ETM connected EL/PL509 in T8 OTL-OCL amplifier - how?

[aparatusonitus]

Hy all,

I am planing to build T8 OTL-OCL from Rozenblit, but I would like to know how to connect EL/PL509 (EL/PL519) output tubes alla Tim de Paravacini ie in Enhanced Triode Mode (ETM)? Would this
particular amplifier benefit from such connection, and how to bias output tubes if everything else is the same as in original design?

Thanks, Ivo

 

[fdegrove]

Hi,

Not that I've actually double tested this but from what I read, ETM augments Zo.

Not something you look forward to in an OTL amp....

Cheers,😉

 

[aparatusonitus]

Hi Frank,

Thanks, I presumed there is a catch somewhere. Anyway, do you know how to connect those tubes in ETM or maybe know where I can get more information about EMT (except in this forum of course).

Ivo

 

[fdegrove]

Hi,

Conventional triode connected pentodes have the second grid tied to the anode, which results in a halfway compromise between a pentode and a triode. This neither has the efficiency of the pentode, nor the linearity of the triode. Tim de Paravicini’s Enhanced Triode Mode (ETM) goes one step further, and creates a true, highly linear triode from a robust pentode tube.

Instead of driving the normal Grid One with signal, this is tied down to the cathode, and thus becomes an invisible, transparent element, and part of the cathode. Signal is fed into the tube’s Grid Two, and in this configuration, Grid Three has no effect - it can either be connected to earth, or left floating. The ETM behaves like a true triode and is in fact more linear than the majority of directly heated triodes. Compared with the latter, it has the additional advantage of being a tough, more robust tube, capable of much longer operational life.


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Dixit Edouardo De Lima:

Enhanced triode mode uses the grids of a pentode in a different manner. When compared to conventional triode connection, the final result usually behaves as a triode with lower levels of distortion but much increased plate resistance and naturally much higher output impedance. The plate resistance in this connection is just about the same as regular pentode operation.

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Dixit Andrea Ciuffoli:

Enhanced Triode mode, big names (Tim de Parravicini) adopted this kind of topology. It has a good output swing, but the inverted usage of the grids generates a very high output impedance. such a situation forces the usage of a power tetrode/penthode for driving the output. Furthermore the stage linearity is pretty questionable.

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Svetlana document re. ETM

http://www.svetlana.com/docs/TechBulletins/technoteNo33.html

Note that G1 is grounded and G3 is tied to the cathode.

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Dixit FDG:

So, the disadvantage seems to be the higher Zo when connected as per TDP.
But, what happens to the Zo if we were to connect G3 to the anode?

This + my comments is what I found...

As I never plotted such valve the way I suggest, I am at a loss.

My logic is only a guess, so don't build on it...

Cheers ,😉

 

[aparatusonitus]

Thanks Frank, this is quite helpfull!
BTW, could someone elaborate Frank's idea:

But, what happens to the Zo if we were to connect G3 to the anode?

Ivo