|[AMRadio] Tube question|
k4kyv at charter.net
Sat Oct 7 16:15:56 EDT 2017
The 845 has always been primarily an audio tube. The RCA manual even says they are not recommended for RF use. One of the earlier uses dating back to the 1920s was as class-A single ended modulators using the Heising “constant-current” configuration. When Loy Barton came up with the Class-B modulator idea circa 1930, 845s were found to make excellent audio drivers because their relatively low internal plate resistance worked well with the widely varying grid impedance of the modulator tubes over the audio cycle. Class A 845s also make good amplifiers to drive hi-fi loudspeakers, since speaker impedance also varies widely, with frequency. Whether as class-B drivers or speaker amplifiers, the 845 provides a well-regulated source of audio driver voltage, minimising distortion as the load varies over the audio cycle.
The 211 also works for this purpose, but not as well as the 845.
I am a silent observer of this forum and am surprised that these large triodes have application in the audio field. I did a fast search in my comm shop and didn't find any 845 but do have a number of the 211 type. I thought I had some that were in military type boxes that were marked V4C but I found them yet. I always enjoy your comments on this forum.
K6OSM since 1958
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