I've been playing with the 826, 327A (big Eimac tube), VT-127A, and 100TH. They're all great and look super cool but the 211 is better.
John Harper commented on electrode geometry, back in 2003 (here: https://www.john-a-harper.com/tubes201/):
-
There are three common tube structures, shown in Figure 5. 5a shows the cylindrical structure, in which the cathode, the grid and the plate are made as three concentric cylinders. This structure has been little used since the 1930s, presumably because it is more expensive to manufacture. (An exception is rectifier diodes, which are almost invariably built like this). It does however have some theoretical properties that may account for the sonic reputation of the classic model 27 and 76 triodes, which are built this way.
-
All generalizations have exceptions.
"little used since the 1930s" ? ?
Just look at some old 6SN7s that have round plates.
Then look at some 6SN7s that have rectangular plates.
Consider all those EiMac Tetrodes with cylindrical cathodes, cylindrical bird cage g1, cylindrical bird cage g2, and cylindrical plates.
I built an amplifier with an all cylindrical output tube . . . The EiMac 4-65A tetrode.
The models in Post # 142, they leave out the unique structure of many RF planar triodes, such as the 416A, and some GE deep space triodes.
All surfaces are flat and parallel, with circular diameters/areas.
"little used since the 1930s" ? ?
Just look at some old 6SN7s that have round plates.
Then look at some 6SN7s that have rectangular plates.
Consider all those EiMac Tetrodes with cylindrical cathodes, cylindrical bird cage g1, cylindrical bird cage g2, and cylindrical plates.
I built an amplifier with an all cylindrical output tube . . . The EiMac 4-65A tetrode.
The models in Post # 142, they leave out the unique structure of many RF planar triodes, such as the 416A, and some GE deep space triodes.
All surfaces are flat and parallel, with circular diameters/areas.
Tony Techson,
Your Post # 135 and Post 136 . . .
True!
Here is the rest of the story:
Single ended operation:
Beam Power tubes, such as a single 829B, 832A, and 5894:
A single output tube can only do Stereo if it is connected in Beam Power mode.
The screen is Common to both Beam Power sections . . .
So a single tube can not be used for Stereo Ultra Linear; or Stereo Triode Wired modes.
You will need 2 output tubes to do Stereo UL, or Stereo "Triode" modes.
Push Pull operation:
Beam Power tubes, such as a single 829B, 832A, and 5894:
A single output tube can only do push pull if it is connected in Beam Power mode.
The screen is Common to both Beam Power sections . . .
So a single tube can not be used for push pull Ultra Linear; or push pull Triode Wired modes.
You will need 2 output tubes to do Stereo push pull UL, or Stereo push pull "Triode" modes.
Your Post # 135 and Post 136 . . .
True!
Here is the rest of the story:
Single ended operation:
Beam Power tubes, such as a single 829B, 832A, and 5894:
A single output tube can only do Stereo if it is connected in Beam Power mode.
The screen is Common to both Beam Power sections . . .
So a single tube can not be used for Stereo Ultra Linear; or Stereo Triode Wired modes.
You will need 2 output tubes to do Stereo UL, or Stereo "Triode" modes.
Push Pull operation:
Beam Power tubes, such as a single 829B, 832A, and 5894:
A single output tube can only do push pull if it is connected in Beam Power mode.
The screen is Common to both Beam Power sections . . .
So a single tube can not be used for push pull Ultra Linear; or push pull Triode Wired modes.
You will need 2 output tubes to do Stereo push pull UL, or Stereo push pull "Triode" modes.
Last edited:
12AH7, anyone tried it? Those plates are round alright. It's a low gain step sibling to the *SN7. Doesn't get a lot of love from the users of this forum. Maybe it deserves another look?
And here’s a snippet from the Valve Museum talking about how flat construction can lead to lower distortion.
“
http://www.r-type.org/exhib/aai0598.htm
“
One source of distortion from a valve amplifier comes about from the variation in inter-electrode distances in conventional valve design. Planar construction has a flat cathode with the grid passing flat across it and the anode as a flat plate parallel with the grid and cathode. For disc seal valves this construction places the grid across the width of the valve and the grid connection is at that plane. See Lighthouse Valves for details. As this valve is not designed for UHF frequencies, or above, the construction is a double sided planar construction.”
http://www.r-type.org/exhib/aai0598.htm
The formulae for space-charge limited operation of tubes (specifically diodes) were derived on the assumption that fringing at the edges of the field could be neglected (Gewatowski and Watson, Principles of Electron Tubes, 1965, page 114). In practice, the spherical diode is the only arrangement without fringing. How much fringing will be determined mainly by geometry, so that concentric cylinders could be expected to exhibit less fringing than (say) an arrangement of flat rectangular cathode, grid and plate, all other things being equal.
The next question is whether fringing has any measurable effect on electrical/electronic performance and, if so, under what circumstances. Any data, anyone?
And, if such data exist, the next question is whether the differences in electrical performance relate to any audible phenomena?
The next question is whether fringing has any measurable effect on electrical/electronic performance and, if so, under what circumstances. Any data, anyone?
And, if such data exist, the next question is whether the differences in electrical performance relate to any audible phenomena?
Attachments
Only 6V6G have oval plates. Newer have a square plate section added on each end and do not sound good like the 6V6G.
Very different? How so? They're all low power transmitting triodes, and you can operate them however you want that makes sense to achieve the goal you are after. The operation parameters are what you make them.
Here is a 1W comparison between the VT-127A and the 211. Identical circuit, just adjusted the operating point of the output tube so that swing was symmetrical and inside max dissipation limits for each tube.
VT-127A inherently had a bunch of low-level high-order harmonics. Each sample had a different harmonic signature (the levels of the various harmonics differed in unique ways) but all had harmonics all the way out like this.
211 had almost no high-order effects like this.
The other cylindrical types I tested were somewhere in between these two types in quality.
I guess my point is that there is nothing inherently superior about a cylindrical plate structure. Each tube type (and sample) should be evaluated on its own merits.
Attachments
1. Build a tube that has the most perfectly formed circles:
Cathodes , control grids, screen grids#, suppressor grids@, and plates*.
Choose the tube elements above, appropriate to the tube type: * is Triode; * # is Tetrode; *#@ is Pentode.
If all those perfect circles are not exactly concentric, than the advantage of the circular elements is negated (imperfect concentric spacing makes electron paths unequal, rp, u, and Gm will vary across different parts of the same tube.
2. As to the Lighhouse 416A, I think that "fringing" is at a minimum.
Just look at the genius design, you may, or you may not, come to that conclusion.
Attention to details counts.
Cathodes , control grids, screen grids#, suppressor grids@, and plates*.
Choose the tube elements above, appropriate to the tube type: * is Triode; * # is Tetrode; *#@ is Pentode.
If all those perfect circles are not exactly concentric, than the advantage of the circular elements is negated (imperfect concentric spacing makes electron paths unequal, rp, u, and Gm will vary across different parts of the same tube.
2. As to the Lighhouse 416A, I think that "fringing" is at a minimum.
Just look at the genius design, you may, or you may not, come to that conclusion.
Attention to details counts.
Last edited:
Surely the best way to compare round plate vs. not is to use a tube type that offers both plate structures. I have access to two types, the *sn7/6f8g and the *j5gt. Tung Sol and National Union made *sn76f8g with round plates. Both are fine tubes but I do prefer others in both of my amps and my preamp. My favorites are the GEC b36 and Sylvania 14n7 but also like GE branded 12sx7 and a few other Sylvania ones as well. Incidentally, I am convinced the 14n7 is equivalent to the metal based Sylvania 6sn7w. The plates are identical and they even have the same copper support rod.
Sylvania made a round plate 6/12j5gt. Once again, it's a quality tube but I prefer a fair number of flat plate 6/12j5gt including ones from Tung Sol, GEC, and Raytheon.
I guess it's possible for round plates to impart some sort of technical benefit even if you can't hear it. I do wonder what the point of that would be. As was said above, if there is an advantage to round plates it is only one of many considerations that go into the sound we get from tubes. I do think it is telling that as tube technology improved round plates disappeared from audio use. Because so few tubes made specifically for audio (6l6, 6v6, el34, kt88, etc.) were made with round plates I assume that if there are advantages they were swamped by something else. I'm willing to believe that sound quality (as opposed to low distortion, not the same thing) wasn't necessarily the end all be all of the design of the 6sn7 because it was used in so many other capacities. Maybe there were manufacturing or longevity benefits for going to flat plates and whatever sound differences that happened because of that were acceptable.
Whatever the case, round plate tubes are mostly relegated to early tube designs and these bespoke, super expensive ones. My experience with round plates doesn't point to an inherent advantage for them over other plate types.
Sylvania made a round plate 6/12j5gt. Once again, it's a quality tube but I prefer a fair number of flat plate 6/12j5gt including ones from Tung Sol, GEC, and Raytheon.
I guess it's possible for round plates to impart some sort of technical benefit even if you can't hear it. I do wonder what the point of that would be. As was said above, if there is an advantage to round plates it is only one of many considerations that go into the sound we get from tubes. I do think it is telling that as tube technology improved round plates disappeared from audio use. Because so few tubes made specifically for audio (6l6, 6v6, el34, kt88, etc.) were made with round plates I assume that if there are advantages they were swamped by something else. I'm willing to believe that sound quality (as opposed to low distortion, not the same thing) wasn't necessarily the end all be all of the design of the 6sn7 because it was used in so many other capacities. Maybe there were manufacturing or longevity benefits for going to flat plates and whatever sound differences that happened because of that were acceptable.
Whatever the case, round plate tubes are mostly relegated to early tube designs and these bespoke, super expensive ones. My experience with round plates doesn't point to an inherent advantage for them over other plate types.
Wondering if cylindrical plates result in more issues with microphonics.
I have noted a number of small signal cylindrical plate pentodes ring clearly and loudly when lightly tapped.
I have often wondered if the plate's shape was a contributing factor in the ringing.
Yes the grid frames may be what I am hearing but hard cylinders do ring nicely and this mades me wonder.
Anyone with a reference on this subject? The "old ones" must know all about this.