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TETRODE amplifiers

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I am quite a tetrode lover. Which always means - beam power tetrodes. "Classic" tetrodes aren't being used anymore since many years. Even the first types quickly were built as "tetrodes with critical electron distance" - which just meant that there was enough space between screen grid and plate to push all secondary electrons back to the plate by collision with primary electrons - and thus preventing the absorbtion of the secondary electrons by the screen which caused the instabilities of "classic" tetrodes. Supressor grid was replaced by a space charge between screen grid and plate. Beam-forming electrode in BPTs simply contributes to this effect by focusing the primary electrons and thus increasing space charge.

Many of the non-linearities of earlier beam-power tetrodes (and pentodes, too) could be removed by reducing the screen grid current as far as possible - by placing the screen grid in the "shadow" of the first grid. So the screen grid wires were touched only marginally by the electron beams passing through the first grid. Additionally, a low screen voltage does contribute to this effect.

So today, if you compare curves and screen current, you'll almost always see that the most linear types are those with the lowest screen current, too - usually much less than 1/10 of the plate current. And I found that this contributes very much to sound quality - tetrodes and pentodes with "non-shadowed" screen (as EL34, 6L6 ....) need to be run in triode or ultralinear mode to sound really well - the "shadowed" types (all sweep tubes) even sound well in classic tetrode/pentode mode.

And if such a tube is fed with a stabilised screen voltage with its AC potential equal to the cathode, then you'll get a very good sounding and very effective amp. I did it this way with my EL36-amp. I tried EL34, then 6L6 (both did sound quite rough. 6L6 even worse than the EL34), then I took 7027A, which was wonderful. Seemd to me to be a "shadowed" version of the 6L6 when looking at its construction. When one tube failed (they were hard to get that time in Germany), I replaced them by the EL36.

Greetings

Uli

amp.jpg
 
Hi Johan,
You have brought to light a question that's been at the back of my mind for years. I nearly posed this as a new thread, in response to the recent thread polling the popularity of differant output tubes. As you point out the EL34/6CA7 is not beam tube (at least in many manufacturers' versions) but a "true" pentode. It's my understanding that the EL84 is also a true pentode. I've never opened one to verify this, but I notice that the published curves plainly show the "kink" that distinguishes this type.
The question that this raises, in my mind is "why"? The beam tube was developed in the middle or late thirties as I understand, to overcome the undesirable "kink" that was a shortcoming in tetrodes or pentodes. This was recognized as a "breakthrough" in tube design, and was incorporated in the design of most power tubes, with the "KT" series of Genelex promoting the kinkless tetrode openly in a highly successful product. The EL-34 and EL-84 were designed years later, probably in the late '40's or early 50's, seemingly ignoring , the at least, supposed advantages of a beam tube. These, of course, went on to be very successful and highly popular tube types
Could it be that the kinked curves of the pentode, are in reality, not much of a drawback in the actual operation of the tubes. Or could it be that there are disadvantages in beam tube operation that are not widely known. Perhaps was it just an issue involving the patent of the beam tube that other companies' designers found a way to avoid with a product that seems to be an equal. I'd love to know! Ideas--anyone?
 
Hi ArtG!

Looking back onto tube history, I think, that this is mainly a historic/geographic rather than an electronic issue. Beam power tetrodes were developed in the United States, while Europe (and especially Germany) has always been pentode country. In the US, TV was introduced much earlier than in war-devastated Europe, and so there was an early need for powerful pulse tubes - and BPTs are ideal for that. Suppressor grid in pentodes always is a bit of an obstacle to free electron flow by increasing internal resistance and limiting a bit their ability to handle large pulses.

In Europe, restoring infrastructure after the war years was mainly for public areas, cinemas and dance halls, TV slowly arrived during the fifties (My parents bought their first TV set in 1961 - and they were quite prosper...). So for cinema amplifiers (EL 34) and radio sets (EL 84 was the classic radio set power tube), traditionally pentodes were used. And if you compare both types from their construction - the EL 34 just looks like a blown up EL 84.....

In Europe, BPTs were introduced on a large scale at the beginning of the 60s. And if I remember well - even one of the first European sweep tubes (EL 38) still was a true pentode.

Btw - the AL 4 I use in my EL 36 amp is also built as a true tetrode - with critical electron distance.... It is the black "coke bottle" on the left side used for screen and pre-stage supply stabilization.

Happy Holidays

Uli
 
hi Art and all,

I believe it was mainly as a result of patent rights. I have an independent publication here somewhere which related the development of the beam tube, but cannot find it at present. As an aside, I may have said before (did not back-check) that "beam tube" is to me actually a misnomer. Or at least, it should refer to horizontal electron beams going through the screen without much getting "absorbed" there, rather than vertical beams formed by the beam confining electrodes.

Anyway, as I recall the inventors protected this mode by patent rights and thus one had the two directions, which never merged afterwards. The EL34 was apparently a successor to and derivative of the EL37, which also came from a line of "ancestors". Judging from the data and specifications, pentodes are not really that inferior to beam tetrodes, although I would believe there must be a price penalty - more electrodes to make and mount. The fact that the 6L6 generation can save on heater current (and a narrow heater) compared to the EL3x generation is something different. But I will look this up - might be on the internet somewhere.

I just saw from a flag that someone else has posted at this time, so perhaps greater elucidation. By the way, EL84 is a true pentode like EL34. The miniature beam equivalent there (not quite similar) is the 6AQ5, equivalent to the 6V6. (I have a display of "dissected" vacuum tubes, just for fun.)

Regards
 
Dave Cigna said:
To add a little more confusion, I have a bunch of 828 transmitting tubes. The RCA data sheet calls them "beam power amplifiers." They have a box structure on both ends of the plate which I presume to be the "beam plates" (though they don't look like the beam forming plates in a 6L6.) Looking in through the glass it looked liked the boxes were attached to the screen support rods. Huh?

The data sheet is from 1948 and the tubes are mid 60's. I suppose they might have changed the construction in later years, but I suspect they were always made that way.

-- Dave

Unforch, they played fast and loose with the terminology. I've seen the same thing concerning 813s. Sometimes these are referred to as "beam power amps", and it isn't. In fact, the 813 was specifically designed so that AM could be produced from it by means of suppressor grid modulation.

You can't do that with a BP VT.

Same deal with 828s: they're true pentodes.
 
ArtG said:
Hi Johan,
The question that this raises, in my mind is "why"? The beam tube was developed in the middle or late thirties as I understand, to overcome the undesirable "kink" that was a shortcoming in tetrodes or pentodes. This was recognized as a "breakthrough" in tube design, and was incorporated in the design of most power tubes, with the "KT" series of Genelex promoting the kinkless tetrode openly in a highly successful product. The EL-34 and EL-84 were designed years later, probably in the late '40's or early 50's, seemingly ignoring , the at least, supposed advantages of a beam tube. These, of course, went on to be very successful and highly popular tube types

Could it be that the kinked curves of the pentode, are in reality, not much of a drawback in the actual operation of the tubes. Or could it be that there are disadvantages in beam tube operation that are not widely known. Perhaps was it just an issue involving the patent of the beam tube that other companies' designers found a way to avoid with a product that seems to be an equal. I'd love to know! Ideas--anyone?

The term "kinkless tetrode" was probably thought up by some marketing weenie. It would be far more accurate to call it a "less kinky" tetrode. If you look at the enclosed plate curves, you can still see those kinks at very low Vpk's and Ip's. The phenomonon of secondary emission never goes away completely, but the beam construction has considerably flattened those kinks, and more importantly, pushed them way down there, far removed from any reasonable loadline.

In a true tetrode, the kinks are enormous, and spread out all over the plate characteristic, well into territory where you'd actually want to run a load. Not only do those kinks represent horrible local nonlinearities, the associated negative resistance characteristic is highly likely to cause RF oscillation. For audio, that is really, really bad. (Sometimes that was desireable, in that this phonomonon could be exploited to make UHF negative resistance oscillators.)

6BQ5 Plate Chars

@ulibub:
tetrodes and pentodes with "non-shadowed" screen (as EL34, 6L6 ....) need to be run in triode or ultralinear mode to sound really well - the "shadowed" types (all sweep tubes) even sound well in classic tetrode/pentode mode.

That is wrong. The 6L6 (and 807s, 6V6s, 6AQ5s) were all aligned grid VTs. These are some of the best sounding VTs.
 
Miles,
I think you may have attached the wrong item as the curve chart is labeled 6BQ6 which is a beam tube. But in comparing these curves to a 6BQ5, at a glance, the 6BQ5 shows flatter curves overall than the 6BQ6. (I know that these are not similar tubes). Nevertheless this, of course, backs up my questioning if the claimed advantages of beam forming plates were as great as the developers thought. Or, as you pointed out, was much of this just "hype". Thanks!
 
ArtG said:
Miles,
I think you may have attached the wrong item as the curve chart is labeled 6BQ6 which is a beam tube. But in comparing these curves to a 6BQ5, at a glance, the 6BQ5 shows flatter curves overall than the 6BQ6. (I know that these are not similar tubes). Nevertheless this, of course, backs up my questioning if the claimed advantages of beam forming plates were as great as the developers thought. Or, as you pointed out, was much of this just "hype". Thanks!

It's the right item, just hit the wrong key. The only advantage that I've seen anyone claim for BFPs is that it got around the patent on the use of the suppressor grid. If anyone claimed an advantage, it would be for bringing the control and screen grids into alignment so as to reduce wasteful DC screen power which doesn't contribute to AC output power, which was done first for the 6L6/807. Getting that screen voltage down certainly does seem to have contributed towards making these excellent audio PAs. (Other BP tetrodes seem to improve when the screen voltage is reduced. A 50C5 with Vsgsg= 90Vdc is a helluvalot more linear than when operated at the usual Q-Point: Vpp= Vsgsg= 110Vdc)

(Actually 6BQ6GTBs also sound quite good, and are in the 6V6 power class.)
 
The story we hear in Europe is that Philips invented the pentode, and the EL34 was the end of a long line of pentode output valves. GEC (i.e. Marconi-Osram) in the UK invented the beam tetrode, but then had trouble making it reliably so they handed it over to RCA (with whom they had a technology sharing agreement) who then developed it into a product. The truth is probably more complicated than that.

Some valves are made in both pentode and beam tetrode forms by different manufacturers, and a data sheet might not be clear. For example, the 6BZ6 remote cutoff RF valve is always described as a pentode but every one I have looks like a beam tetrode internally.
 
The term "kinkless tetrode" was probably thought up by some marketing weenie. It would be far more accurate to call it a "less kinky" tetrode. If you look at the enclosed plate curves, you can still see those kinks at very low Vpk's and Ip's. The phenomonon of secondary emission never goes away completely, but the beam construction has considerably flattened those kinks, and more importantly, pushed them way down there, far removed from any reasonable loadline.

In a true tetrode, the kinks are enormous, and spread out all over the plate characteristic, well into territory where you'd actually want to run a load. Not only do those kinks represent horrible local nonlinearities, the associated negative resistance characteristic is highly likely to cause RF oscillation. For audio, that is really, really bad. (Sometimes that was desireable, in that this phonomonon could be exploited to make UHF negative resistance oscillators.)

6BQ5 Plate Chars

@ulibub:


That is wrong. The 6L6 (and 807s, 6V6s, 6AQ5s) were all aligned grid VTs. These are some of the best sounding VTs.

The 6W6GT is very superior in sound quality.
The 6L6, 807, 6V6 and 6AQ5 have solid state sound in comparation to 6W6GT.

Aldovan
 
For example, the 6BZ6 remote cutoff RF valve is always described as a pentode but every one I have looks like a beam tetrode internally.

They're not beam formers. A lot of pentodes, especially the high frequency types like the 6BZ6 or 6BC6 or 12BY7A have suppressor "grids" in the form of a silver plate with an opening in it that's the same size as the plate itself. This is made possible due to the wide spacing between the plate and screen needed to keep the internal capacitances down. Given that, it really doesn't require all that much suppression to tame the secondary emission problem, and so you don't see the wire wound suppressor grids, such as the 6AU6 has.

Beam formers don't have openings, and are perpendicular to the plate, not parallel with it.
 
There is a wierd russian tube called 6E5P. It is a pure tetrode. What strikes me is the huge distance between g2 and anode. I guess this is done to reduce a kink.
I used this tube in my headphone amp (triode mode). Works well so far. Have not tried as an output tube yet, but there are some projects around on internet using 6E5P as an output device.
 

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I think the orientation of the 'suppressor' plates in a 6BZ6 is less important than their size and position - just on the edge of the electron stream. They will tend to concentrate the stream, yet will have little direct suppressor effect in the centre of the electron stream. I agree that the wide spacing to the anode reduces the need for secondary suppression.

The 'opening' in a conventional beam former is the gap between them.
 
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