Polepiece effects on tone

[David Schwab]

I think the difference is that tubes are slower reacting then the transistors, and tend to round off the wave more.

[hewo]

i recall faintly mesa boogie had fet modules that substituted the preamp tube socket, can't imagine those days had high voltage fet headroom capability because that's what would be required. that would mean the differential between the gate to either source or drain would have to withstand quite a voltage potential.

is this still done today? if it is, does the fet still have to be replaced like a tube whose heaters eventually get cooler with use? it would surely revolutionize retubing hassles.

[ken]

The MB FET module never saw high volts, it had large I/O coupling capacitors to remove the DC volts from the AC signal voltage.

Peavey uses either a FET or silicon transistors with a biasing network from a germanium transistor to 'slow down' the signal response to mimic tubes in a Transtube amp.
I found out how they were doing this in a 1960's electronics textbook.

Ken

[hewo]

i don't know what peak to peak output ac voltage those mb fet modules generated, but surely it would still have to be in the realm of what a vacuumtube would have to push would'nt it?

speaking of the delay inherent to tube signal generation output, does this mean in real time there actually is a slight time shift between output versus input?

i originally thought there was no time shift between I/O, instead, the output had "extra" waves that weren't really intrinsic to the original input waveform.
i think i read somewhere that even order harmonics were characteristic of vacuumtube sound appeal. whereas, odd order harmonics characterized semiconductor coloration which do not appeal to human hearing. then later, i read on the web that even & odd order harmonics have nothing to do with the reason why vacuumtube generated output appeals to the ear. instead it was explained how the gain behavior, output versus input behaved, especially in the region of exceeding the confined voltage rails. i still do not fully understand why this is so because when i listen to the single coil free of any vacuumtube saturation distortion, it rings crystal clear, however, there is something different about it compared with a fet amp. it seems the strings are "sticky" sounding, as if alive beyond normal string vibration. the closest descriptive would be shimmering. could be this signature is a compilation of many components, including the speaker transformer & paper speaker too.

[Glass Snuff]

Regarding the role of the volume control in a guitar, it's primary contribution is to act as a 'load' on the pickup. The pickup is a coil and a magnet, and when a string moves in its magnetic field a voltage is induced across the leads. Of course, if the leads aren't connected, there's no circuit and nothing happens. If the leads are shorted together, there can't be a difference in voltage and again nothing happens. To get a useable output we need to put a resistor across the leads to act as a "load". The load allows some current to flow while keeping the voltage different (maintaining a potential differential, in more poetic geekspeak). This load is the volume pot. The greater the load, the greater the difference in potential, i.e, the higher the voltage will be. Thus, a higher value pot will produce more output from the pickup.

Now, since a pickup is a coil, it will have a resonance*, and the resonance will cause a peak in the output. In the case of a guitar pickup, this peak is above the fretboard frequencies (the 'fundamentals') in the area of the overtones. The size of the load (value of the volume pot) will determine the size of this peak, and a larger load** creates a greater peak. That's why a 1 Meg pot has more highs than a 250K.



* Forgive me for skipping a few pages of explanation here.
** Again, I'm taking a short-cut. Engineers would say a smaller value was a larger load, but that tends to confuse musicians.

[NightWinder]

Well said!

[hewo]

but the need for, by user preference, for the volume pot hi-pass cap, e.g. 1000 pico, compensates for inadequate highs when the volume is turned down. does not this show that the highs aren't getting through the carbon track as easily to the wiper tab unless we allow for a hi-pass detour?

[David Schwab]

Regarding the role of the volume control in a guitar, it's primary contribution is to act as a 'load' on the pickup.

Just to be pedantic, the primary role of a volume control is to control the volume of the pickup. The pickup doesn't need a load, that's more of a byproduct of the potentiometer, and one could argue it has a somewhat undesirable effect of shunting high frequencies to ground.

The pickup is a coil and a magnet, and when a string moves in its magnetic field a voltage is induced across the leads. Of course, if the leads aren't connected, there's no circuit and nothing happens. If the leads are shorted together, there can't be a difference in voltage and again nothing happens.

That's not entirely true. If you wire up one coil as a closed loop, and have it close to another coil that is in the circuit, it will have an effect due to mutual inductance. The closed loop is in the magnetic field, so just as any conductor it will generate current, even if it's not going anywhere. The loop does have resistance, it's not a total short.

To get a useable output we need to put a resistor across the leads to act as a "load". The load allows some current to flow while keeping the voltage different (maintaining a potential differential, in more poetic geekspeak). This load is the volume pot. The greater the load, the greater the difference in potential, i.e, the higher the voltage will be. Thus, a higher value pot will produce more output from the pickup.

You don't need the load of the volume pot to get an output, and this can be demonstrated by the fact that you can wire the pickup directly to the output jack. The pickup will always be feeding a load as long as you have it plugged into am amp. The input impedance of the amp is analogous to a resistance between the input and ground, while the output impedance of the pickup is analogous to resistance in series with the pickup. The junction between these two resistances is your potential, and you can see where it looks just like a voltage divider or potentiometer (because it is). Ideally you want a high resistance as the input and a low resistance as the output.

The problem with a volume pot is as you turn the volume down you increase the resistance on the output of the pickup, and decrease the input resistance between the signal and ground, which changes the load, so to speak, and we loose highs.

Now, since a pickup is a coil, it will have a resonance*, and the resonance will cause a peak in the output. In the case of a guitar pickup, this peak is above the fretboard frequencies (the 'fundamentals') in the area of the overtones. The size of the load (value of the volume pot) will determine the size of this peak, and a larger load** creates a greater peak. That's why a 1 Meg pot has more highs than a 250K.

This all depends on the impedance of the pickup as well. An 800 Ohm pickup is not going to get loaded down too much by a 250K pot, and the resonant peak will be much higher than an 8,000 Ohm pickup. Also keep in mind that right after the resonant peak, the response of the pickup starts to drop off fairly rapidly. A larger load creates a lower peak... not sure how much larger the peak would be though, but on average it probably increases in amplitude as it gets lower, just based on the fact that the impedance decreases as the frequency does.

but the need for, by user preference, for the volume pot hi-pass cap, e.g. 1000 pico, compensates for inadequate highs when the volume is turned down. does not this show that the highs aren't getting through the carbon track as easily to the wiper tab unless we allow for a hi-pass detour?

No, it shows that the load on the pickup, i.e., the resistance between the pickup and ground, are shunting the high frequencies. Because the impedance increases with frequency, you can think of it that the highs are impeded more.

Put an active buffer between the pickup and pot, and you eliminate that whole scenario.

[NightWinder]

Schwab!!! Wuzzsup man! Breakin it down once again!! YEah!! LOL.
I have a super strat wired directly to the jack, no pots nada!! Sounds brutal and retains highs,,,, and the pickup is at 19.2k. I use this guitar for agressive showmanship and Haters........Floors them every time!!

[Glass Snuff]

I was trying to clarify some misconceptions. Your need to invoke transformers as pickups, no volume control at all, and 800 ohm windings (how did that work for you?) in order to reply says quite a bit about why I felt compelled to post in the first place.

[David Schwab]

I was trying to clarify some misconceptions.

Oh I know you were... as was I.

Your need to invoke transformers as pickups, no volume control at all, and 800 ohm windings (how did that work for you?) in order to reply says quite a bit about why I felt compelled to post in the first place.

Well I was pointing out that you really don't need a resistive load on the pickup to get a signal. You don't even need a coil... look at ribbon mics and Lace transsensor/alumitone pickups for examples. Pots will load the signal, and we all know that lower value pots will give a warmer tone.. depending on the DC resistance of the pickup. That and the tone control make a nice little LCR filter...

800 Ohm windings work fine. You will find similar winding in most low impedance microphones (probably much lower actually). Since we have standardized on high Z amps, you need to either use a transformer, like Les Paul did with the recording guitars, or a more modern approach and use a buffer amp, as you would find a company like Alembic doing.

In my case I did 500 Ohms on each half of a P bass pickup, and used a FET preamp to boost the signal up to hi-z levels. You get very wide and flat frequency response. Real smooth and warm sounding.

My current bass pickup designs use a bit more turns, but not that much.

[hewo]

okay i get the treble leakage picture.
whatever highs that make it to the wiper rather go to ground at the zero volume position ground tab than go to the amp's hi impedance input. it's an easier path.

[hewo]

another question:
can i freehand machine a hole in my nightwatch just by using a dremel carbide cutter and maybe a reamer to mount switchcraft's wde13?
it's the jack that looks like a very long threaded bushing.
the nightwatch has a horrible factory jack located topside.
it makes the cord point outwards, imparting undue strain on the conductors.
if anybody knows how to do this without risking trouble please respond. the nightwatch is mint mint.

[David Schwab]

Why not get a 90° angled cord?

[Glass Snuff]

Well I was pointing out that you really don't need a resistive load on the pickup to get a signal.

You don't think an amp qualifies as a load?

800 Ohm windings work fine.

If they "worked fine", you wouldn't need a preamp.

I understand, you're suffering from the "big fish in a little pond" syndrome. You're used to wowing your customers with your techno-babble, but I'm not one of your customers. At this point you have so little credibility, I wouldn't even accept an apology.