Electronics theory question

[nandrewjackson]

Sorry if it's not the appropriate section. I have a theory question with no known (to me) practicality.


In one electronics class we had some formulas and a mnemonic device for the phase shift between current and voltage in AC.  We never got around to any practical applications for it. So aside from doing the equations it was and remains a real world mystery to me.


The mnemonic device is "ELI the ICE man"


ELI being voltage (E) comes before current (I) in a strictly inductive (L) circuit.


And ICE being current (I) comes before voltage (E) in a strictly captacitive (C) circuit.


And with the long forgotten by me equations, the phase shift between current and voltage could be calculated with circuits including various inductive and capacitive components.


My question. Is anyone aware of any practical applications for this in guitar amps? (Fender, Marshall, vox) .


A second, somewhat related question. Does the current/voltage phase shift have anything to do with phaser/flanger effects stompboxes, or is that phase shift different?


Guitar amps are generally full of capacitance in the tone shaping, and then a whopper of inductance at the output transformer.


Thanks in advance for any replies.

[thetragichero]

not an answer to your question, but just to add: inductors can be used in tone shaping (check out some ampeg bass amp or 60s/70s orange amp schematics. while not tube amps acoustic control corp used a ton of poorly secured inductors for tone shaping. if you open up a well-traveled 370 you're likely to find at least one of the inductors floating around inside)

[tubeswell]

So in an inductor with a changing source voltage following a sine wave function, there will be peaks and troughs where potential voltage reaches maximum +ve (or maximum -ve), and the 'mid-point' (between the peaks and the troughs) where the change in charge is occurring at the quickest rate (i.e where the gradient of the line is steepest) albeit that this is through the 'neutral' voltage point for an AC voltage sine wave. Now, as it happens, this mid-point (on the source voltage sine wave) happens to be where the magnetising current is reaching its peak (which is 90 degrees out-of-phase with the source voltage) because inductance is a function of rate of change in charge from the source - hence, when you compare the two functions (the source VAC vs the magnetising current) the 'voltage comes before current' in an inductor. Is that what you were asking?

[nandrewjackson]

Tubeswell,
No, I understand the basic concept of the phase change between current and voltage when inductance and/or capacitance is present.


I'm really wondering if there is any applicable real world practicality if that specifically in the guitar amps discussed here on the EL34 forum.

[nandrewjackson]

I think reading through the recent "average screen current" thread sparked that long lingering question from myself.


That "average screen current" thread is what I would call a deep dive into some tube specifics.


I'll still sleep ok whether or not there's a practical application of the voltage/current phase shift in tube amp builds.
Lol


😎


In class we spent a lot of time analyzing circuits (on paper) , plugging in numbers into equations, drawing little phase graphs, with absolutely no real-world practical explanation for the work we were doing.


Thetragichero,
I've come across a couple inductors in the guitar amp world. The only one I've built was a crybaby wah circuit. I wanted a stompbox version of the wah-wah, where it could be set at a favorite position and switched on/off. I got it working mostly, but it was easy to overdrive the transistors with strong chording, and it wasn't a desirable overdrive. It was "farty". I never did work that out. I built it and rebuilt it, checked and double checked everything until the cows came home. It did the wah-wah thing, just had sensitive input and occasional farty overdrive.

[HotBluePlates]

... never got around to any practical applications ... it was and remains a real world mystery to me.

The mnemonic device is "ELI the ICE man"

ELI being voltage (E) comes before current (I) in a strictly inductive (L) circuit. ...

EMF (voltage) comes first, because the inductance "generates a counter-EMF" and resists a change of current caused by the voltage-change.  We come to understand an inductor tries to keep current constant.

     -  You will read in some places that "a choke-input filter has good regulation" meaning the supply voltage stays steady from idle to full-power.

     -  In reality, we find a Class AB or Class B amp (with big peak currents that greatly exceed the average current draw) don't work well with choke-input power supplies.  The output voltage tends to collapse when the amp tries to draw a high peak current through the choke-input supply.

     -  What works better is attaching a Class A amp to the choke-input power supply.  Idle current is high, and full-power current swings between (almost) zero and 2x idle current.  As a result, average current stays nearly the same as idle current, and the choke feels like "current stays the same."

... a whopper of inductance at the output transformer. ...

Small inductance means current-change happens faster after voltage-change.  So think of that as the inductance has effect at some high frequency, but isn't opposing current-change at some lower frequency.

     - Resistance/Impedance is also an opposition to current flow.

     - That output transformer has a "reflected impedance" at the primary due to the turns ratio & the attached speaker-load.

     - The lowest frequency the output transformer can handle well is determined by its primary inductance.  The reactance has to be large at that "lowest frequency" because this reactance is in parallel with the reflected impedance.  If the primary inductance (and its reactance) is too small, it reduces the effective primary impedance of the output transformer.

     - The primary stops being "4kΩ" (or whatever the marked, expected reflected impedance is), is suddenly a lower impedance, and power output to the secondary drops.

     - Hewlett-Packard has some oscillators with 2 output transformers.  When outputting high frequency, a smaller transformer could be used for the rated power output.  For low frequency, a second, larger output transformer (with a bigger core & higher primary inductance) was used.

... never got around to any practical applications for it. ... remains a real world mystery to me.

The mnemonic device is "ELI the ICE man"
...
And ICE being current (I) comes before voltage (E) in a strictly capacitive (C) circuit.

Electrons carry the charge that enables a capacitor to build up a voltage.  Electrons are the "current" and to charge a capacitor to a voltage, the electrons must be pulled off the + plate (by connection to a positive voltage) and allowed to pool on the - plate (because they are attracted to the + plate's positive voltage, and by connection to the rest of the circuit from which electrons are drawn).

Overall, capacitors resist a change in voltage (by pulling in more electrons/current, or by releasing electrons/current into the circuit).

     - Compressors have Attack/Release settings, and maybe a variable control.

     - The Attack/Reelase circuit determines the time is takes for the compressor to start/stop having effect.

     - Mostly, the Attack/Release circuits use an R-C circuit, where they leverage "ICE":  One option for a faster Attack/Release is to use a smaller cap, with smaller plates, which then requires less charging current to reach a voltage.

     - Many circuits will use a signal capacitor & a variable resistor to get adjustable Attack/Release.  But now the resistor performs its function of "slowing current" (remember "Ampere" is "Coulombs per Second", and a coulomb is a quantity of electrons, so less-current is fewer electrons moving in a unit of time)

     - A larger resistance (by way of the rheostat/pot) slows charging-current ("ICE" again) and causes the capacitor to take longer to reach the applied voltage.  Attack (or Release) time has been lengthened.

[HotBluePlates]

A different example for "ICE" is the "maximum capacitor rating for rectifier tubes."

     - A filter cap tries to charge to the rectified voltage applied to it by pulling a charging-current through the rectifier tube.

     - Bigger caps pull a bigger charging-current, and may exceed the rectifiers rating/capability.

     - We can limit the charging-current to that which the rectifier can handle by adding resistance to the circuit (slowing/lowering current).  This means the cap will take longer to charge to full-voltage, which may/may-not have an impact when the amp is played loud & is drawing a lot of current through the power supply.


Both types of reactive components "take time to reach the applied voltage (or current)."  Then we learn there is a frequency-dependent element to this time that they take, and that reactance changes with frequency.  Turns out, "ELI the ICE man" is the microscopic view to explain "why reactances take time," and "why inductors seem to do exactly the opposite of what capacitors do."

In class we spent a lot of time analyzing circuits (on paper) , plugging in numbers into equations, drawing little phase graphs, with absolutely no real-world practical explanation for the work we were doing.

When you're in class, you're there to learn How to Do Stuff.  Not every student has the brain-space to also take on "why do we do that."

When you get out in the world, you learn "why do we do that" from the senior engineers that have been around a while.  The junior engineer just knows how to do the calculations.  Kinda like PRR (senior engineer) vs me (junior engineer); he's been around a while & knows the "why."

... the phase shift between current and voltage could be calculated with circuits including various inductive and capacitive components.

My question. Is anyone aware of any practical applications for this in guitar amps? (Fender, Marshall, vox). ...

Every coupling cap that sets every bass roll-off?  The phase-shift (that we ignore) is the other side of the coin for "higher reactance" which creates a voltage-divider with circuit-resistance, which lowers the signal-level of that lower frequency.

It's all related, we learn it all, then only pay attention to the one facet that seems most-useful most of the time.

... Does the current/voltage phase shift have anything to do with phaser/flanger effects stompboxes, or is that phase shift different? ...

"Phase shift" is "time delay."

But capacitors & inductors have a maximum (theoretical) phase shift of what?  90º?  And that's 90º (1/4-cycle) of an applied frequency.  So 1/4-cycle of 1kHz is "1/4 of 1 millisecond" = 250 µSec.

The Strymon Deco manual (Page 4) says Flanging is up to 3 milliseconds, and chorus is 3-50 milliseconds of delay.  A phaser pedal does something a bit different, with a more-complex all-pass filter.

You're not gonna mimic either one just with a single capacitor or inductor.

[PRR]

... ... ... .... ... ... ... Is anyone aware of any practical applications for this in guitar amps?... ... ... .... ... ... ...

No.

[nandrewjackson]

HBP,
Thanks, it seems like the concept is alive and well inside the guitar amps.  Whether or not it approaches any critical aspects or not.


I always figured that section of class was a foundation for something, alas, that something never reared it's head.

[shooter]

that something never reared it's head.

if you really want to see it mess up your day switch to fix'n transmitters n receivers