tag:blogger.com,1999:blog-4348292068036323235.post4237432947054267309..comments2023-08-29T03:53:29.334-07:00Comments on Bones Hi - Fi: Nuvistors: The Forgotten Thermionic Vacuum Tube Technology?Ringohttp://www.blogger.com/profile/09506068154852505840noreply@blogger.comBlogger2125tag:blogger.com,1999:blog-4348292068036323235.post-84152186180262427612013-07-10T02:47:08.616-07:002013-07-10T02:47:08.616-07:00When it comes to the limitations of a "plain-...When it comes to the limitations of a "plain-vanilla double triode phase splitters" in push-pull thermionic vacuum tube, as well as nuvisor triode based amp designs, Dr. A.R. Bailey was clever enough to utilize triode-pentode tubes for phase splitting in his Radford designs instead of the double triodes oft used in Mullard 5.20 type amps like the famed Marshall electric guitar amplifier. Even though the Nu Vista 300 hybrid power amp uses a solid state bipolar transistor output stage by the nature of the beast it is very reliant on large(ish) amounts of negative feedback for its operation - like all Golden Age of Stereo era push-pull tube hi-fi amps. The problem with the conventional triode phase splitter is its rather high input capacitance caused by the Miller Effect (the Miller Effect not only affects tubes and nuvistors but also solid state devices like bipolar transistors, FETs and MOSFETs). The Miller Effect causes undesirable high frequency loading on the first active device - whether vacuum tube, bipolar transistor or FETs - or in the case of the Nu Vista 300, the first nuvistor input triode - thus reducing useful bandwidth and making it very difficult to use appreciable amounts of negative feedback without the resulting instability due to the phase shifts incurred. A pentode tube - or a pentode nuvistor if you are lucky enough to still find one - has a very low input capacitance and high gain due to the shielding effect of the screen grid. This means that the loading on the input tube or nuvistor is greatly reduced, increasing bandwidth and decreasing troublesome phase shifts. I, too, wonder why Musical Fidelity's Antony Michaelson didn't use a pentode nuvistor type (probably because pentode nuvistors are now virtually extinct?) as a phase splitter for the Nu Vista 300 hybrid power amplifier.Ringohttps://www.blogger.com/profile/09506068154852505840noreply@blogger.comtag:blogger.com,1999:blog-4348292068036323235.post-64129622838983913182013-07-09T04:07:26.934-07:002013-07-09T04:07:26.934-07:00Given that Musical Fidelity's Antony Michaelso...Given that Musical Fidelity's Antony Michaelson used the Mullard 6CW4 triode nuvistor when he designed the Nu Vista 300 hybrid power amplifier, would the inherent "Miller Effect" problem of triodes rear up its ugly neck here? After all, you cannot use - inherently - large(ish) amounts of negative feedback with triode type vacuum tubes like the 6CW4 nuvistor, ESPECIALLY when the Musical Fidelity Nu-Vista 300 uses bipolar output transistors which by nature requires appreciably large amounts of negative feedback compared with the Golden Age of Hi-Fi era pentode and tetrode output power tubes. Nadinehttps://www.blogger.com/profile/00863337525873209650noreply@blogger.com