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Thread: Amp Wattage Measurement - Approximations

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    Amp Wattage Measurement - Approximations

    Saw a video several months back and a gent claims you can measure the A/C voltage from your speakers with a multi-meter, measure the ohms of the speaker(s), and then use Ohms law to approximate the power (at any level of distortion you like).

    I recently applied this technique to one of my amps and while playing I could keep it right around 15 volts without too much trouble (sometimes higher, sometimes lower), so at least in terms of my meter, and for what I was playing (single notes in the key of B, the average was 15 volts.

    So using ohms law I would come up with 15vac / 8ohm = 1.875amps and then, 1.875 amps x 15vac = 28 watts (with a highly distorted signal in this case).

    Or just use a shortcut of squaring 15vac divided by ohms ; (15^2) / 8 = 28 watts

    One thing I got from all this is that wattage is a moving target, and depending on what you play it's quite variable, not to mention the efficiency of the speakers and cabinet you use. So SPL can vary tremendously regardless of what the wattage spec 'says' the amp should be doing.

    Is there any validity to this method of measurement as an approximation, or is it all rubbish unless you measure things with a scope, a dummy load, and with a specific frequency ?

    Thanks for any input !

    Here's the video from Gerald Weber.
    https://www.youtube.com/watch?v=1b2jQWK8xlQ

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    Last edited by HaroldBrooks; 06-17-2019 at 06:04 PM.

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    Old Timer Leo_Gnardo's Avatar
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    Thanks for keeping the punchline 'til the end.

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    Senior Member Old Tele man's Avatar
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    Just remember, it's AVERAGE power in Watts, not RMS power!

    Root-Mean-Square (RMS) is merely the mathematical conversion technique that's applied to sinusoidal VOLTAGE and CURRENT to derive their "average" equivalent value(s). And, thus, an average value will consist typically of values above and below that average value.

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    But... But... But what was the answer???

    I have a much simpler way of measuring power that only requires my guitar, an amp, & me.
    "I can play this in my bedroom without waking anyone...." = GARBAGE!
    "This works for jamming in the living room..." = 25-50W.
    "Hey, I can invite my rowdy drummer buddy over!" = 50-100W.
    "What? WHAT? SPEAK UP!!!" = Holy Crap that's awesome!

    In all honesty, I've never had to measure output power. Granted I'm just a tinkerer who can usually help your old tube amp work right, but my ears do good telling me whether an amp is "working" or not.

    Jusrin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    The standard method requires:

    - a load resistor corresponding to nominal output impedance
    - a continuous sine input signal of around 100mV and 400Hz
    - an AC voltmeter (preferably true RMS), simple AC voltmeters mostly give viable RMS readings at 400Hz sine but not much higher; they can't deal with multi-frequency or non-sinusoidal (distorted) signals - while a good true RMS meter might.

    RMS (sine wave) output power is given by Vrms*Vrms/Rload. A scope is used to determine the onset of clipping. You may avoid the scope by connecting a speaker (or headphones) wired in series with a 470 Ohm resistor and a 0.1µ cap over the load resistor and detect the onset of clipping by ear.

    It is no good idea to:

    - use a guitar signal as its not continuous/sinusoidal and contains many frequencies which is likely to give a wrong meter reading
    - use a speaker load as its impedance changes with frequency and speakers typically are not able to stand full sine power.

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    Last edited by Helmholtz; 06-17-2019 at 07:55 PM.
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    Member HaroldBrooks's Avatar
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    Quote Originally Posted by Justin Thomas View Post
    But... But... But what was the answer???

    I have a much simpler way of measuring power that only requires my guitar, an amp, & me.
    "I can play this in my bedroom without waking anyone...." = GARBAGE!
    "This works for jamming in the living room..." = 25-50W.
    "Hey, I can invite my rowdy drummer buddy over!" = 50-100W.
    "What? WHAT? SPEAK UP!!!" = Holy Crap that's awesome!

    In all honesty, I've never had to measure output power. Granted I'm just a tinkerer who can usually help your old tube amp work right, but my ears do good telling me whether an amp is "working" or not.

    Jusrin
    I too am more impressed by an amps actual performance, and less by just wattage, as I know speaker efficiency, cabinets, and the chosen frequency curve exert a tremendous influence on the attainable "Loudness" and perceived volume...

    But it's also nice to know if your amp is functioning within the realm of normalcy given it's design. I would feel depressed if I had an amp with two 6V6's or 6L6's that wasn't churning out half of what it should wattage wise, and want to know what was wrong so I could fix it.

    So I was hoping the Weber method was a quick approximation, or something that could be used a comparison, not an absolute and not required to be all that accurate, but hopefully of some use.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by Justin Thomas View Post
    But... But... But what was the answer???

    I have a much simpler way of measuring power that only requires my guitar, an amp, & me.
    "I can play this in my bedroom without waking anyone...." = GARBAGE!
    "This works for jamming in the living room..." = 25-50W.
    "Hey, I can invite my rowdy drummer buddy over!" = 50-100W.
    "What? WHAT? SPEAK UP!!!" = Holy Crap that's awesome!

    In all honesty, I've never had to measure output power. Granted I'm just a tinkerer who can usually help your old tube amp work right, but my ears do good telling me whether an amp is "working" or not.

    Jusrin
    Very few of us have a multimeter that has VERY FAST reading rates. Most economical Multimeters are 2-3 readings per second, to begin with. Using a Guitar as your signal source produces a complex waveform that has an initial peak envelope, then rapidly decays from that initial waveshape to those of the notes/harmonics that trail off, prior to striking more notes or chords. The loudspeaker used with the amp is NOT a fixed impedance, but has an impedance cuve that gets into the vicinity of the 'nominal' impedance in the range of 150Hz, with a broad Q that's in that impedance for about an octave (1/2-oct either side of 150hz, typically).

    Now, if you had two FAST reading multimeters, one to read AC Volts, the other to read AC Amperes, then you wouldn't need to be concerned with the impedance curve. But, then your task is WHEN do you take the reading of both the AC Volts and AC Amps of the chord you struck on the guitar? You would still have to take those two numbers and multiply them to arrive at a power level. The meter reading rate is the basic problem, as is the envelope of the guitar 'signal'.

    While I have an AC Power Analyzer that reads True RMS, that measures both AC Volts and AC Current, and displays the results in Watts (Valhalla 2101), it too has slow reading rates....also in the 2-4 readings per second. Point is, you'll never get a steady reading that's meaningful. Now, you could sit there and play the same chord over and over and decide to choose the highest reading. Doing this with your two multimeters, that may only be AVERAGE reading and not True RMS reading, trying to get the max readings on both while you're playing the same chord over and over is still a crap shoot. Neither instrument is reading at the same time, most likely, but still, you'll eventually arrive at some peak reading on both, which then have to be multiplied to arrive at that Average Wattage for that moment.

    It's really a complex instrumentation problem. System Multimeters have reading rates upwards to 100 readings per second, and due to their sophistication, will also have TRUE RMS conversion, and can either store or output the data stream. And, can be sync'd so one is reading voltage, the other reading current at the same time. Feeding both into a multiplier circuit that can output the results in a calibrated DC level, scaled in Watts RMS, it can be displayed on a CRT as an instantaneous varying wattage over time. Very few of us have such facilities at our disposal.

    Oh, you can get some numbers with just a multimeter reading the AC voltage off the loudspeaker, and compute what that number represents. As to it's accuracy.....best of luck.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by Helmholtz View Post
    It is no good idea to:

    - use a guitar signal as its not continuous/sinusoidal and contains many frequencies which is likely to give a wrong meter reading
    - use a speaker load as its impedance changes with frequency and speakers typically are not able to stand full sine power.
    I think the bolded part here may be the most critical reason why this method is not reliable. You need a verified load resistance.

    Have a look at this graph. The lower curve (and numbers on the right) are impedance of the speaker.


    Click image for larger version. 

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    but has an impedance cuve that gets into the vicinity of the 'nominal' impedance in the range of 150Hz, with a broad Q that's in that impedance for about an octave (1/2-oct either side of 150hz, typically).
    AFAIK, nominal speaker impedance is measured/defined at 400Hz. It often stays within around 20% between 200Hz and 1kHz and rises below and above. A 4x12 Marshall cab has an impedance of around 40 Ohm@150Hz rising to 65 Ohm at its 120Hz resonance.

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    Old Timer J M Fahey's Avatar
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    GW is a confirmed trick man, just like that.
    Or uses typical Politician tricks to catch audience... same thing.

    His video is crap useless, all it tells you is that amp is "workung" ... which you can check yourself by ear.

    Proper way to measure is to simplify, to reduce variables to minimum, not wildly varying elements inn the equation, so:

    * with a plucked guitar you never know: frequency - amplitude - duty cycle while a continuous sinewave is well defined and, most important, repeatable.

    * a speaker load has wildly varying impedance, while a resistor is stable and uniform across the frequency band.

    * a sinewave has a defined peak to average valuie; a plucked guitar has *anything* depending on picking style.

    * a sinewave has constant level, a guitar literally jumps all over the place.

    If anything, and in a musical environment, a pressed down organ key will be acceptable.

    Helmholtz trick to listen to clipping edge works well, in fact not even the series resistor is needed, since the small capacitor is high impedance enough in this test and all you need to hear is appartiton of buzz.
    Personally I use a cheap piezo tweeter connected straight to load and kept near my ear.

    That said, transistor amps have sharp defined clipping, they either have 0.01% distortion or 10% clipping just 1 volt away so this method works well; tube amps have 5% distortion even without visible clipping, and still sound relatively "sweet" , to boot Pentodes compress, period, that again blurs the frontier , so for tube amps it´s better to use scopes.

    Since you wantb to "see" , not that much "measure" (with the scope that is), any software scope works fine.
    I don´t mean those USB "digital scope boards" widely available today but simple software which turns your PC into a basic scope.
    Only 2 problems: thay can not display DC levels nor go beyond 20kHz, ... because PC soundboards can not either, no big deal to *watch* aguitar soundwave.

    I also published a simple and safe attenuator to connect speaker level signals to microphone/soundcard input, it works very well

    search for scope-died-anyone-use-the-pc-based-ones at The Gear Page.

    Can not straight link there because my IP is blocked on that board.

    FWIW this is the deluxe multi range version, but the simplest one has just 2 resistors, 1 capacitor, 2 cheap diodes.

    Click image for larger version. 

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    Last edited by J M Fahey; 06-17-2019 at 09:13 PM.
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    Member HaroldBrooks's Avatar
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    Mr. Fahey, glad to see your back on-line.

    Not to hijack my own thread, but I heard there was a huge power outage, not unlike the one on the east coast of the USA many years back.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by Helmholtz View Post
    AFAIK, nominal speaker impedance is measured/defined at 400Hz. It often stays within around 20% between 200Hz and 1kHz and rises below and above. A 4x12 Marshall cab has an impedance of around 40 Ohm@150Hz rising to 65 Ohm at its 120Hz resonance.
    I don't recall the 400Hz Standard Test Frequency for defining nominal impedance. I think speaker mfgrs use the 0 phase frequency in the low midrange trough where impedance Q is broadest. Changes with each speaker type. I had stated 150Hz as the most recent impedance curve I had recently looked at & posted was that of an Ampeg BXE-410HL4. Here's a variation of speakers besides that one, three being free air, plus a Yamaha NS-10 speaker and that Ampeg bass speaker.

    Click image for larger version. 

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    Regardless, in the exercise that was initially suggested, changing to a fixed resistor makes it far more predictable.

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    Senior Member Old Tele man's Avatar
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    Just to add some more mud to the waters: AMPS and SPKRS are (unfortunately) spec'd differently, for example, on many Fender schematics, it's 400Hz for bass amps and 1kHz for guitar.

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    Quote Originally Posted by HaroldBrooks View Post
    Mr. Fahey, glad to see your back on-line.

    Not to hijack my own thread, but I heard there was a huge power outage, not unlike the one on the east coast of the USA many years back.
    Thanks.
    Luckily it lasted only 4 hours and apparently followed the exact same path as the US one: branches trying to cover for main line sections and dropping themselves.

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    I think speaker mfgrs use the 0 phase frequency in the low midrange trough where impedance Q is broadest.
    A speaker impedance has (at least) two zero phase frequencies. The lower one(s) at the bass resonance(s) and the next typically around 400Hz for a guitar speaker.
    According to Wikipedia some manufacturers just use the middle frequency impedance minimum.
    A Q-value can only be defined for a (bass) resonance, defining the half-value width of the peak.

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    Last edited by Helmholtz; 06-17-2019 at 10:26 PM.
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    Just decided to read some of the comments under the YouTube video I sent the link to. Sounds like there's a bit of controversy swirling around regarding the video. I don't know him from Adam, so I can't say one way or the other.

    Ok, just thought I could take the easy way out, but I see it's not that simple to measure wattage, but not all that complicated if you are serious about it.

    What I do generally is setup a DB meter one meter on access from my amps and let it rip to record the maximum SPL peaks. But that also has serious flaws in that amps with an upper mid focus have big SPL number, but sometimes very little bass, so it's a mixed bag measuring sound pressure levels as well.

    I've found from gigging with different amps and the same moderately loud drummer, that I need about 105db of midrange to just start to poke a hole in the wall of sound, and have my guitar heard in a solo. A 10 watt amp with the right speaker can do this, if you dime it and the speaker is efficient, but it's easier with a 20 + watt twin speaker amp, for sure.

    Nothing is that simple in this world !

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    Senior Member vintagekiki's Avatar
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    https://www.google.com/search?q=How do you measure the power output of an amplifier
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    People also ask
    How do you calculate RMS power of an amplifier?
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    Senior Member Old Tele man's Avatar
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    *IF* you're wanting to 'guess-ti-mate' an approximate output power for a Class-AB push-pull amp by simply looking at its schematic, here's an equation:

    Po ≈ (%)*(Zoo/4)*(gm*Vg)^2

    where:
    Po = estimated output power, watts (avg)
    % = Zo loading effect on tube rp, dimensionless (typical value 0.85-0.9-.95)...see below
    Zoo = OT plate-to-plate impedance, ohms
    gm = output tube average transconductance
    Vg = output tube BIAS voltage, absolute value


    Actual (%) formula: (%) = (rp/(rp+Zo))^2...note: Zo versus Zoo here; rp = nominal output tube plate resistance, ohms.

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    Last edited by Old Tele man; 06-19-2019 at 06:24 PM.
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    Quote Originally Posted by Old Tele man View Post
    *IF* you're wanting to 'guess-ti-mate' an approximate output power for a Class-AB push-pull amp by simply looking at its schematic, here's an equation:

    Po ≈ (%)*(Zoo/4)*(gm*Vg)^2
    Doesn't that give peak power as gm*Vg is peak current?
    Average power will be half of that.

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    Senior Member Old Tele man's Avatar
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    Quote Originally Posted by Dave H View Post
    Doesn't that give peak power as gm*Vg is peak current?
    Average power will be half of that.
    No, as Vg is BIAS (DC voltage), not Vgg which is AC peak-to-peak, which I believe you are thinking about.

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    One of those commenters is a member here. We know the Great Bandmaster Saga well. That's what turned me off permanently to GW...

    Though I remember poring over the schematics & Trainwreck Pages in his book.

    Justin

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    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Quote Originally Posted by Old Tele man View Post
    No, as Vg is BIAS (DC voltage), not Vgg which is AC peak-to-peak, which I believe you are thinking about.
    The bias voltage is the peak voltage isn't it? That's what I was thinking about. Peak to peak is twice the bias voltage.

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    For power calculation you want RMS current. (gm*Vg) gives peak plate current or amplitude. (gm*Vg)^2 = 2*Irms^2.

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    Quote Originally Posted by Helmholtz View Post
    For power calculation you want RMS current. (gm*Vg) gives peak plate current
    Yes, that's what I thought. When I use the approximation formula on my 15W EL84 amp it give the output as 33W.

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    Senior Member Old Tele man's Avatar
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    First, there is no such thing as WATT(RMS)!

    Why? Because RMS is just the mathematical-equivalency operation (0.707*pk) that's applied to the peak value of sinusoidal VOLTS and/or AMPS to convert them to their equivalent AVERAGE values:

    SINUSOID(avg) = 0.707(rms)*SINSUSOID(pk).

    POWER(average) = VOLT(rms)*AMP(rms)...where RMS*SINUSOID(pk) = DC.heating(average).

    When working with sinusoidal volt and amp waveforms: WATTS(avg) = VOLT(rms)*AMP(rms)...where AVG watts is the "time-averaged" DC-heating equivalent of an (assumed) sinusoidal waveform.

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    Quote Originally Posted by Old Tele man View Post
    First, there is no such thing as WATT(RMS)!
    True, that's why I said average power in post #19

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    Quote Originally Posted by Old Tele man View Post
    First, there is no such thing as WATT(RMS)!

    Why? Because RMS is just the mathematical-equivalency operation (0.707*pk) that's applied to the peak value of sinusoidal VOLTS and/or AMPS to convert them to their equivalent AVERAGE values:

    SINUSOID(avg) = 0.707(rms)*SINSUSOID(pk).

    POWER(average) = VOLT(rms)*AMP(rms)...where RMS*SINUSOID(pk) = DC.heating(average).

    When working with sinusoidal volt and amp waveforms: WATTS(avg) = VOLT(rms)*AMP(rms)...where AVG watts is the "time-averaged" DC-heating equivalent of an (assumed) sinusoidal waveform.
    The average (or mean value) value of a pure sine is zero. RMS is not average. The unit Watt (W) is defined as the product of the units Ampere (A) and the unit Volt (V).

    The output power of an amp is usually measured using a load resistor. This reflects to a resistive (real) plate load Rp (=Zoo/4). Consequently Rp*Irms^2 gives the real (as opposed to apparent) power. Nothing wrong with calling the result RMS power in a steady state situation.
    Anyway your formula gives peak power which corresponds to twice the value you would get from (Vout^2)/Rload (Vout being RMS output voltage).

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    Quote Originally Posted by Helmholtz View Post
    Rp*Irms^2 gives the real (as opposed to apparent) power. Nothing wrong with calling the result RMS power in a steady state situation.
    Are you sure, isn't that average power?
    RMS power would be the RMS value of the power waveform which isn't useful.

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    Class-AB1 PP amps do not conduct 360-degrees, only about 190-205-degrees, or more generally 'slightly more' than 50% of a FULL sinusoid, so referencing a 'pure' sinewave is only valid prior to the PI and output tubes...or, at the load.

    The ONLY place the two quasi-halves of the full pre-PI sinewave become a FULL 360-degree sinusoid again is *inside* the OT -- specifically in the secondary (output) winding(s) -- because anything prior to that is (basically) pulsing alternating ± half waves (PI, output tubes, each primary half of OT).

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    Quote Originally Posted by Dave H View Post
    Are you sure, isn't that average power?
    RMS power would be the RMS value of the power waveform which isn't useful.
    You're completely right. It's correctly mean power or average power. But I am used to call Vrms*Irms "effective power" (which would translate into RMS power), knowing that it's actually apparent power and that the RMS value of time-varying power is of no use. I know it's sloppy.

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    The ONLY place the two quasi-halves of the full pre-PI sinewave become a FULL 360-degree sinusoid again is *inside* the OT -- specifically in the secondary (output) winding(s) -- because anything prior to that is (basically) ± pulsing alternating halfwaves (PI, output tubes, each half of OT).
    But both OT primary halves carry full cycle voltages. The voltages in both halves must be identical as the windings are tightly coupled.

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    Quote Originally Posted by Old Tele man View Post
    Yes, but the author correctly uses RMS grid voltage and not bias voltage.

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    ...and he (me) also later explains that the DC bias voltage is assumed to equal the AC(rms) driving signal for estimating Po from the schematic values of Zoo (OT), gm (power tube), and applied BIAS (Vg.dc) voltage:

    http://www.tdpri.com/threads/estimat...1-amps.776469/

    Q: Why use BIAS voltage?
    A: Because BIAS voltage is almost universally specified on schematics while PI output drive signal levels aren't. It's an approximation about simplicity, not about exact accuracy.

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    DC bias voltage is assumed to equal the AC(rms) driving signal
    But that's wrong as the the DC grid bias corresponds to the max. grid signal peak voltage.

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