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.

SE transformer recommended:

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


Beautiful piece of glasswork. Details inside show absolute quality



Looooong plate!


Filament glow