First, I refer the reader to this thread, which discusses the citation II:
http://www.audiocircle.com/circles/index.php?topic=49209.0
I'm sorry, I can quote just as many - in fact more - noted audio designers who disagree with Roger. Those who disagree include Bob Carver and Dr. Norm Thagard (astronaut, Audio Express author, and co-designer sometimes with Nelson Pass) who come to mind right away. BTW, Norm has at least 8 Cit II amps, all using stock audio path circuitry...
Also in that thread, "Flyquail" completely mischaracterizes my responses to the topic as "personal attacks". Over the years I've learned what that usually means is I didn't support his position. But he's entitled to his opinion too - so be it.
I have also found another truism - you can find an "expert" to support almost any reasonable position you take. That's why I tend to look for a preponderance of evidence/opinion - and the preponderance of opinion regarding the "Deuce" is in a place where Roger is not to be found.
Roger speaks obtusely about the need to stagger the time constants of the poles in a (I'm surmising here) more than two stage amp. The time constant of the first to second stage coupling in a Cit II (time constant = R*C) is .47 uf * 1 Megohm, or .47 secs. The second stage to output section is .22 uf * 100K, or .022 secs. The constants differ by a factor of 20; it's generally considered that a factor of about 4 is required for good stability. Clearly the amp meets that criteria handily.
Jon s said:
"Last time I had it running, I attempted to drive my Citation II into oscillation as described by Roger, and I saw no spurious cone movement even at insane levels driving my 85dB sensitive AR 302s - this is with all new caps, however. I still suspect that he may have had problems with his power supply during those tests. Poor decoupling between stages can have the same effect."
That's the same "personal attack" observation (!) I made regarding Roger's testing. I have tested these amps innumerable ways (some tests were completely accidental!
) and I have yet to observe the phenomenon he describes. I have found, as did Stu Hegeman, that the amp is stable under any load - including open circuit. I must qualify that statement by saying that an amp with a compromised power supply (the original parts are at LEAST 45 years old!) may not meet the absolute stability criteria. Norm Thagard has reported to me that he's seen healthy looking 50 uf power supply caps he removed from Cit IIs that had been in daily use measure 1uf!
That amp could indeed be unstable and misbehave in the way Roger described.
If Roger was to test an amp fully restored with one of my power supply kits I would certainly take more notice and attach more credence to his findings. Let me be perfectly clear - I do not question his skill and knowledge, I question the validity of the test for reasons he may not have considered at the time.
Oh, lastly I might add it is VERY difficult to get a healthy Cit II to clip in the real world.
As jon surmised, the amp benefits enormously from added decoupling of the voltage amp stage. Stu didn't have nearly the quality parts we have today to apply to that task. His choice of a dual 50 uf cap was remarkable not only for the size (in uf) of the capacitance for each channel, but for the cost of a cap of that size back in the day. What we can use now is far superior - and cheaper to boot!
Jon, I have tested many a Deuce, and found that once properly redone, they will exceed their RMS power rating, and do so while producing about 20% of the rated THD. I've seen peak power in the 150 watt/channel range (with my top of the line PS kit installed). At normal listening levels trying to measure distortion products is very hard to do.
A 10KHz square wave is a very difficult test for an amp - a healthy Cit II will produce a square wave with virtually no tilt, unbelievable rise times, and a near "textbook" appearance. There's very little deterioration even at 20KHz! And of course, square waves at 10 or 20KHz test the amp well above a sine wave of the same frequency, we're talking 100K and up response. This is the area where the triode front ends (including some I've built myself) can't match the wideband performance of the video pentode 12BY7As and the nested NFB circuitry. BTW, the Cit II uses very little overall loop NFB.
Incidentally, Norm and I discovered the V1 and V4 (voltage amp) tubes should be at the high end of the Gm range for the 12BY7A. Sylvania and RCA tubes
generally have higher Gm than GEs, for instance. For the drivers it makes no observable difference, but in V1/V4 a higher Gm tube will slightly increase power output and lower distortion.
Once you get to the very top of the list of great amps of all time, it's hard to say one is definitively "better" than the others. But I feel completely comfortable in saying a properly redone Cit II using the stock audio circuitry is at least as good as any amp ever built.
Great post jon, thanks for sharing the info. :thmbsp: