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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: RE: +2 posted by 91derlust on May 27, 2015 at 19:46:27:
"Also, at least one EE and another enthusiast... have measured the current draw changing during playback."That's kinda funny. I'm pretty sure that tens of thousands of engineers have seen this, probably going back to at least the '30s. Anyway, as Paul pointed out, it can be caused by non-linearities in the output tube. Another potential cause is the fact that source material can contain significant content at frequencies to 20Hz and below. Most mechanical ammeters will respond to those frequencies if installed in series with the anode or cathode of the tube. Meter fluctuations in this regard do not indicate changing average current, only that the average must be calculated over a longer time. A third possible cause for changing meter readings is simply the fact that music isn't sinusoidal. Various patterns can cause ammeters to respond differently.
As an aside, this attempt by the output stage to "modulate" the power supply at the frequency of the source material is the reason the power supply must be designed to maintain the lowest possible output Z over frequency. Lcrit regulation, a longer-term characteristic, is only peripherally related to this. I personally consider Lcrit regulation to be one of the least necessary requirements to designing a high quality SET. In fact, if not done exactly right, it can seriously degrade sound quality. Chokes are essentially current sources, the antithesis of the voltage source that's needed to maintain steady B+ against instantaneous current changes.
Edits: 05/27/15 05/27/15 05/27/15 05/27/15
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