarity trying various cables with the SS amp, which has an input impedance at least one order of magnitude lower than the tube amp. ("Acoustic" amps excluded, they usually have 5meg Zin.)
Right! I think that's what I was trying to get to... with tube amps (actually most amps) I normally keep the vol on the guitar (any guitar) at 10 to avoid the treble clarity rolloff (or the counter effect of a treble boost cap across the vol control terminals) that I attributed to cable capacitance and the typical high input impedance of the amp.
However, I recently picked up a Kustom 200B... and it maintains treble clarity with volume control position (same cable, same guitar) much better than any amp I've played through.
I'm assuming the Kustom has a low input impedance... the bias resistor on the first transistor (base to ground) is 49.9K with an input coupling cap value of .1mfd. My Acoustic 260 has a 4.7meg in this position, and doesn't maintain clarity with guitar volume control changes. I'm not sure if this is the "Acoustic" term you meant, but it seems true.
So... (acknowledging the 'system effects' but looking for general trend statements) I guess the question is... could the use of relatively high impedance amp input stages be hazardous to your tone?
|12/22/1999 5:49 PM|
Right. The "acoustic" I was referring to was any amp specifically designed to best accommodate high impedance piezoelectric pickups, not the brand of amps bearing that name.
|12/24/1999 4:49 PM|
Please forgive my ignorance, but this thread has brought some sort of question to my mind. I run a mic w/ a 5meg volume control into a BF Pro Reverb which has a 1 meg resistor at its volume pot. I have read about a mod specifically for a harp amp that requires changing this resistor to 5 meg.
My question, what will this do to my sound? Will I be louder, clearer, more defined? Is this something that I should consider doing? Do I have an impedence mismatch the way I have it now?
Thanks for all replies
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