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I have a pair of Audio Note TRANS-152 OPTs which are gapped for 90mA and 2.5K into 8 ohms. I was thinking of 300b but revised this idea to PSE 2a3s. I intend to use the biplate versions, or the biplate Russian 6C4C, which is a 6B4G.
The 6B4G data sheet states 250v and 60mA in SE, class A. It also states 325v and 40mA in PP, class AB1
I'd like to get somewhere around 45mA each tube, but with these biplates how high could I take the Anode-Cathode voltage? Any suggested operating points? Who has tried out 2a3 in PSE?
Andy
Follow Ups:
especially the Russian tubes which can usually be run WAY over spec...
how they sound this is the question....generally I share Jim Shane's view that many tubes can be significantly run over their voltage ratings if their power ratings are respected...you will get a bit more 2nd harmonic at this point, but generally the bass is more solid at higher voltage/higher impedance operating....
You will have to find out also if AudioNote means 90ma really, when they rate their OPT's, but I think generally they are a bit conservative...
so fine...I think you will like what you hear....
This is looking hopeful! I've never tried PSE. Do I use separate cathode resistors and bypasses, or can I use a single cathode resistor and bypass?
Andy
Andy, the advantage of separate Cathode Rs is that you can use these to balance the idle currents. Also, if you choose to trade off the slightly better technical performance afforded by bypassing (all or part of each) then you will gain better AC signal matching.
Personally, I'm not a fan of 100% Cathode Biasing. In my 845PSE monos I use (mainly) fixed biasing through Pos and Neg PSUs and just enough Cathode Biasing via adjustable Rs to balance idle current of each tube. I use DC coupling and my main (coarse) control for idle current is via adjustment of the DC grid voltage, set in the driver stage.
Happy to share if can be bothered with the additional complexity. Advantages are A2 drive on peaks, DC coupling and great balance.
Cheers,
Naz
Hi Naz - you use PSE a lot I think? The whole issue of fixed bias for me is a difficult one - I tried several types and all of them seemed to degrade the high frequencies. The best I could come up with was Schottky diodes and a couple of CR stages, the last cap being polypropylene. I thought even that was audible in the highs. I didn't go into tube rectification and choke input.
So I remain to be convinced about fixed bias!
With cathode bias, I only use polypropylene caps - electrolytics are horrible.
With small tubes I use filament bias. Seems to be the best solution - small cathode resistor. All the tubes I use are DHTs, and I use Rod Coleman's filament supplies.
andy
I use PSE because I need the power but notwithstanding, properly implemented, to me it sounds better because you can halve the primary impedance of the OP tranny. I also use DC coupling almost exclusively in SETs to give me the extra overhead and less aggressive overload characteristics.
The fixed bias I use is not the normal simple bias supply connected to the grid with a high value R. I use damper diode tube rectification and small C-> LC filtering to feed my driver stage(s) and the driver sets the bias voltage for the OP tube. Definitely no loss of highs here, in fact extended but smooth highs with lots of air is a must for me!
Cheers,
Naz
Hi Andy,320 VDC P-K at .045A into 5K is not a great compromise op point IMHO.
First of all, its is PSE so you need to drop output trannie Z in half, to 2.5 K or as low as 1.25 K for two tubes in parallel. One Electron makes a UBT-1 which is 1.6 K as I recall, not too bad, not too costly.
Also, the previously suggested op point dissipates 14.4 Watts across the plate, so tubes will last only about 2,000 hours, and degrade continuously.
I would keep the current about 41 to 43 mA. (-49 to -50 VDC self bias) and reduce the B+ to about 245 to 250 VDC P-K, and the tubes will sound less STRESSED THERMALLY. With 10.5 Watts plate dissipation, tubes could last 10,000 hours, five times longer, and sound better.
If you need a matched quad of used 6B4Gs, I have them.
My very first from-scratch, self-designed SE amp WAS a PSE 2A3 using UBT-1 outputs and a 5687 front end, a two stage amp. Forum Member "Electron" auditioned it, a 'long long time ago.
I would think you can do the Rks either way, combined or separate.
I probably would tend to do them separate and use TWO Mills paralleled Rs, closely matched in Ohmic value, for each Rk. This gives you two wire paths, and hopefully a short return path to star ground.
Place your star ground point NEAR the 2A3 cathodes, and use the body and wiring of the Rks to bridge the span between the Ef XFR and star ground. Use the center tap wire of the 2.5 VCT trannie, KEEP IT SHORT, and no hum pot. You should shoot for film caps across the Rks, multiple bypassed as per Marzio and Jelasi in the old Sound Practices article on Building a Single Triode DC Amp. Short simple return paths to ground for DHTs sound superior.
Your amps I know will not be DC coupled, but the Italian boys' Rk bypassing technique, written in 1994, is on the right track.
Have fun.
Jeff Medwin
Edits: 05/26/12 05/26/12 05/27/12
Parallel 2A3 with higher than specified load impedance should sound great. I noticed when I use higher than manufactures recommended impedance for maximum power out at usually 5% distortion, the tube sounds better. I find the magic starts around 50% higher impedance on a few amps I built using different taps. At 100% increase you could push the plate voltage to 275-290 volts and set bias for 45ma. The JJ 2A3-40 would be very comfortable at that voltage & stock 2A3 or 6B4G should be fine. Longest stock 2A3s life are likely at 250 volts & 35ma. But, we are not talking a seriously rare tube here.
I built a SE 2A3 with 260 volts plate @ 50ma using 3.5K primary impedance. Audio is crystal clear, bass strong enough & midrange delightful. I do not need another amp. Speakers are 98dB efficient.
One 6B4G requires 2K5 loading!
One 6B4G requires 2K5 loading!> >
Yes - two should sound really nice! Would lose a bit of power, though.
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 05/26/12
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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