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TELEFUNKEN RS291 in Triode Mode
We have this pair for sale, which is believed to be NOS.
PRICE FOR THE PAIR: 420 Euro, with guaranteed Test Data.
Tube Socket. The original socket is hard to find, and if you do it's probably a bit bulky. However it is very easy to make such a socket yourself. All you need is a round plate, and you mount 4mm banana plugs to it, chassis type. Like the ones here on the left, they can be found on Ebay for 3 Euro, one. The footprint is similar like a large B4 socket (same as RGN1064, but larger)
Power consideration: RS291 is a nice tube for HiFi purposes, but certainly not commercial from the shelf. The Japanese like to use tubes like 211 and 845, which are transmitter tubes also. However these are very hard to find as NOS, and the Chinese replicas do not last very long. Moreover, 211 and 845 are "supposed" to output 25 Watt, which is distorted. Without distortion ist stops pretty much above 22 Watt, provided you work at 1280 Volt, or above. If below, it drops down to 8 Watt only at 820 Volts. . The RS291 is much more convenient with the anode voltage. Though it has it's best performance at 1000...1500 Volts, it works well at 400...800Volts also. So just not to stress the tube too much, use it at lower voltage, lower dissipation, and you still can get easily enough power out of it. .
The datasheet of this tube is for tetrode connection, because that's what this tube was originally intended for. 4 pages - full datasheet here
However, for HiFi you should use this tube triode connected, which means you just connect the second grid to the anode. There was never an official data published for this. The test report that comes now, is in Triode mode, and I did so for BOTH TUBES TOGETHER in one sheet. This appears to be a perfectly matched pair.
What do we see from this report? This is a HUGE 45 tube if used in triode mode. All data is like the 45, only the RS291-Triode-mode runs at a much higher operating point.
You will see the excellent matching of the pair. I have set the tubes for a normal HiFi value of 400 Volt, 80mA. As you can see that produced a bias of 79.3 Volt for tube #1 and 76.4V for tube #2.
Bias: In real auto bias circuit, the tubes will bias the same way, each at 80mA exact. So bias at 400V/80 mA is just what I used at the tube tester, not stressing the tubes and the tester. However, the crazy thing is, these are 110 Watt triodes. :) So at 400V/80mA they got only warm, but not hot. Actually if you try it, you will see the 45 bias point fits somewhere on those curves too. You should bias them somewhere like 480V 120mA.
Gain. It is coincidence, they did not think of this TFK of course, but the Triode gain is very nice, the gain is 2.9 for each tube. The low gain of 2.9 will ensure low distortion.
In triode mode, this tube looks like a big 45 tube.
LL1620-80mA, 6k to 4, 8 or 16 Ohms. Medium Power Design
LL1688-120mA. 5k5 to 4,8, or 16 Ohms. High Power Design. We can supply 120mA at special order
There you are, the TRIODE characteristics of the RS291, measured with my good old SOFIA, which testis the tube at full dissipation.
From this above files, I have extracted the S-Parameters. (File here). Then, with the S-Parameters, I could re-print the curves at any current and voltage needed, and then simulate the working tube. In Triode mode it seems to work best at 6k Ra. If the anode voltage is chosen higher, the tube begins to develop a LOT more output power. At 1000V it will give 25 Watt without positive grid voltage, and with positive grid voltage it gives 42 Watt. However, that has distortion of 11...13%. With enough feedback you can get that under control. At a lower voltage of 650V as shown here, output power is 10.46 Watt, and distortion stays low. So for none feedback, I would say that is a good voltage.
The AMAZING thing is, there is NO 3rd. Harmonic distortion. Only second harmonics. This trannsmitter Tetrode in Triode mode, gives perfect Triode sound indeed.
Working point at 650V Anode. No positive grid voltage. Gives 10.46 Watt at 4.8% distortion, at 6k Primary. It is entirely second harmonics only, which is remarkable, and very good of course. If reduced to 2 Watt only, which is for soft room loudness, we have 2.4% distortion, also entirely 2nd. harmonics only.
Since this tube is MADE for positive grid voltage, I found a crazy maximum of 25.7 Watt, at 7% distortion. Now this needs definitely some feedback, but not even that much, I would call the 25.7 Watt theoretical, but the simulation shows at least a gigantic boost is possible like this. However drive a tube with positive grid voltage is not easy, it needs a very low impedance driver.
Or, most probably also very good: Create an Ultra Linear SE stage, using the screen grid, but for that I have no simulation. A little something you need to do yourself :)
In any case, this WW2 airplane transmitter tube has fantastic HiFi probabilities. If you buy a used one, imagine what flights it was, that it has been used for.
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Beautiful piece of glasswork. Details inside show absolute quality
Looooong plate!
Filament glow