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In Reply to: Always grateful for your input Brian! posted by Naz on November 20, 2006 at 20:57:18:
Naz,If you're still watching...
Yes, a DC filament supply will remove the modulation and additive hum effects in a DHT. But the cathode current density will be much higher at one end of the cathode than at the other. After a while, the poor end doing all the work will weaken before the loafing end does. At least AC heating keeps the wear and tear even and averaged across the filament length. Morgan Jones suggested in a forum once that you could use DC to heat DHTs, but then flip a heater polarity reversal switch before every listening session to even out the wear. That is actually not a bad idea. I suppose, per your thinking, that you could conceive of a DHT intended for DC heating where the geometries of the cathode (and maybe the nearby grid and plate too) are tapered to compensate for a cathode with voltage spread. Quick – get a patent! But I don’t see how that could help in AC heating.
Furthermore, again using our parallel tubes analogy, with DC heating you’ve got a string of tiny tubes all running at slightly different operating points. How this might affect linearity when they’re added up is not straightforward. With line AC heating this non-linearity is modulated by 60 Hz, giving the effects mentioned before. With RF heating, Nyquist’s rules apply. During any one period of audio sine wave (arguably the shortest being 50us at 20kHz), the filament will undergo several cycles of heating voltage reversal, effectively averaging the non-linearities out during any one cycle of audio waveform.
By the way, in a DC-heated DHT, the end with higher current density (the more negative end) might actually become slightly cooler than the other end since a greater number of high energy electrons are removed from that end’s space charge cloud by the plate than from the other end. I’d have to convince myself that this effect is not swamped by greater IR heating in the cathode itself. Just a thought…
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Follow Ups
- Likewise Naz! - BBeck 15:42:11 11/23/06 (1)
- Thanks Brian ... - Naz 16:17:46 11/23/06 (0)
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