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In Reply to: RE: Neutralization of Miller capacitance posted by Tre' on March 12, 2017 at 16:53:42
Indeed, grid resistors of the output stage may limit driver's Rp. Grid chokes might help though.
As to RF oscillation due to positive feedback, a good 2A3 design, with or without neutralization, should incorporate grid stoppers, or RF plate chokes, or a combination of both. These measures should prevent oscillation.
PFB is not necessarily evil in an audio amplifier, as has been discussed on this forum.
Here is a link to a practical high quality 2A3 amplifier utilizing neutralization in both driver and output stages:
Follow Ups:
I like the choke feeding the IT to help force differential behavior in the driver stage.
The output tube cathode resistor with a fuse bypass in case of neg bias supply failure is a neat trick.
Thanks for the link.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
is the choice of driver tube. The author claims that 6C4 was the best available driver tube at the time (1950). 6C4 was primarily designed as oscillator. It is similar to 6J5 or 6SN7, but with one important distinction: its plate-to-grid capacitance is 60% less. According to the article, this low Cpg, together with Miller neutralization, is important to keep ringing in phase inverting transformer above the audio range. 6C4 is used at 280 V, close to its rated Ua, and biased to its maximum plate dissipation of 3.5 W.
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