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RENS1234 operation

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Forum » Valves / tubes - Semiconductors » VALVES/TUBES / SEMICONDUCTORS in RADIOMUSEUM » RENS1234 operation
           
Joe Sousa
Joe Sousa
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13.Aug.10 21:41

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Fellow radiophiles,

I became interested in the operation of the RENS1234 hexode as an IF/RF amplifier.

The typical operating circuit on the tube page shows an extra output IF transformer winding coupling back to the third grid, which is usually near ground potential, or more negative for gain control. Is this feedback for neutralization, or regeneration?

The published curve families on the tube page are all for operation with a fixed anode voltage.

I would be interested in seeing a plate curve family with the third grid as a stepped variable. In a pentode, the third grid (suppressor) will move the knee of the plate curves from left to right, with a negative grid voltage. there is not much effect of the suppressor grid on plate current above the knee.

The data sheets suggest that the combined AGC effect from G3 and G1 can change gain 10000:1, which is very impressive for a single tube. The effect of G1 is that of mu variation, while that of G3 is multiplication.

Is the effect of G3 on the plate current sweep of the RENS1234 similar to pentode operation, or is it similar to the effect of the control grid G1?

Best regards,

-Joe

 p.s.:Upon further reflection, the effect of mu variation at the control grid reduces the AGC effect, but extends the linear signal range with large negative bias levels, so that strong signals can be attenuated linearly by the gm reduction that comes with increased negative grid bias. Variable mu reduces the AGC effect, but it is the price that must be paid for linear attenuation of large signals. This is most important at the last IF stage.

This article was edited 13.Aug.10 22:19 by Joe Sousa .

Jacob Roschy
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14.Aug.10 08:29

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Hello Joe,

the diagram “Abb.655”, shown at the RENS1234 page, is tricky in a way: the first RENS1234 operates, as intended, as an AGC controlled hexode. However, the 2nd RENS1234 is operated in an abusive way, i.e. as hexode- oscillator - modulator, which actually should be done by the RENS1224 !

To understand the intended operation of the RENS1234 and the RENS1224, I attach some original Philips pages of the Philips equivalents to the RENS1234 and the RENS1224, the E449 and E448, repectively.

Best Regards, Jacob


 


 

Attachments:

This article was edited 14.Aug.10 08:32 by Jacob Roschy .

Wolfgang Holtmann
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14.Aug.10 12:14

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Dear Joe and Jacob

To my opinion, the circuitry Abb, 655 shown on the tube page is not a superhet!
The two RENS1234 are correct and have the function as:

- V1: HF amplifier with AGC applied to grids 1 & 3. The ground connection of the cathode is missing!

- V2: HF amplifier with feedback (regeneration) to grid 3.

The REN 914 (V3) is the anode-bend detector.

Kind regards, Wolfgang

 

 

This article was edited 14.Aug.10 13:42 by Wolfgang Holtmann .

Wolfgang Holtmann
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14.Aug.10 13:39

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.... an even closer look revealed,

the screen grids (2 & 4) of V1 and V2 are tied together, which is quite normal.
A bypass capacitor to ground is necessary and of great importance!

 

 

This article was edited 14.Aug.10 17:27 by Wolfgang Holtmann .

Joe Sousa
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15.Aug.10 04:45

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Dear Jacob and Wolfgang,

Thank you for the very good information you gave.

The Philips data sheet was very useful in highlighting the different approach to gain control that Hexodes had, as opposed to the simpler pentode with remote cut-off that was favored in the USA. I always wondered what made the hexode popular in Europe.

The advantage of the Hexode as an RF/IF amplifier is that it essentially gave the equivalent AGC action of two pentodes in cascade; compare 10000:1 for an Hexode to 200:1 for a Pentode. The drawback of the Hexode vs the Pentode was inherently lower Transconductance because of the cathode current split away from the grid and into more grids with potentially higher partition noise and greater construction complexity.

Another important aspect of Hexode operation that became clear in the Philips data sheet, is that it can be quite different, depending on bias levels at G3 and G4.

Frequency converter service was described for the E448 Hexode, with G4 biased negative as oscillator control grid, and G3 serving as the non-inverting oscillator output plate. The transconductance from G3 to G4 is negative, and the voltage gain is non-inverting. The positive feedback between G3 and G4 is analogous to the positive feedback between G2 and G3 in a Transitron Pentode oscillator. G1 and G2 work conventionally as tetrode elements.

If the bias voltages of G3 and G4 are reversed, G4 becomes a positive tetrode-like screen and G3 is the second control grid. This is the recommended operation for the RENS1234 and the equivalent E449 as RF/IF amplifiers.

After my original post, I looked at the equivalent types listed in the RENS1234 page and found a plate curve for the RENS1834, which simply has a different heater voltage. This curve shows the classic tetrode kink when the plate voltage nears the voltage of the last screen grid G4. The plate is normally biased substantially more positive than G4 to avoid the gain loss at the kink.

It is now clear that G3 steers current between the G4&plate pair and G2 under the recommended bias conditions. When G3 is brought negative, more current is blocked from G4&plate and held back for G2. This accomplishes gain control action without reducing the linear input range for large signals at G1. If even more AGC is desired, then the voltage at remote cutoff G1 can also be controlled to achieve the 10000:1 control range.

As to my original question about the effect of G3 on plate current, it is now clear that it controls the plate current well beyond the knee, which is unlike the effect of G3 in a pentode, which is largely on the shape and location of the knee.


The illustration that Wolfgang shows is useful to help understand the operation of the Hexode, in particular the positive feedback from plate to the second control grid G3. This positive feedback is not the same as what is used for Miller capacitance neutralization because neutralization requires that the nature of the neutralizing feedback also be capacitive.

The regenerative feedback in the application circuit does not exhibit phase shift in the band of interest and serves to boost stage gain from G1 to the plate by boosting the apparent plate impedance of the Hexode. The voltage gain from G3 to the plate is inverting, so the IF transformer should show phasing dots for the necessary inversion from the plate to G3.

The 1uF capacitor that Wolfgang added helps illustrate the screening effect and fixed voltage bias that is intended at the bypassed grids G2 and G4.

Regards,

-Joe

This article was edited 15.Aug.10 04:46 by Joe Sousa .

Dietmar Rudolph
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15.Aug.10 12:34

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The RENS1234 and RENS1224 hexodes were produced only for a short time in 1933, and only a few sets used them. One example is the (famous) SIEMENS "Aetherzepp" 47WL which has the same schematic as AEG "Ultra-Geadem" 304WL and TELEFUNKEN "Admiral" 346WL. Indeed the chassis of all three types is nearly identical albeit the cases and scales are quite different.

The schematic of this types is identical, and as far as I can see also correct. The schematic Abb 655 seems to be an imperfect copy thereof.

The feedback coil at the 2nd RENS1234 is labeld "Rü" which means "Rückkopplung" (reaction). A fixed reaction then was the state of the art with 3 circuits TRF sets.

Additionally, a hair pin coil which indeed is a very small capacity is used to provide a combined electrical and magnetical coupling of the 3rd tuning circuit. This will improve the synchronization of the circuits, and additionally provide a greater bandwidth of the 3rd circuit which otherwise is reduced by the reaction.

Regards,

Dietmar

Georg Richter
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15.Aug.10 21:52

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If you like to have another circuit diagram in the collection of different ones for the AEG Ultra Geadem 304WL here we are with the original one, issued in "AEG Hilfsbuch für den Rundfunkhandel" (undated, the addendum is talking about models from 1935):

(As all circuit diagrams of this book there are not values but item numbers written)

An extract from the AEG Super Geadem 34 shows the automatic volume control (Schwundausgleich) details:

a = 1st preselection circuit

b = Fading Hexode RENS 1234

c = 2nd preselection circuit

d = Mixer Hexode RENS1224

e = oscillator circuit

f = IF Filter

g= Binode RENS 1254

i = network for automatic fading control

Best Regards,

GR

Jacob Roschy
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15.Aug.10 21:51

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Dear fellow radio enthusiasts,

I admit I did only a superficial glance at diagram “Abb.655”, so I didn't realised the actual function of the 2nd RENS1234. I suspect, the way it is, it's a waste to use a hexode only to apply a feedback winding to its 2nd control grid g3 ?

It has probably to do with the big hype about hexodes at the time, as the manufacturer could claim then: “he, people look, we have a radio which even has 2 hexodes !”

My my assumption, the 2nd hexode acts as oscillator- converter tube was not by chance, as such circuits did actually exist during this rather short hexode era, as obvious by looking at sets like these:
 

AEG Geadem-Super 34WL

Körting Hexodensuper S3410WL

Telefunken Bayreuth 654WLK
 

The first hexode RENS1234 operates as an AGC controlled hexode, while the 2nd hexode, the RENS1224, operates as oscillator - frequency converter.

AGC is applied only to the RENS1234, but not so to the RENS1224, as oscillation would stop very soon, once the ACG voltage increases. 

The announcement of the hexodes was a huge hype, especially in Germany, since they were invented by Telefunken. This hype was only short living, until much better frequency converter tubes were introduced, i.e. the triode-hexode ACH1 or the octode AK1, accompanied by AGC controlled penthodes such as RENS1294 = E447 or AF2.

Best Regards, Jacob


 

This article was edited 15.Aug.10 22:02 by Jacob Roschy .

Dietmar Rudolph
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16.Aug.10 10:51

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The "Hexoden" RENS1234 and RENS1224 were short lived tubes. Siemens issued a modified model of the "Laenderband - Aetherzepp" 47WL, the "Laenderband -Aetherzepp" 47aWL  within the same season which no longer had hexodes but penthodes RENS1214, and looked identical to the model with RENS1234. The reason for this modification is not known to me.

In the following season, new tube types appeared which had new numbers (AB1, ACH1, AF3 etc), and (exept the ACH1) had a different base called "Topfsockel".

Regards,

Dietmar

  
rmXorg