The 6BE6 was intended for operation as a "pentagrid converter" in an AM radio. It is an oscillator (a triode comprising the cathode, G1 and the "screen grid" G2 which acts as the oscillator plate) and a mixer (a pentode made of the cathode, G3 the mixer grid, where the RF signal is applied, the screen grid G4, the suppressor G5, and the plate) in a single tube.
The 5th grid is tied internally to the cathode. Both of these "triodes" operate on the same electron stream which affords the frequency conversion action needed in a superhetrodyne radio receiver.
The triode curves published tie the "screen" G2 and G4 to the plate, but the "mixer grid" G3, is grounded, and the suppressor G5 is tied internally to the cathode. So our "super grid" tube still has two of the five grids tied to a low voltage potential, two tied to the plate, and one acting as the grid, biased up more or less as it is intended.
I did test this tube in Pete Millett's Engineers Amplifier, but found other tubes that did work better. The 6GU5 (4 grids) was the overall "winner" out of about 50 different tubes I tried in that amp, based on gain and distortion. It however was operated as a pentode with two suppressor grids.
I also remember experimenting with the 6BE6 and other pentagrid converters as audio amplifiers many years ago, but don't remember exactly how I had them connected up.
Another tidbit of useful information:
The more grids you have inside a tube, especially with signal applied to them, the more microphonic the tube will be. This again is a generalization observed primarily by extracting a bunch of gain out of a single tube for use as the input stage in a guitar amp.
The "gain" (Vout / Vin) of a triode amp can never be higher than the Mu of the triode. However the gain of a pentode tube is dependent on its Gm and the load impedance.
Making a near infinite load impedance with solid state technology and connecting it to a high Gm pentode can create an amplifier stage with gain approaching 10,000. Unfortunately a single stage with that much gain amplifies EVERYTHING, internal noise included. You will "hear" the sounds of the internal elements squeaking and creaking as the tube warms up and even the guitar cable becomes microphonic.
The 5th grid is tied internally to the cathode. Both of these "triodes" operate on the same electron stream which affords the frequency conversion action needed in a superhetrodyne radio receiver.
The triode curves published tie the "screen" G2 and G4 to the plate, but the "mixer grid" G3, is grounded, and the suppressor G5 is tied internally to the cathode. So our "super grid" tube still has two of the five grids tied to a low voltage potential, two tied to the plate, and one acting as the grid, biased up more or less as it is intended.
I did test this tube in Pete Millett's Engineers Amplifier, but found other tubes that did work better. The 6GU5 (4 grids) was the overall "winner" out of about 50 different tubes I tried in that amp, based on gain and distortion. It however was operated as a pentode with two suppressor grids.
I also remember experimenting with the 6BE6 and other pentagrid converters as audio amplifiers many years ago, but don't remember exactly how I had them connected up.
Another tidbit of useful information:
The more grids you have inside a tube, especially with signal applied to them, the more microphonic the tube will be. This again is a generalization observed primarily by extracting a bunch of gain out of a single tube for use as the input stage in a guitar amp.
The "gain" (Vout / Vin) of a triode amp can never be higher than the Mu of the triode. However the gain of a pentode tube is dependent on its Gm and the load impedance.
Making a near infinite load impedance with solid state technology and connecting it to a high Gm pentode can create an amplifier stage with gain approaching 10,000. Unfortunately a single stage with that much gain amplifies EVERYTHING, internal noise included. You will "hear" the sounds of the internal elements squeaking and creaking as the tube warms up and even the guitar cable becomes microphonic.
Connecting all three grids of a power pentode together is likely to lead to one of the following problems:
1. if the 'grid' is negative, or insufficiently positive, then little anode current will flow - this is because the valve needs a significantly positive g2 to conduct.
2. if the 'grid' is sufficiently positive for conduction then g1 will probably overheat - this is because in most cases it is not designed to take much current.
You can produce something like a 'Class B zero bias triode' by connecting g2 and g2, then connecting g1 to these via a resistor (maybe 22-47k). The resistor limits the current in g1. Input drive is connected to g2+g3 - but of course you need to provide drive current as well as voltage.
1. if the 'grid' is negative, or insufficiently positive, then little anode current will flow - this is because the valve needs a significantly positive g2 to conduct.
2. if the 'grid' is sufficiently positive for conduction then g1 will probably overheat - this is because in most cases it is not designed to take much current.
You can produce something like a 'Class B zero bias triode' by connecting g2 and g2, then connecting g1 to these via a resistor (maybe 22-47k). The resistor limits the current in g1. Input drive is connected to g2+g3 - but of course you need to provide drive current as well as voltage.
For your possible interest:
6BE6 also used for tremelo in a Git Amp.
http://www.ozvalveamps.org/playmaster/pm102circuit20090624142858_00002dlch.jpg
Cheers,
Ian
6BE6 also used for tremelo in a Git Amp.
http://www.ozvalveamps.org/playmaster/pm102circuit20090624142858_00002dlch.jpg
Cheers,
Ian
No problems. Just a "right handed triode", with pentode -like curves, or so called "high Mu mode".
In a "left triode" connections it is better to connect both G2 and G3 to anode. G3 in such case almost adds nothing, just a little bit, since it does not sit between high potentials.
I am right now at the stage experimenting with EL34 UL vs. Triode. As a first step I did:
- Connect G2 through a 1uf4007 and a 100 Ohm resistor to the Anode (Kevin Carter-Style)
- Connect G3 to the cathode
- Raised the fixed bias to about the same current/voltage as with UL
- raised the voltage swing of the input signal to about the same level
The GOOD: What I get is a more natural, smoother musical experience compare to UL, a deeper soundstage and smaller actors (UL bloats, makes bigger).
The BAD: There is a mellow, lush, less transparent, less detail presentation with very clear information losses compared to UL.
I have read that there is an explanation and a cure for this behavior, will build this next weekend: Pentodes & Tedrodes to Triodes
What I found surprising: He suggests to give -5V to g3 from the bias supply...and this shall make g3 unvisible / bring the EL34 more to a real triode...
...can someone give this statement some explanations ?
I would have thought either:
- g3 to anode like g2 to anode gives more triode like behavior or
- g3 to cathode /0V is actually good enough to control the electrons bouncing back
- or a somewhat lowered HV at the grids like Anode=380, G3= 370, G2=350...
....but -5V at g3 ??? Worth the trouble ?
- Connect G2 through a 1uf4007 and a 100 Ohm resistor to the Anode (Kevin Carter-Style)
- Connect G3 to the cathode
- Raised the fixed bias to about the same current/voltage as with UL
- raised the voltage swing of the input signal to about the same level
The GOOD: What I get is a more natural, smoother musical experience compare to UL, a deeper soundstage and smaller actors (UL bloats, makes bigger).
The BAD: There is a mellow, lush, less transparent, less detail presentation with very clear information losses compared to UL.
I have read that there is an explanation and a cure for this behavior, will build this next weekend: Pentodes & Tedrodes to Triodes
What I found surprising: He suggests to give -5V to g3 from the bias supply...and this shall make g3 unvisible / bring the EL34 more to a real triode...
...can someone give this statement some explanations ?
I would have thought either:
- g3 to anode like g2 to anode gives more triode like behavior or
- g3 to cathode /0V is actually good enough to control the electrons bouncing back
- or a somewhat lowered HV at the grids like Anode=380, G3= 370, G2=350...
....but -5V at g3 ??? Worth the trouble ?
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Ok, I am continue reading more articles and try to extend my understanding...and will for sure build the different scenarios and listen...
If I understand correctly, the reason why g3 still would be correct on ground may be :
"Secondary emission can be undesirable such as in the tetrode thermionic valve (tube). In this instance the positively charged screen grid can accelerate the electron stream sufficiently to cause secondary emission at the anode (plate). This can give rise to excessive screen grid current."
Source:
Secondary emission - Wikiwand
So, if we connect g2 to the anode, g2 still is accelerating electrons which will bounce in to the anode with high energy and will generate secondary emission. As g2 is near anode potential, we have a second thing which is positive, additional to the anode.
And this means we have secondary emission, no ?
"When an energetic, fast-moving electron hits a metal surface, the impact dislodges some of the other electrons and causes them to be emitted. An electron arriving at a 250V plate has an energy of 250eV, whereas the metal’s work function means that only around 4eV is required for an electron to be emitted. This phenomenon is called secondary emission, and occurs each and every time an energetic electron arrives. Under normal conditions, the plate is the most positive thing around, and these secondary electrons are simply attracted back to the plate where, having only a little energy, they are re-absorbed without provoking any further emission. This is benign and has no effect on the electrical operation of the tube, and is indeed unmeasurable.
Secondary emission only becomes a problem when the emitting surface is not the most positive thing nearby. In this case the emitted electrons are captured by the electric field and form a secondary current between the two electrodes. For example, if the grid of a triode is made positive rather than negative, and more positive than the plate, so that electrons flow directly to it, then the secondary electrons will be attracted to the grid, causing a flow of current away from the plate.
Secondary emission - or more accurately, secondary current flow - is almost never a good thing. It occurs unavoidably in the tetrode (as described below) and is the principal reason why tetrodes have been replaced by pentodes. It was exploited in photomultiplier tubes, as a way to multiply a very feeble initial current. There was also a tube in the late 1930s, the Philips’ EFP60, which used secondary emission from a target electrode as a way to increase the Gm, but it proved difficult to build predictably and was not successful. The problem is that although secondary emission can be measured for a particular metal, and in principle allowed for, in practice it is heavily affected by surface contaminants and the like. It cannot therefore be used as a reliable element of a tube’s operation."
Source: Tubes 201 - How Vacuum Tubes Really Work
Having written this, I guess the key question will be, how much lower the g2-potential needs to be that electrons would naturally bounce back to anode rather than being attracted by g2...which raises the question:
Why not simply neutralize g2 and g3 by grounding them altogether ? Than no secondary emission is possible at all...and is this not than the most Triode-like-Scenario of all ?.
By the way: I found in the El34 Mullard data sheet that the Power_max goes up from 25W to 30W when connecting g2 to Anode...this is some good news, so I will raise current further in triode mode.
If I understand correctly, the reason why g3 still would be correct on ground may be :
"Secondary emission can be undesirable such as in the tetrode thermionic valve (tube). In this instance the positively charged screen grid can accelerate the electron stream sufficiently to cause secondary emission at the anode (plate). This can give rise to excessive screen grid current."
Source:
Secondary emission - Wikiwand
So, if we connect g2 to the anode, g2 still is accelerating electrons which will bounce in to the anode with high energy and will generate secondary emission. As g2 is near anode potential, we have a second thing which is positive, additional to the anode.
And this means we have secondary emission, no ?
"When an energetic, fast-moving electron hits a metal surface, the impact dislodges some of the other electrons and causes them to be emitted. An electron arriving at a 250V plate has an energy of 250eV, whereas the metal’s work function means that only around 4eV is required for an electron to be emitted. This phenomenon is called secondary emission, and occurs each and every time an energetic electron arrives. Under normal conditions, the plate is the most positive thing around, and these secondary electrons are simply attracted back to the plate where, having only a little energy, they are re-absorbed without provoking any further emission. This is benign and has no effect on the electrical operation of the tube, and is indeed unmeasurable.
Secondary emission only becomes a problem when the emitting surface is not the most positive thing nearby. In this case the emitted electrons are captured by the electric field and form a secondary current between the two electrodes. For example, if the grid of a triode is made positive rather than negative, and more positive than the plate, so that electrons flow directly to it, then the secondary electrons will be attracted to the grid, causing a flow of current away from the plate.
Secondary emission - or more accurately, secondary current flow - is almost never a good thing. It occurs unavoidably in the tetrode (as described below) and is the principal reason why tetrodes have been replaced by pentodes. It was exploited in photomultiplier tubes, as a way to multiply a very feeble initial current. There was also a tube in the late 1930s, the Philips’ EFP60, which used secondary emission from a target electrode as a way to increase the Gm, but it proved difficult to build predictably and was not successful. The problem is that although secondary emission can be measured for a particular metal, and in principle allowed for, in practice it is heavily affected by surface contaminants and the like. It cannot therefore be used as a reliable element of a tube’s operation."
Source: Tubes 201 - How Vacuum Tubes Really Work
Having written this, I guess the key question will be, how much lower the g2-potential needs to be that electrons would naturally bounce back to anode rather than being attracted by g2...which raises the question:
Why not simply neutralize g2 and g3 by grounding them altogether ? Than no secondary emission is possible at all...and is this not than the most Triode-like-Scenario of all ?.
By the way: I found in the El34 Mullard data sheet that the Power_max goes up from 25W to 30W when connecting g2 to Anode...this is some good news, so I will raise current further in triode mode.
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Confirmed...I am just sitting in front of my utracer and measuring the theory.
Basically No Anode current when g2 is connected to cathode or not connected at all...which is interesting. I can see that if g2 is a counterforce for electrons that this might be an issue and reduce current, but when you do not give g2 any potential, the tube simply does not work either ??? So without the accelaration of g2 as a kind of speed booster for the little electrons the thing would not work ??? OK...this reduces the chance that a penthode will ever sound like a real triode, I fear.
With G3 connected to the cathode I get at 350V Anode/-25V grid:
- 69.25mA
- 1.04K
-9.6ma/V
- mu 10
With g3 at Anode:
- 86.4mA
- 890ohm
- 10.6ma/V
- mu of 9
I am just not sure if we really do not care (soundwise) if the electrons belong to the anode or g2 if you look at the physical layout of such a tube: Unbenanntes Dokument
g2 is very near g1 and anode is faaaaaaaaar away from both.
Nevertheless, I connected g3 originally to the cathode and I am missing resolution / detail now (compared to UL)...but I guess coming back to my original question WHY did the author of the article Pentodes & Tedrodes to Triodes suggest not only to ground g3, but even give it -5V simply means he wants to kick back secondary emission electrons back to the anode earlier, more rigid and more narrow to the surface of the anode, he has basically strengthen the force of g3 as a negative "magnet" kicking back the negative elementary "magnets" (electrons).
Basically No Anode current when g2 is connected to cathode or not connected at all...which is interesting. I can see that if g2 is a counterforce for electrons that this might be an issue and reduce current, but when you do not give g2 any potential, the tube simply does not work either ??? So without the accelaration of g2 as a kind of speed booster for the little electrons the thing would not work ??? OK...this reduces the chance that a penthode will ever sound like a real triode, I fear.
With G3 connected to the cathode I get at 350V Anode/-25V grid:
- 69.25mA
- 1.04K
-9.6ma/V
- mu 10
With g3 at Anode:
- 86.4mA
- 890ohm
- 10.6ma/V
- mu of 9
I am just not sure if we really do not care (soundwise) if the electrons belong to the anode or g2 if you look at the physical layout of such a tube: Unbenanntes Dokument
g2 is very near g1 and anode is faaaaaaaaar away from both.
Nevertheless, I connected g3 originally to the cathode and I am missing resolution / detail now (compared to UL)...but I guess coming back to my original question WHY did the author of the article Pentodes & Tedrodes to Triodes suggest not only to ground g3, but even give it -5V simply means he wants to kick back secondary emission electrons back to the anode earlier, more rigid and more narrow to the surface of the anode, he has basically strengthen the force of g3 as a negative "magnet" kicking back the negative elementary "magnets" (electrons).
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Ok, I did some listening test where I have build an external socket /adapter where I can always listen and compare by switching between A and B. I am not yet completely finished and all my observations are obviously completely subjective, so here we go:
- 100R and diode and g3 on cathode: Mellow, lush a bit fuzzy, warm, boring
- 1000R g3 on cathode: A bit better defined, but stiffer and less dynamic, even though bias did not change a lot (2mA)
- 3000R and g3 on cathode: feels like too stiff and loss of dynamics, but 3D is best, but musical flow / tone a bit too neutral
- Now: 100R/diode with g3 connected to g2: Very, very nice !!! All the mellow, lush sound is gone, the fuzziness is nearly completely away, the tube focusses very well, the space is deep, transients are good and very dynamic...best ? Maybe...but for my taste beaten by:
- 100R/Diode and no connection of g3 at all. Just leave it open. This gives razor sharp imaging and nearly nothing irritates the sound, I guess this is triode sound now.
The last idea was not mine: I found an article in l'audiophile from 1992 where this connection was suggested and I guess the guy knew what he was talking about: Document sans titre
I will continue to play with some alternatives, resistor values and as well the suggested zener string, but I believe that the more the EL34 is allowed to work just like a triode, so no irritating g3 at work at all, maybe best. It transformered my amp for sure, much better resolution and more musicality at the same time. Good deal.
- 100R and diode and g3 on cathode: Mellow, lush a bit fuzzy, warm, boring
- 1000R g3 on cathode: A bit better defined, but stiffer and less dynamic, even though bias did not change a lot (2mA)
- 3000R and g3 on cathode: feels like too stiff and loss of dynamics, but 3D is best, but musical flow / tone a bit too neutral
- Now: 100R/diode with g3 connected to g2: Very, very nice !!! All the mellow, lush sound is gone, the fuzziness is nearly completely away, the tube focusses very well, the space is deep, transients are good and very dynamic...best ? Maybe...but for my taste beaten by:
- 100R/Diode and no connection of g3 at all. Just leave it open. This gives razor sharp imaging and nearly nothing irritates the sound, I guess this is triode sound now.
The last idea was not mine: I found an article in l'audiophile from 1992 where this connection was suggested and I guess the guy knew what he was talking about: Document sans titre
I will continue to play with some alternatives, resistor values and as well the suggested zener string, but I believe that the more the EL34 is allowed to work just like a triode, so no irritating g3 at work at all, maybe best. It transformered my amp for sure, much better resolution and more musicality at the same time. Good deal.
I should perhaps start with an apology if this is duplication; I have not read the whole thread again. But in reaction to Blitz's interesting posts -
What happens in triode connection of a beam power tube, where there is no g3 - the g3 function being performed by an 'electron cloud/screen' at the right position between anode and g2? Will this automatically adapt itself to duplicate an open g3? (It is a little difficult to try guess from graphs, as I do not find a beam tube and power pentode close enough in characteristics to be able to reach a conclusion. Nothing seems to be different in the triode graphs of relevant tubes - but then there are no pentode-as-triode graphs leaving g3 open.)
It might lead to a comparison between true pentodes and beam tubes as triodes - but let me not start that now. One thing at a time!
What happens in triode connection of a beam power tube, where there is no g3 - the g3 function being performed by an 'electron cloud/screen' at the right position between anode and g2? Will this automatically adapt itself to duplicate an open g3? (It is a little difficult to try guess from graphs, as I do not find a beam tube and power pentode close enough in characteristics to be able to reach a conclusion. Nothing seems to be different in the triode graphs of relevant tubes - but then there are no pentode-as-triode graphs leaving g3 open.)
It might lead to a comparison between true pentodes and beam tubes as triodes - but let me not start that now. One thing at a time!
Hi Johan,
Good question, I asked myself the same. My current understanding based on the experiments witht the El34, which is clearly NOT a beam power tube is: G3 has a measureable impact in what will happen with the electrons, but more important: You can hear the action of g3 quiet significantly, my guesstimate is:
- g3 at cathode brings the fuzziness and warm character because electrons get impacted negatively in their travel from g2 to anode. G3 works with its negative charge to them like a quirl. It is a big diffusor when electrons try to fly between the g3 to the anode, they get into the gravity of g3 being bounced around as g3 is as well negative (as the negative loaded electrons are), one electron is bouncing to the next until the applied force of g3 is absorpt, so we have a big billiard game.
- g3 being positive: Better as now g3 tries to pull electrons through and accelerate them further on their way to anode, emission is highest. But doing this, it applies as well a force to the electrons and changes their "flight characteristics". So maybe not a complete chaotic billard game like above as it does not push back electrons, but pulls them faster to the anode, but still a bit of a diffusor.
- No g3 connection: Electrons which have passed g2 can fly a straight line. Lets not forget that g2 still is an element which is not needed in a real triode to make the electron flow work between cathode and anode. So, maybe a real triode has still the most natural and straight electron flow as a g2 is as well a mini squirl when pulling electrons through, but looking how tightly woven g2 actually is in an EL34, it might be not that bad.
Now, I would hesitate to predict how this will sound with a beam power tube. My guestimate would be is: We want the beam plate to work like a black hole or a big rubber: What flies onto them as electrons shall simply be captured and stay there and it should not be a big "magnetic " force again which diffuses the main electron flow/work between g2 and anode like a squirl. So, it shall form a beam, but electrons within the main beam itself should allow electron to fly a straight line like in a real triode without being confused by any force of the g3-plates.
I would have thought that grounding the beam power plates would do this. But not shure if this is true as it all depends on the size and distance betwenn the plates, the anode and g2....(...or if the plates are at anode potential, they suck the electrons into them better). I guess it depends on the physical layout of the tube. I would try both an listen carefully.
Good question, I asked myself the same. My current understanding based on the experiments witht the El34, which is clearly NOT a beam power tube is: G3 has a measureable impact in what will happen with the electrons, but more important: You can hear the action of g3 quiet significantly, my guesstimate is:
- g3 at cathode brings the fuzziness and warm character because electrons get impacted negatively in their travel from g2 to anode. G3 works with its negative charge to them like a quirl. It is a big diffusor when electrons try to fly between the g3 to the anode, they get into the gravity of g3 being bounced around as g3 is as well negative (as the negative loaded electrons are), one electron is bouncing to the next until the applied force of g3 is absorpt, so we have a big billiard game.
- g3 being positive: Better as now g3 tries to pull electrons through and accelerate them further on their way to anode, emission is highest. But doing this, it applies as well a force to the electrons and changes their "flight characteristics". So maybe not a complete chaotic billard game like above as it does not push back electrons, but pulls them faster to the anode, but still a bit of a diffusor.
- No g3 connection: Electrons which have passed g2 can fly a straight line. Lets not forget that g2 still is an element which is not needed in a real triode to make the electron flow work between cathode and anode. So, maybe a real triode has still the most natural and straight electron flow as a g2 is as well a mini squirl when pulling electrons through, but looking how tightly woven g2 actually is in an EL34, it might be not that bad.
Now, I would hesitate to predict how this will sound with a beam power tube. My guestimate would be is: We want the beam plate to work like a black hole or a big rubber: What flies onto them as electrons shall simply be captured and stay there and it should not be a big "magnetic " force again which diffuses the main electron flow/work between g2 and anode like a squirl. So, it shall form a beam, but electrons within the main beam itself should allow electron to fly a straight line like in a real triode without being confused by any force of the g3-plates.
I would have thought that grounding the beam power plates would do this. But not shure if this is true as it all depends on the size and distance betwenn the plates, the anode and g2....(...or if the plates are at anode potential, they suck the electrons into them better). I guess it depends on the physical layout of the tube. I would try both an listen carefully.
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...obviously you can do this only with beam power pentodes where you can decide what you want to do with the plates, like the 814a. For the El34/Kt88 etc family I would just compare those beam power tetrodes triode strapped with an El34 triode strapped and see what sounds best. Maybe I will do this next.
Ok, I plugged in a set of Psvane Kt-88, Treasurary Series (the grey ones) and compared, both at 345/60mA:
- Overall, the characteristic is like the EL34 when g3 is not connected.
- Subjectively, little less dynamic and a little haze, less drama. I prefer the EL34.
BUT:
- The Psvane are new out of the box. Chinese vs. Old stock from Philips from the 60s.
- My driver is maxed out with the EL34 already and we have here now mu of 8 vs. mu of 10.5, no wonder that the EL34 seems to be more dynamic. Maybe the haze impression would go away when the KT88 would get a larger voltage swing.
So, the good news is: Beam Tetrodes seem to work quiet well as well, better than pentodes with g3 at cathode and maybe not as good as if you would not manipulate the electron stream at all, like g3 not connected pentodes or real triodes. My 5ct.
By the way: Intersting reading: 813 Beam Power Tube and Data Sheets
...basically the beam was the trick why g3 was not needed anymore as the electron field gets manipulated so that secondary emission will be killed by the anode itself. So you manipulate the electron flow to simulate what happens in a triode anyhow.
- Overall, the characteristic is like the EL34 when g3 is not connected.
- Subjectively, little less dynamic and a little haze, less drama. I prefer the EL34.
BUT:
- The Psvane are new out of the box. Chinese vs. Old stock from Philips from the 60s.
- My driver is maxed out with the EL34 already and we have here now mu of 8 vs. mu of 10.5, no wonder that the EL34 seems to be more dynamic. Maybe the haze impression would go away when the KT88 would get a larger voltage swing.
So, the good news is: Beam Tetrodes seem to work quiet well as well, better than pentodes with g3 at cathode and maybe not as good as if you would not manipulate the electron stream at all, like g3 not connected pentodes or real triodes. My 5ct.
By the way: Intersting reading: 813 Beam Power Tube and Data Sheets
...basically the beam was the trick why g3 was not needed anymore as the electron field gets manipulated so that secondary emission will be killed by the anode itself. So you manipulate the electron flow to simulate what happens in a triode anyhow.
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