|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
76.90.39.24
In Reply to: RE: Neutralization of Miller capacitance posted by sser2 on March 12, 2017 at 13:40:42
One could think of a [differential] two-stage 2a3 amplifier with overall gain in excess of 100.
Neutralization caps only work cross coupled across a differential stage.
BTW the 7b4 has a mu of 100 but in circuit the stage has a gain of about 55.
In a differential amp, CCS plate loads on the driver tubes would get you a gain of 100 (7b4, 12ax7, etc.) if direct coupled or close to 100 if cap coupled with the largest grid resistors allowed on the output tubes.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
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:
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.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: