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In Reply to: RE: SUN SV 2A3 stereo SE amp .... MEASUREMENTS posted by FenderLover on April 01, 2015 at 09:35:04
Hi,
I don't know the Class designation for sure.
I have asked EE Paul Joppa, a educated guy, above in this thread, lets see where that goes !!
I am not sure it matters. Because ................
What does matter is that I run the tubes where they sound the best, less thermally STRESSED SOUNDING, and they have long life.
Jeff Medwin
Follow Ups:
I did not locate Jeff's question to me, but having been invited here I'll take this opportunity to give my thoughts. (I'm an engineer, but not specifically electrical - worked mostly in acoustical engineering FWIW.)
At the operating point mentioned in this thread and shown in the curves provided by Caucasian Blackplate (250v/37mA/2500 ohms) the tube is operating at about 11% second harmonic distortion. As long as the reflected speaker impedance is greater than the nominal, distortion may be high but technically it is Class A - the tube is never cut off. But it is at the edge - if the impedance of the speaker dips below nominal (they all do!) then the operation will be Class AB, with the tube cut off for a portion of the cycle but less than 50% of the cycle.
The interesting thing to me is that the output power is 2.9 watts at the edge of grid current, whereas if it were really linear then 37mA would produce at most 1.7 watts. In addition to the extra power, the high second harmonic gives a psychoacoustic effect of greater loudness, often described as "more dynamic". The price paid is masking of the music by the distortion products. With enough distortion at full power, the distortion of real overload clipping is probably less obvious.
The average current at full power is nearly 50mA, 35% more than the 37mA at no signal. This is a reasonable explanation of the need for a tightly regulated and fast power supply - to control envelope modulation. Rapid power supply recovery is much less important when the tube is operated as RCA suggests, where the variation of average current is less than 10%.
At smaller signals, the distortion can be reduced by using a driver stage with similarly high levels of second harmonic, which is what you will get from a notoriously nonlinear tube like the 12AX7, with a resistor plate load. Of course, this distortion cancellation actually increases the higher harmonic distortion products.
Using direct coupling eliminates blocking from grid current into the coupling capacitor, but blocking from the cathode bypass capacitor is still present, and is increased by the large variation in average current. Balancing the blocking against the power supply regulation is a complex challenge, since the time scales are psychoacoustically important as well.
Hi Paul,You show a fine understanding of electronics and you write clearly. I respect that, and do not have that attribute.
The 37 mA. at 250 VDC P-K ( 9.3 Watts dissipation) was simply the Golden Ratio ( 62% ) of 15 Watts, with 15 Watts being the maximum 2A3 plate dissipation rating. I mentioned that low current number to Fender Lover only as an example, but it is NOT at all what I have ever used.
Caucasion Blackplate, in his rush to try to discredit me ANY way he can, seized upon the "lowest" number mentioned, ( and inadvertently perhaps,... so have YOU ! ) without figuring-out what I really liked to employ.
The favored op point of mine was in my description to Fender Lover. In practice, I typically will use 42 mA. on a 2A3, when given free reign to pick the spot.
At 250 VDC P-K, 42 mA. is 10.5 Watts of plate dissipation, or 70 percent of maximum. It is dissipating 8% over the Golden Ratio of 62%. It certainly is NOT operating at 95 to 100% of maximum ( 60 mA. times 250 is 15 Watts, 100% ). The stock SUN SV 2A3, and MOST 2A3 amps running 60 mA. and 250 VDC P-K exhibit a negative-to-my-ear STRESSED SOUND trait, due to thermal issues.
The other thing, ( which I think is totally hilarious ), is that you and Caucasion Blackplate, and MOST other EEs TOTALLY FAIL to ever consider in your analysis is this :
WHERE does the tube operate, on its load line, 90% or more ( 99% ?? ) of the time??
At RMAF shows, LOUDLY playing demanding music (Symphonic Orchestra with Organ), we may reach 3/4s of one Watt. In my home's listening room, ditto, 1/4 of a Watt to 3/4s of a Watt.
CB's graphical load line analysis, and your analysis, is quoting distortion figures at FULL SWING and maximum possible power.
THAT is what BOTH of your analysis' is "all about". Max swing.
I question you two guys, and you TRADITIONAL way of "looking" at audio design. Almost ALL of the time, I am swinging at LESS than full power out, in the linear mid-point on the load line, and at LOW distortion.... BUT..... 42 mA. does NOT sonically exhibit the thermally-stressed "thumbprint" of a 60 mA. bias.
Simply put, my load line SOUNDS NICER, more RELAXED, at normal listening levels we would employ.
If everyone else wants to conventionally design and listen to 2A3 amps with a STRESSED presentation, and burn up the output tubes doing so, so be it. I just simply LAUGH at that !!!
I also must seriously question your impartiality when you describe a 12AX7 as being a non linear tube. If one looks at the characteristic curves, it certainly looks MOST linear to me! Furthermore, it has a RICH proven history of being used in audio devices with State of the Art performance level, for decades in our lifetimes. Think of the ARC SP-3 and SP3A-1, when we were younger, or the highly regarded ARC SP-8.
Now Paul, ALL OF THE ABOVE, ....ALL OF IT...., is just hypothetical engineering gobble-de-gook to me !!!
The proof of the pudding is in the eating. I live in Missouri Paul, the "Show Me" state. "Don't tell me about the pregnancy, just SHOW ME the baby".
Are you coming this year to RMAF 2015, October 2-4 ? If so, look me up and you can HEAR precisely what I am talking about !! Its superb, and beautifully honest to the recorded music. If it sounds good, and is reliable, it IS good !!
Regards and best wishes,
Jeff Medwin , DIY audio amateur .... mostly
Edits: 04/03/15 04/03/15 04/03/15
nt
I just posted this to dadbar in a thread below.
dadbar had said,
"The point I was making is that I don't really care about what the music should "really" sound like as long as I am totally enjoying what I hear out of my own system."
My response to him was,
"That is the way most people feel about the subject but the original goal of the hobby, Hi-Fi, was to reproduce as accurately as possible the sound of the original event.
Since we can't control the recording process, reproducing the recording as accurately as possible is all we can do.
There is nothing wrong with having a different goal but having a different goal means that it's a different hobby.
I call it [the] "Happy, Happy sound machine" [hobby] with no real disrespect intended."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
From Wikipedia, the free encyclopedia
De gustibus non est disputandum is a Latin maxim meaning "In matters of taste, there can be no disputes" (literally "about tastes, it must not be disputed/discussed").[1][2] The implication is that everyone's personal preferences are merely subjective opinions that cannot be right or wrong, so they should never be argued about as if they were.
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