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> Tubes Vs. Transistors & Quantization Distortion, Hawksford quantization distortion theory
Scifi
Posted: October 26, 2010 01:27 am
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Has anyone here heard of quantization distortion similar to pulse code modulation in solid state circuits?
See http://www.audiokarma.org/forums/showthread.php?t=327777

Hawsford discusses this type of distortion (JAES, vol.31, no.10, pp.745-754, October 1983) and references some HiFi News articles that discuss differences between tube and transistors amps.
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johansen
Posted: October 26, 2010 01:45 am
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see post #8 on the same forum you linked.


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Sch3mat1c
Posted: October 26, 2010 06:29 am
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Tubes work on identical principles as FETs, the fundamental difference being, electrons are given enough energy to leave the cathode entirely (past the work potential), instead of being elevated only to the conduction band. This allows ballistic transport through vacuum, instead of resistive drift through a crystal lattice.

Electrons are generated thermally, which is no different from a minority carrier device (like the "emitter" in a BJT). Maximum current capacity varies with temperature, as one would expect (life expectancy also drops preceptitiously with rising temperature, so keep your cathodes at ratings!).

Since ionic charge dominates the field between active regions in transistors, they are naturally shielded from output-terminal (drain / collector) voltage effects, except at low voltage where the depletion region is small, or other effects become significant (like saturation).

Tubes, on the other hand, have free space between electrodes, so the electric field is determined directly by electrostatics. What's more, there's only a minimal shielding effect from the grid (it's a bad shield -- it's full of holes!), so the plate voltage has a substantial impact on the amount of current going through the grid. This gives rise to negative feedback and results in the triode's naturally low plate resistance. Subsequent shields can be added (screen, suppressor) to increase plate resistance, and accomplish other effects as well. A screen grid is very useful because it effectively biases the control grid positive, without having to actually bias it positive. It's like an electrode that continuously varies the tube's cutoff point, from "enhancement mode" to "depletion mode". This has the added bonus of generating plate current even at very low plate voltages. The 6V6 has plate saturation voltage as low as 20V!

The downsides to vacuum tubes also becomes apparent:
-- They must operate at high temperature, because there are no low temperature emissive coatings available for the cathode. There are materials with a low work potential, like cesium, but these would ruin the vacuum by their high vapor pressure. There are also field-emission cathodes, but these are very sensitive to voltage instead of temperature, as well as construction and contamination. You think transistors have a lot of variation? I'd hate to see the parameter spread on brand new field-emission cathodes...

-- Shooting electrons through empty space requires very low current density, otherwise the electron beam feels its own space charge and becomes self limiting. In contrast, the electrons in a transistor are floating in a sea of atoms and ions, so the charge is substantially neutral at all points, and the current density can be immense (schottky diodes have the highest density, while IGBTs have the highest density in a controlled device). This, combined with the small size and lack of heater power, is the primary reason transistors have ruled for over half a century. (Cost is also part of it, *now*, but transistors used to be more expensive than tubes. In fact, GE made a tube TV as late as the 70's!!)

What I find amazing about tubes is how much worse they are compared to transistors -- that is, not that they're so awful, but that they're as close as they are! Consider a typical 60s era sweep tube: it might do 1A peak at 50V plate, 150V screen. With a DC rating of 880V, that's 880VA total switching capacity (which implies you could build a switching supply of about 220W real capacity, not bad for a 30W-rated tube, eh?), and an effective "on resistance" of 50 ohms. Now consider a comparable transistor: IXTP1R6N100D2 is a 1000V, 1.6A MOSFET, rated for 10 ohms Rds(on) typical. That's "only" five times lower. The tube is not even an order of magnitude worse in performance!

But it gets better: to be on a fair level with the transistor, we should really consider peak voltage capacity, not average. That's how transistors are rated. Now... that sweep tube can stand off a whopping 7kV peak! MOSFETs don't even *exist* that can handle that much.

The most I've ever seen is 2.5kV (and they have awful resistance). To switch that much voltage, you have no choice but to cascode at least 7 of these MOSFETs, which would end up having far more off-state leakage current (because you build a cascode using a resistor divider chain), and Rds(ON) would be 70 ohms, which is actually worse than the tube!

One more thing to point to: capacitance. This MOSFET is rated as 645pF gate capacitance (in the off state, at fairly low voltage). The capacitance is a lot lower at higher voltage, but still noticable. (This is a direct result of that depletion region, which generates the useful shielding effect, but expands and contracts, thus varying capacitance.) In contrast, the tube has no more than 30pF between any combination of electrodes, *and* has substantial grid-plate shielding, minimizing Miller effect, *AND* the capacitance is completely constant with respect to bias. (Typical values for 6JS6: 24pF input (G1 to K, H, G2, G3), 10pF output (P to G3, G2, K, H), and only 0.7pF G1-P.

Of course, the lower gain means you need about as much driving power for tube or MOSFET. The MOSFET will do its job with 10Vp-p; the tube needs 100-150Vp-p. But more current means more loss to resistance, and common MOSFETs poop out above a few MHz. The capacitance of a tube is essentially lossless, so they can be operated in a resonant circuit at essentially any frequency, until further effects become apparent (like lead and electrode inductance, and electron bunching).

Even today, tubes are still dominant in certain radio frequency niches -- microwave magnetrons are one, and television transmitters are another (although IOTs might be getting phased out by now). (NASA is also fond of TWTs on satellites and probes; I don't know if they switched to semiconductors by now. It says something about TWT reliability if fricking NASA uses them!)

Tim


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Scifi
Posted: October 28, 2010 09:56 pm
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Thanks for your response. I put some more information here
http://www.audiokarma.org/forums/showthrea...t=327777&page=2
about comments on Hawksford's article and information about traps.
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