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In Reply to: RE: Can someone school me on cathode feedback? posted by twystd on March 24, 2015 at 19:52:28
It is just feedback that happens to be on the cathode instead of the more common global feedback. Most push pull amps utilize global feedback that is more than 8dB so the driver section in almost any amp schematic you look at will be more than up to the voltage gain task. Your point on the driver current, more current is always better, in any amp. The MC-60 used the Gutsy 12AX7 Cathode follower to put the iron hand of fear into the cathode feedback output section.
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Thanks, but I'm not the kinda guy that would use any of the classic driver stages, I think they are too colored, and unnecessarily complicated. I like the minimalist brute force approach, much more suited to my lack of education.I'm thinking of a simple 6e6p-dr (triode mode) choke loaded, differential driver stage. The minimalist parts count, high linearity, high current, low output impedance, coupled with a relatively high mu, appeal to my audio dogma. Just trying to figure if I can get there from here :-)
BTW, I was thinking of making this a 3 stage integrated amp, with a 6n6p parafeed line stage, utilizing a custom 3:1+1 parafeed autoformer/splitter (already have them). That could drive a 6e6p-dr choke loaded (already have the PP chokes) differential driver stage, cap coupled to a 6p36s ultralinear PP power stage.
Utilizing cathode feedback on the OPT for the damping factor and lower distortion is inviting. Maybe I'll try Blumlein's garters as well. Just spitballing it at this point, but I'm nutz enough to try it.
twystd
Edits: 03/26/15
LOL--Why did Mac do that, instead of, say, another 12BH7?
The main issue with the 12AX7 as a driver is its high output Z. That problem is resolved in the CF configuration. Why waste additional filament current on a more robust tube when it isn't needed?
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Buy Chinese. Bury freedom.
Same filament draw.
CF output impedance is approximately Rp/mu.
12AX7 gives about 600-800 ohms.
12AT7 gives about 180-200 ohms.
Designers sometimes simply use what they have on hand, or what they're most familiar with. It could be as simple as that, rather than the result of some in-depth technical analysis. If they use the 'AX7, and it sounds good and measures good, what would be the point in a production environment of investigating other tubes?
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Buy Chinese. Bury freedom.
...because they could? At first glance the 12AX7 seems an odd choice esp considering that it's operated beyond its ratings in this position. OTOH, Mac successfully used this config. from the early 50s thru the end of the 60s.
I read someplace that Mac claimed the 12AX7 driver stage was capable of 1W output when terminated into a proper load which assured low distortion drive when g1 was driven +. Curiously, Mac switched to the 12AT7/12AZ7 in the driver position for the MC75/MC275 series.
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