heptode/pentagrid compressors and ECH8x tubes

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vinyvamos

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Is there anything to say that a pentagrid compressor cannot be built around a pair of ECH81 or ECH83 with the cathodes grounded, and the triode sections used as the next gain stage? A reference negative grid bias would be needed on both the heptodes and triodes due to the cathodes sharing a pin. Or... how about using LEDs to ground from the cathode pins to provide a fixed bias point, which would have to suit both sections of course. Or would this simply not work? I am assuming here that the use of LEDs means that no bypass caps are needed and that the voltage will be constant regardless of changes in Heptode current during gain reduction.

On that topic, how does the heptode cathode current vary WRT gain reduction compared to a more common vari-mu triode/pentode stage? Is it pretty much the same scenario?

Why were pentagrid compressors forgotten about? Do they exhibit higher distortion/lower GR capability than the later designs using triodes and pentodes?
 

abbey road d enfer

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vinyvamos said:
Is there anything to say that a pentagrid compressor cannot be built around a pair of ECH81 or ECH83 with the cathodes grounded, and the triode sections used as the next gain stage? A reference negative grid bias would be needed on both the heptodes and triodes due to the cathodes sharing a pin. Or... how about using LEDs to ground from the cathode pins to provide a fixed bias point, which would have to suit both sections of course. Or would this simply not work? I am assuming here that the use of LEDs means that no bypass caps are needed and that the voltage will be constant regardless of changes in Heptode current during gain reduction.

On that topic, how does the heptode cathode current vary WRT gain reduction compared to a more common vari-mu triode/pentode stage? Is it pretty much the same scenario?

Why were pentagrid compressors forgotten about? Do they exhibit higher distortion/lower GR capability than the later designs using triodes and pentodes?
What do you expect from using such a tube over a simple remote cut-off triode or pentode?
These tubes were designed to solve problems particular to superhet receivers, in particular assuming two functions in one (oscillator and mixer).
In a vari-mu compressor, the extra grids would be tied to other electrodes or to fixed voltages, which would result in operation similar to a triode or pentode.
 

ruffrecords

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abbey road d enfer said:
In a vari-mu compressor, the extra grids would be tied to other electrodes or to fixed voltages, which would result in operation similar to a triode or pentode.
I know it is three months later but a recent thread here about a Wilcox limiter which uses one grid for signal and another for control:

https://groupdiy.com/index.php?topic=77133.msg981352#msg981352


Cheers

Ian
 

vinyvamos

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Cheers Ian! I will give that one a read now :). Alas I didn't do anything more on the pentagrid idea yet, but maybe soon it will get breadboarded
 

abbey road d enfer

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ruffrecords said:
I know it is three months later but a recent thread here about a Wilcox limiter which uses one grid for signal and another for control:

https://groupdiy.com/index.php?topic=77133.msg981352#msg981352


Cheers

Ian
My understanding is that it is an attempt to avoid thumping by separating the audio and control signals.
This arrangement, which works extremely well in superhet receivers, does not seem to bring a distinctively better performance in audio limiters.
 

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