The primary current across a whole bang is the only tuning evidence a loaded QCW gives you, and almost nobody photographs it. It is also a coarser instrument than it looks. It will tell you the current is running away at the top of the ramp and point you at which pole the driver is on. It will not tell you how far to move the tap.
Why the envelope is the only place to look
Everything else about a QCW's tuning can be settled cold. Dead time and phase lead are arithmetic and both are set at the start of the ramp, where no arc is needed. Detuning is the opposite case: the rule is to detune by as much as the arc detunes you, and an arc is exactly what an unloaded coil has not got. So the check has to happen while the coil is making something, and the primary current is what a running coil will show you.
The frequency page puts it that way and sends the reader here, and this page is the instrument end of it. What that page does not claim, and what this one spent a long time claiming, is that the instrument reads out the rule it was brought in to check. It reads out something coarser.
Three things set the shape, and only one of them is tuning
- The bus climbs several-fold across the ramp. On its own that carries the current up with it, which is why a QCW envelope is never flat and why a perfectly flat one is not the target. The MMC page rules a flat top out as unphysical for this machine, for exactly this reason.
- The resistance the bridge sees climbs while the arc is short. More load on the secondary is more reflected resistance and less current for the same bridge voltage. That pulls against the bus, and it is what stops the envelope from simply following it.
- And then it stops climbing. The resistance rises with arc length, turns over, and falls again. The curve is on the primary's page: the turnover sits at 0.88 m at
k= 0.40 and at 1.25 m at 0.50, and this coil's 0.421 lies between them, so between 41 and 17 per cent of a 1.5 m ramp is spent past it. Past the turnover the load falls away again and the current has to climb to put the same power in.
Which is the first thing this page used to get wrong. A tail that turns up is already in the envelope of a coil with nothing wrong with it, because at the couplings a QCW runs at, 0.30 to 0.50, the turnover falls inside a 1.5 m ramp. The tail on its own is not a tuning fault. That curve is a model rather than a measurement, and the page that carries it says the closed form behind it has never been fitted to a built coil, so take the turnover as a shape and not as a length to the centimetre.
What it does read out
What survives is a reading about which pole the driver is on, and it survives because the two accounts of the mechanism that disagree about everything else agree about this.
Anders Mikkelsen's account, quoted in full on the poles page: on the lower pole the tank draws more power as the arc adds capacitance, which is what makes it hard to ramp the power up slowly, and in his words "most people prefer to run QCWs on the upper pole as it gives a more linear response of power vs bridge voltage". Keep the QCW in that sentence: the same page has the lower pole as the ordinary choice on a DRSSTC, which has no ramp to break. That is a current getting away from you at the top of the ramp.
This corpus's own arithmetic, on the same page, disagrees about the sign. At a fixed bus it gives negative feedback on both poles, the reflected resistance rising through the ramp and the current holding itself back, with the self-limiting about 3.7 times weaker on the lower pole. A runaway against a slack brake. That page says plainly which of the two to doubt first: he has built the coils, and the model has one calibrated constant in it.
Both accounts bend the envelope the same way, steeper on the lower pole than on the upper. They differ on how much steeper, and that difference is the part a photograph cannot settle. So the reading is steep tail, suspect the pole first, and the reading is not a number.
What it cannot do, which is most of what it was brought in for
- It cannot give you the detuning percentage. The turns arithmetic is on the primary's page, and its advice is to cut the taps generous in both directions because until you have measured the detuning you do not know the number. The envelope does not measure it.
- It cannot separate detuning far enough from detuning too far. Both leave the system coming into tune across the ramp, and both give a weakly rising envelope. Separating them would need the current level rather than the shape, and a level means nothing without a reference current for the arc you got.
- It cannot be read at the start of the bang. The first stretch is the wick, a bus shelf set as a duty in the ramp generator, so the envelope does not begin at zero and the height it begins at is a setting rather than a symptom.
This page used to carry a list of shapes against prescriptions, telling a reader whose envelope curled up at the tail to add primary turns and a reader whose envelope was high and flat to take turns off. Both halves failed. The first reads the impedance turnover as a tuning fault. The second describes the same envelope that the section above it had just called correct, so the page gave one observation two opposite verdicts.
Two ways to misread the picture
What the shape costs the bank
The envelope's shape decides the tank's heating far more than its peak does, and the two figures that settle it are with the MMC: a linear rise gives I_pk/sqrt(6), 0.408 of the peak, and a quadratic one I_pk/sqrt(10), 0.316.
0.408 / 0.316 = 1.29 RMS current
1.29 squared = 1.67 dissipation, which goes as the squareSo carrying a coil from the second shape to the first is 1.29 times the RMS current and 1.67 times the heat in the same parts [derived, from those two figures]. Note which two shapes that compares. The flat case is deliberately not in it: the same page rules a flat top out as unphysical here, and an MMC overload figure computed on one has already been withdrawn from this corpus once.
Where the mechanism comes from
Gao Guangyan sets out three ways a conventional feedback driver can be tuned, and what each does as the spark grows, on his QCW 1.5 page. Primary at the secondary excites both poles and notches the current at the beat. Primary below the secondary drives the lower pole, and the coil starts out of tune and goes further out as the spark loads it. Primary above drives the upper pole, and as the secondary falls the upper pole follows it down and the system comes closer in tune, which he calls the best of the three and still not right. His own trick goes further, tuning the primary below the secondary while forcing the upper pole, so that the coil becomes "more and more in tune as the spark grows".
The figures he puts beside that are why any of this is worth photographing: five feet of spark at 110 A on QCW 1.5, against about 200 A for the same length on Steve Ward's QCW 1 under conventional upper-pole tuning. Two machines, and the comparison is his rather than ours.
Uspring, on Mads Barnkob's build thread (HVF 24), states the feedback the other way round for a coil tuned far above its secondary: on the upper pole the arc moves the secondary's resonance away from the operating frequency, the primary current rises, and that is positive feedback, while the lower pole gives negative feedback and a steadier current. Set beside Anders' account the two disagree about which pole is the one that runs away, which is the second reason this page reads coarsely rather than to a number.
Nobody has published the picture this page is about. Mads Barnkob's thread comes closest: four primary taps on one coil, 96, 90, 86 and 65 kHz against a loaded secondary he could only estimate, near 88 kHz off a wire surrogate, with the current on the scope at each and the lowest tap performing best for reasons he says he does not fully understand (HVF 24). He recorded those four runs as video rather than saving the individual captures, and the shapes were never written down. Until somebody reads an envelope off against a known tuning on one coil, this page is a mechanism and not a calibration.
The rings it is measured on are on the department diagram, and there is a piece on them. The trajectory the tuning is set against is on the frequency page.