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In Reply to: RE: PSE 2a3 operating points? posted by andy evans on May 26, 2012 at 04:50:47
Hello Andy,First of all, what is your speaker load? By that, I am asking how efficient is it, and what range will the amps be used for..full range or ??.
Have you ever actually MEASURED in your listening room how much power your speakers need when you play them loud?? That is your starting point - to determine power needed Andy.
IF your speakers are efficient, 98 dB / 1 Watt / one meter, you might be better off with a simple SE amp. This avoids paralleling's "choir" effect.Forget 6B4Gs, they require DC on the filaments, which asymmetrically biases the filament, and this simply is inferior sounding compared to AC operation of a 2.5 VAC (2A3) filament.
Also, biplates, be they a 6B4G or a 2A3, generally will not equal a JJ 2A3-40 in performance, due to (1) the later's monoplate construction, aspects of the JJ's design such as (2) large cathode surface area, (3) symmetrical filament spacing, etc.
IMHO, its hard to beat the cost-versus-effectiveness of the new JJ 2A3-40.
Better to (1) FIRST determine your power needs by direct measurement and (2) use the best possible, simplest design choice, rather than spend time and money on needlessly compromised implementations. Have fun.
Jeff Medwin
Edits: 05/26/12 05/26/12 05/26/12Follow Ups:
Forget 6B4Gs, they require DC on the filaments, which asymmetrically biases the filament.
If the asymmetry of a static 2.5V DC shift in bias from end to end bothers you then how do you explain the good sound of the dynamic 7V shift in bias for the AC heated 2A3?
I prefer AC filaments but unless someone can present a compelling argument, I call the bias shift claim for the inferior sound of DC "bunk" since 60 times a second you have a range of bias shift with AC that is 2.8X that of DC.
dave
Dave,
You stated this above:
"I call the bias shift claim for the inferior sound of DC "bunk" since 60 times a second you have a range of bias shift with AC that is 2.8X that of DC."
Now, let me ask you Dave ..... how can a 6B4G be considered "symmetrical" when the filament is DC operated ? One side of its cathode is at plus 6.3 VDC and the other side of the cathode is at ground, or 0 VDC ???
At least, with AC, it is symmetrically moving about a zero point, across the entire length of the filament.
That is just one reason why we run AC filaments instead of DC filaments on DHTs. It is why designers select 2A3s and 45s, and these tubes are still being made, but not 6A3s and 6B4Gs.
Respectfully,
Jeff Medwin
I don't buy the argument that filament voltage should be a primary reason for choosing one tube over another. Asymmetry has long been touted as being much more detrimental in tubes with high voltage filaments, say an 845 with 10V Vs a 2A3 with only 2.5V.
However there's more to it than that, eg under typical operating conditions the RATIO of input signal compared to voltage drop across the filament is not that much different between a 2A3 an 845 despite the four fold difference in filament voltage.
Another case of taking one small piece of the puzzle and drawing a conclusion as to what the big picture looks like.
Naz
Your argument doesn't make any sense since ground and symmetry have everything to do with hum cancellation but little to do with how the tube biases up.
When you consider AC heating you need to bring the aspect of time into the mix to consider the possible voltage gradients. assuming you start a sine at 0, in 4ms you have a voltage gradient of +3.5V. At 8ms the filament is at the same potential from end to end. 12ms puts you at -3.5V and 16ms (ok (1/60ms) puts you back to the beginning to repeat. This means a 2A3 gives a 7V change in bias over time which is worse than a DC heated 6A3.
Furthermore, the bias in a DC heated tube is fixed. In an AC heated tube it is a moving target. Take a 1Khz signal and look at how the bias shifts in time. With a DC heated tube every trace of the sine will see the same stable bias. With AC, the single 1Khz waves at about 4ms and 12ms will see an overall shift in bias of 7V which some actually call IM distortion.
By no means am I saying that DC is better and in fact I like AC whenever I can get away with it. I am just stating my view that the concept of the "voltage gradient" or "bias shift" on a DC heated tube is a silly one to me since with AC it is actually worse.
dave
Nicely presented Dave.
Jeff Medwin
"I call the bias shift claim for the inferior sound of DC "bunk" since 60 times a second you have a range of bias shift with AC that is 2.8X that of DC."I agree.
What do you think of the "pre primed pump" argument?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/28/12
I am a big fan of analogies for first order approximations. I have a friend Mike in DC who is a bit of a night owl. We show up at mikes place at 1AM and inevitably lose a couple of hours in his basement talking. On the occasional cigarette break outside he told this story.
We were sitting on his front porch at around 2am and we could clearly hear two things. The 60hz buzz of the streetlight and some neighbors clearly talking on a back porch 3 houses up. I jokingly said I wish the light was a bit louder to drown out the neighbors and Mike insisted just the opposite. He went on to tell the story of how every weekend evening in the summer the same thing happens. The people sit out back and talk and during the daylight hours it is just a murmur. However as dusk sets in and the second the streetlight clicks on, the conversation immediately becomes intelligible.
I didn't get the chance to actually verify it with my own ears and I wasn't about to grab a rock and see. I do have a number of others who tell of anecdotal tales like this and they often follow a pattern similar to preturbulence reducing turbulence.
The other possible explanation is the people on the back porch were drinking and there is an interesting pattern of people talking louder as they drink more.
dave
could be a useful with power supply ripple..
shane.
Dave,Be nice to me.
NEVER EVER ask me to explain things up here, my detractors will go wildly amuck !! :-) OK :
(1) It sounds good because it bucks and boosts symmetrically around a zero point.
(2) It also sounds good because it is working, in motion, so the speak, producing a little AC, we call it low-level hum.
WHAT WHAT !!! A percussionist in the orchestra does not hit a gong, at the precise point in the score where it is called for. NO, they lightly tap it to get it moving in advance, ..... THEN they WHACK it good !!
A dead quiet DC 2A3 filament supply will MISS getting parts of the music off, instantaneous things. The amp will be dead, miss things.
These are "my" two reasons why AC sounds good at 2.5 VAC on a Type 45 and 2A3. YMMV, that is OK too !! My best to you Dave.
Jeff Medwin
Edits: 05/26/12
"A dead quiet DC 2A3 filament supply will MISS getting parts of the music off, instantaneous things. The amp will be dead, miss things."
That's clearly an educated guess on your part, and I don't mean to criticize you for putting it forward. However, it's incorrect, as those of us who have used ultrasonic AC filament supplies can attest. The "magic" of AC heating is still present in such cases, even though the amp is dead quiet. My experience with this derives from my own 211 SET amplifiers. For anyone interested, ultrasonic filament heating is relatively simple and inexpensive to try. Search my past threads on this topic for the details.
--------------------------
Buy Chinese. Bury freedom.
Take a "cleansing breath", re-read your post, and see if your argument still makes sense to you.
I have Mark Audio Alpair 10 fullrange speakers - similar to Jordan JX92S but better sounding. Probably 89db or so. They need about 8W. OK with 300b SET but better with 2a3 PP which is my current favourite. I was just wondering what to do with this SE Audionote TRANS-152 which allows for 90mA and is 2.5K. I didn't initially think of PSE 2a3 - was thinking 300b. But I'm preferring the sound of 2a3 PP to 300b SET. Not an equal comparison of course. I'd run the 6B4G with AC with hum pots. Don't have much of an issue with hum.
Andy
Thanks for posting. Wow, 89 dB is very low for tubes, especially SE tubes.
Yes, I could see how you would like paralleled 2A3s better than a 300B, due to the instantaneous peak currents it musters, on your load.
And you are correct, AC hum is much less a problem on 89 dB speakers.
Those speakers will actually be best with a real GOOD solid state amp, like a Spectral or Boulder, as USA-manufactured amps go.
Those speakers will actually be best with a real GOOD solid state amp, like a Spectral or Boulder, as USA-manufactured amps go.> >
Behind me Satan!! I don't listen loud and have a smallish room, so no problems with a PP 2a3 amp. Sounds great and plenty loud enough. I have considered just using a solid state output stage driven by DHTs with a transformer stepdown for PP out. Thought of UcD180 with input stage bypassed or a circlotron mosfet stage.
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