On the use of names...

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jack
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#1 On the use of names...

Post by jack »

When is a pentode not a pentode? When its a beam pentode, which is really a "beam tetrode" as there is no 3 grid - no suppressor - only beam forming plates.

So should all "beam pentodes" or "beam power tubes" really be called "beam tetrodes" - technically, without the third grid, they are tetrodes...

The EL34 is an example of this - Mullard called it a pentode, but others who made them (e.g. as a 6CA7) called it a "beam tetrode" or a "beam power tube".

The name makes no difference to how they sound, but it is a bit of a grey area.
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Nick
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#2

Post by Nick »

Not really, it was because Mullard/Phlips patented the pentode, so the beam forming plates were invented to get the same results without the third grid.

http://www.r-type.org/static/grid14.htm

I dont think a 6CA7 was ever officially described as a beam tube.
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jack
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#3

Post by jack »

Nick wrote:I dont think a 6CA7 was ever officially described as a beam tube.
Not in most data sheets, no, however...
Wikipedia on Beam Tetrodes wrote:Some tubes described as pentodes are actually beam tetrodes. The ubiquitous Mullard EL34 (6CA7), although manufactured by Mullard as a pentode, was also produced by other manufacturers as a beam tetrode instead.
This seems to be because although the EL34 was a pentode, the "American EL34s", i.e. 6CA7s, seem to have been, initially at least, beam tetrodes with similar characteristics. There's quite a lot of stuff on the www about this, but little in the way of hard facts that I could find...

JJ Electronic actually describe their 6CA7 as an A. F. Beam Pentode, and, as we all know, a Beam Pentode is actually a Beam Tetrode

http://www.jj-electronic.com/pdf/6CA7.pdf
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Paul Barker
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#4

Post by Paul Barker »

The true Mullard El34 is a wonderful piece of skilled precision work. That suppressor grid has to be perfectly wound for a true pentode. The beam construction much cruder and simpler to produce.

The greatest benefit of the true el34 ( I can't say with any authority but perhaps, the only version of the El 34 which is true) is that that skilfully constructed extra grid is not clumsily internally tied to the cathode, this makes it much more flexible, in particular triode mode has the option of connecting this grid also to the anode as the screen, which gives a different sound to when it is tied to the cathode, you can experiment for yourself but I definitely prefer it with the anode.

Of course it is well know this facility is available with the 813, and I tried many variations when I was experimenting with the 813. But that is if course a beam tetrode, the beam forming plates are free of internal connection. One sound I really liked was to ground the anode and make the beam's the anode. But obviously you have made a low powered valve. A bit of fun.

Proper el34 very good valve as a triode. Actually the sound which first awakened my interest in valves at a friend's house one seminal evening.
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#5 Re: On the use of names...

Post by rowuk »

The 1975 RCA Receiving Tube Manual that I have states that the 6CA7/EL34 is a "Power Pentode". It also shows all three grids independently accessible.

Here is a page on the different EL 34s: http://www.tubecollection.de/ura/EL34_Varianten.htm

As far as the semantics of tube descriptions, considering the vast differences in actual knowledge of all that visit here, the truth is probably more useful than convention.
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