Bigun,
In post # 38, were you calling a 2A3 a transmitting tube? Or were you referring to a different project using actual transmitting tubes?
The 2A3 was designed to be an audio tube, and it has also been used by the US military as a Regulator Pass tube, but I have never seen it referred to as a transmitting tube before. The 2A3 is a DHT, and there are Transmitting tubes that are DHT, but the similarity ends there. Most transmitting DHTs use Thoriated Tungsten. I wish someone would tell me of a Barium coated DHT transmitting tube. Anybody?
I just built a quick and dirty parallel single ended 2A3 mono-block, using a One Electron UBT-1 OPT. 250V plat to filament, 49V self bias. The driver is 1/2 of a 12AY7; I use RC coupling. All this was just easy. It is up and running, sounds great. But . . . I have to finish the filament balance circuit, it is using AC filaments. I hate AC filaments on DHT. I hate DC filaments on DHT. Each has its tradeoffs.
In post # 38, were you calling a 2A3 a transmitting tube? Or were you referring to a different project using actual transmitting tubes?
The 2A3 was designed to be an audio tube, and it has also been used by the US military as a Regulator Pass tube, but I have never seen it referred to as a transmitting tube before. The 2A3 is a DHT, and there are Transmitting tubes that are DHT, but the similarity ends there. Most transmitting DHTs use Thoriated Tungsten. I wish someone would tell me of a Barium coated DHT transmitting tube. Anybody?
I just built a quick and dirty parallel single ended 2A3 mono-block, using a One Electron UBT-1 OPT. 250V plat to filament, 49V self bias. The driver is 1/2 of a 12AY7; I use RC coupling. All this was just easy. It is up and running, sounds great. But . . . I have to finish the filament balance circuit, it is using AC filaments. I hate AC filaments on DHT. I hate DC filaments on DHT. Each has its tradeoffs.
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no... I was saying that a 2A3 amp is relatively simple (e.g. Loftin White) and I was initially hoping this Krakatoa amp could also be simple - just using the 2A3 as an example of what I meant by "simple". Having said that, I have had the parts for a 2A3 amp for many years and still haven't got around to it.
Filaments you say - well 2A3 can use AC fairly well can't it, with only 2.5V. But the SV811 is a 6.3A filament and 4A of magnetically radiating current ! With my plan to go close to zero bias the voltage drop across the filament is going to be significant in comparison. The 2A3 with -50V grid bias doesn't suffer from that issue.
Great - thanks, it works just fine now !
I've modified the Spice instructions (attached) to provide a clean FFT and I've added a load to the driver consisting of a diode and a resistor in series. The diode and resistor represent the power tube grid impedance so that I can see the impact of this load on the driver circuit. It's not that I was expecting a surprise but it's sometimes a comfort to see that the math (Spice) aligns with my world view.
Bigun,
You are right. AC on a 2.5V filament is not always as much of a challenge.
But 2.5A AC current (3.5A peak) on one of those new production mono-plate '2A3' tubes can cause problems due to the filament's electromagnetic attraction of an unevenly spaced filament to the two sides of a magnetic plate.
I had a pair of SV811-3 tubes for years (lower rp, and higher bias V than the SV811-10),
I finally sold them because I did not think I would ever get around to building an amp from them.
But you are also right, in post # 41, I mistyped what I actually built . . . a quick and dirty modern production mono-plate '6A3' PSE. I actually wanted to build a PSE 2A3 instead (with the modern production mono-plate '2A3' they are for an amp later on).
With the mono-plate '6A3' 6.3V 1A filaments, I do have to put a hum balance pot in.
I guess that is next on my latest quick and dirty amp.
Happy building, and happy listening!
You are right. AC on a 2.5V filament is not always as much of a challenge.
But 2.5A AC current (3.5A peak) on one of those new production mono-plate '2A3' tubes can cause problems due to the filament's electromagnetic attraction of an unevenly spaced filament to the two sides of a magnetic plate.
I had a pair of SV811-3 tubes for years (lower rp, and higher bias V than the SV811-10),
I finally sold them because I did not think I would ever get around to building an amp from them.
But you are also right, in post # 41, I mistyped what I actually built . . . a quick and dirty modern production mono-plate '6A3' PSE. I actually wanted to build a PSE 2A3 instead (with the modern production mono-plate '2A3' they are for an amp later on).
With the mono-plate '6A3' 6.3V 1A filaments, I do have to put a hum balance pot in.
I guess that is next on my latest quick and dirty amp.
Happy building, and happy listening!
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I think a real 6B4G has 2 plates, series filaments, and a 3.15V filament in each plate structure.
I have used push pull 2A3 with AC filaments, and was not pleased with the level of AC hum, even with balance pots.
However, that was on a magnetic steel chassis, and hum from magnetic spray to-and-from the chassis was part of the problem.
I may have to try push pull 2A3 later, on an aluminum chassis.
I have used push pull 2A3 with AC filaments, and was not pleased with the level of AC hum, even with balance pots.
However, that was on a magnetic steel chassis, and hum from magnetic spray to-and-from the chassis was part of the problem.
I may have to try push pull 2A3 later, on an aluminum chassis.
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A quick look shows that at 600V/100mA into a 5K load, 25+ watts would be available.
You'd have to deal with the grid current, and by deal with it I mean they don't even tell you what it is in the datasheet, so you'd need to put one of these on a bench supply and take some measurements to get an idea of what the demands will be.
The 200V P-P drive isn't that big of a deal if you use a pentode or a cascode up front.
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Hey Bill, well not much has been happenin’ on this front. I wandered off reading about the Sakuma amplifiers for some weeks and ended up procrastinating myself into a dead end. I think I got myself a big 845 triode during that period. I’d love to get stuck back in on a project, although first up I need some sort of pre for switching between sources in my family room system and that just begs for a bottle somewhere in the box, maybe a cathode follower just to make sure there’s a glow. Of course, the big bottles still beckon, and the high voltage seems less dangerous than the virus or the stock market these days!
Congrats on the quad there, your tubes are the higher mu version, so less swing at the grid which will be nice.
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No, they do not and yes, this is of utmost importance. My experience is SV811-10 show exactly the same grid currents as SV572-10 and those are published. More than that, I found that they are the plate curves of SV572-10 that I have observed with SV811-10. And they were from all sorts of production years, in both straight and coke bottles and with Svetlana and Ryazan plant logos.
I was trying to find a way of getting into A2 with this tube without inflicting unacceptable pain from 2012 to 2019 and eventually gave up. But I got meanwhile hooked on strong 2nd harmonics so badly that I needed a special detox procedure to shake it off after...
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yes, that 845 is a beauty for sure, "go big or go home", as they say.
Now, one of those SV811-10's would make a sweet driver for my 845
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