5842-417A Highest gm

Radio Age magazine editor Ed Lyon has asked me if I have ever seen a small-signal VHF/UHF receiving tube with more transconductance than the 5823/417B. At one point in his career he used these tubes as cathode followers to drive coaxial cables. The 417B designation is for Western Electric, and 5823 is US military.

The 5823 has a specified transconductance between 19mS and 29mS, for the Philips version.

The Raytheon version specifies 25mS at 150V, 25mA. mu=43

1mS/mA at 150V is not rare. The 12AX7 does that, but the size efficiency of the 5823 is much greater than the 12AX7.

This is extraordinarily high gm for a very small VHF tube. VHF receiving tubes in this class have gm no higher than 15mS.

The plate length of the 5823/417A is approximatelly one third of the length of the plate of the 12AX7, yet it has 25x more gm and current at comparable plate voltages.

The small size means that the capacitances will be modest (cga1.8pF, cgk=9pF) and make it possible to realize very high gm/C bandwidth. With an infinite load impedance the internal impedance of 43/25mS=1.7KOhms and the plate capacitance of 1.8pF gives a 500MHz bandwidth with a grounded grid stage, if parasitic inductance and capacitance effects could be controled.

Perhaps there are triodes with a faster transit time, but transit time is not specified.

Are there receiving triodes with a higher gm/C bandwidth?

Is there a higher performing UHF pencil or ceramic receiving tube?

The plate size in combination with the high gm of the 5823 also suggest extermelly small internal grid-cathode spacing.

I expect that these questions can be answered in different ways, depending on what parameters are taken as most important.

Best regards,

-Joe

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Your question involves many considerations, also including the internal geometry of the tube and the external circuitry. I can only add a little information about high gm planar tubes.

Planar tubes offered the most advanced designs for conventional UHF amplifiers and oscillators. In these devices the grid could be placed very, very close to the cathode surface and the electrodes were easily connected in a grounded grid configuration to external resonating cavities. Of course any planar type was designed to comply with specific customer requirements, including size, mounting, frequency range, gain, power dissipation and noise.

Low power planar triodes, first designed in STC and later improved in the so-called lighthouse, pencil and rocket types, eventually evolved in the General Electric planar cermet families. Some types, about the size of a nuvistor, were capable of operation at 7.5 GHz. Some, as the GE 7768, offered a transconductance between 40.000 and 60.000 micromhos.
 

Regards, Emilio

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Thank you very much for the response. I just got a few minutes to respond.

Did the 5842 have a wire ladder grid, or a planar mesh grid? Was it the fastest tube with a wire grid?

You mention STC. I presume you mean STC/Brimar English tube maker.

I took your leads on planar tubes, and found that I have a few in my collection that have gm up to 50mS. All of these are Cermet or other variants, with opaque construction.

A few examples:

D3A=7721 minuature glass RF pentode has 40mS and extensivelly specified at 100MHz. Pictures suggest wire grid construction.

A cermet 7588 with 45mS mu=175 for 500MHz RF amp service:

 

A metal/glass-seal 416B by Western Electric has 50mS mu=300 and 4GHz operating frequency. I own a couple of these. I hope to take a picture soon.

Best regards,

-Joe

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Dear Joe,

obviously I do not have the answers to the many questions you entered. I have only a superficial knowledge of the vacuum tubes, since solid-state devices were already replacing tubes when I was young.

The development of high frequency tubes started very early in many countries. The main guidelines were the reduction of the transit time and the reduction of parasitic parameters, as interelectrode capacitance and inductance of the leads. Close spacing between cathode and grid, to reduce transit time, also results in higher transconductance. It was soon evident that planar structures could have made possible to precisely position the grid very close to the cathode surface. I have a paper from B.J. Thompson and G.M. Rose of RCA, reprinted from Proc. of I.R.E., Dec.1933, where they describe some all-glass planar tubes of small dimension, capable of amplification at 600MHz. See fig. RCA_1, triode, and RCA_2, tetrode.

S25A, later CV16, was designed by Smyth and Foster of British STC early in 1941. It is believed to be the first disk-sealed UHF triode, followed shortly later by S28A or CV88, capable of operation up to 1GHz. More details can be found in ‘Metres to Microwaves’, from Callick.

416A was the Western Electric interpretation of the disk-sealed structure, designed after WWII for applications in microwave relay equipment. It was derived from the prototype shown in the fig. WE_1. Grid to anode spacing was 0.012 inch, grid to cathode was just 0.0006 inch or 0.015 mm; grid wire dia. was 0.00033 inch, while grid-wire spacing was 0.001”. The plate to cathode capacitance was as low as 0.012 pF. Of course, with electrodes so close to each other, the thermal expansion of the supporting materials limited the cathode temperature and the anode dissipation. The use of low thermal coefficient BeO ceramic spacers, as in the latest 416C and 416D version, made possible to increase the output power to 5 W at 4GHz, while raising the Gm to 65 mS.

Other planar structures, using ceramic spacers, were developed by Sylvania in the mid ‘950s, these also intended for improved reliability and automatic assembly. The exploded view of the ceramic body prototype SN-1724D is given in fig. Syl_1. The 7245, a premium quality 6J4 equivalent, is the only production tube I know that used this structure. But I never tried to investigate if other tubes of this family went in production with glass or ceramic body.

Tubes for television service evolved in a quasi-planar design around 1960. A frame grid surrounded the rectangular section cathode, the tiny grid wires being stretched between two side rods, close to the cathode surfaces. See fig. Phi_1. This structure was used for UHF, VHF and also for IF amplifiers, since it granted higher transconductance. The 5842 is of the frame-grid type, according to Sibley. I do not have mechanical details about it, but probably it has a ladder grid, like most of these tubes: the mesh is more tick and its distance from the cathode surface is irregular.

I hope that other people with specific experience would complete the answers to your questions.

Regards, Emilio

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• Pic. RCA_1 (22 KB)
• Pic. RCA_2 (39 KB)
• Pic. WE_1 (66 KB)
• Pic. Syl_1 (23 KB)
• Pic: Phi_1 (55 KB)

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Dear Emilio,

Thank you for the comprehensive answer. My curiosity exceeded my ability to ask good questions, however, your exposition gives a very clear outline of the history of these tubes. The illustrations are great too.

One very important clarification was the meaning of "planar". It refers to the flatened elements, arranged in a parallel fashion. This contrasts with older cylindrical concentric designs.

Another clarification is the meaning of "frame" grid. The frame grid has it's wires wrapped around a rectangular frame for good control of tension, as opposed to being wrapped around two posts supported by mica discs, like a ladder.

Another type of tube structure that is unique to Russian subminiature tubes is a structure made entirely of parallel rods. One example is the 1J17B subminiature. The cathode is a single filament, the control grid is two flattened rods on either side of the filamentary cathode, the screen grid is made with  two pairs of additional rods, more rods for the suppressor, and finally two anode rods in diametric opposition to catche the two opposing sheet beams emanating from the filamentary cathode. They have about 1mS of transconductance, but much faster transition times that the equivalent western 5678. I measured 1.8ns delay for a 2V step delay through the 1J17B, and 3ns delay through a 5678. Sub-ns rise and fall times for the 1J17B were sharper than the faster delay time might suggest. I have not seen a Western filamentary tube that could match the performance of the Russian rod tubes.

I was also surprised to hear that ladder grids, as opposed to mesh grids, were very common with these tubes. And the 8micron thick ladder grid was spaced only 15microns from the Cathode. Pretty amazing. ( I am using microns=0.001mm because I work with microns in my IC design work. I think this is the first time I ever referred to tubes and transistors in the same dimensional scale.)

This evening I got a chance to photgraph 3 planar tubes. The top one is a 5675 pencil, the Western Electric 416B is to the lower right, and a 7820 nuvistor-like tube completes the set. I have not been able to find any info on the 7820.

Best regards,

-Joe

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Dear Joe,

As you can see, I know very little about European tubes and about the many types developed for television applications.

I do not know the 1J17B and I am not able to read its data sheet and understand its use. Nevertheless the parallel-rod grid structure, usually referred to as ‘squirrel-cage’ grid, is quite common in VHF and UHF power transmitting tubes, glass and ceramic ones, including I believe the 7820 of your picture. In the picture ‘Grids’ below from RCA TT-5 manual, there are two examples of squirrel-cage grid (c).

The fast pulse response is usually related to the emission characteristics of the cathode. In tubes designed for pulsed operation, the cathode surface is the greatest possible. Its temperature is raised to have plenty of electrons all around the cathode, ready to be attracted by the plate as soon as a positive pulse on the control grid drives the tube out of the interdiction. Pulse rated cathodes were capable of delivering up to 30A per square cm; receiving tubes operated with less than 0.1A per sq. cm. Fairly good devices for fast rising pulses were the transmitting power tubes. Tektronix in its 519 oscilloscope used a 4CX250F as linear sawtooth generator operating down to 2ns/division in the fastest speed. You should compare the 1J17B with other UHF transmitting tubes.

About the fascinating grid structures of high frequency tubes, GE developed very refined designs in its wide range of microwave planar tubes. Grid tiny wires were tied to the grid ring-shaped frame. But one or two layers of supporting wires or legs were often added to keep the grid wires properly aligned. Fig. GE_1 shows two typical grid structures, one with the tiny wires just held by a brazed washer, the second one with some large orthogonal support wires. In this case, the grid frame was mounted with the support wires facing the anode and the grid wires close to the cathode surface. Fig. GE_2 shows an improved design with electro-etched frames. Here the diagonal grid wires ran diagonally between the vertical legs of the main frame and the smaller horizontal support bars etched in a counter-plate. The large vertical bars were nested into slots in the cathode, so that grid wires ran very close to the cathode active surface. Triodes with large cathode surface, very high dissipation capabilities and transconductance as high as 1 mho were built with this structure.

Best regards, Emilio
 

 

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• GE_2 (87 KB)

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Dear Emilio, thank you again for further expanding my understanding of VHF/UHF tubes. Your modesty claiming that you don't know very much about these tubes, indicates to me that you know that there is much more to know about these tubes. Your knowledge, however, is quite impressive!

Recently, I wrote an article about Russian subminiature filamentary battery Pentodes with rods used for all the grids and anode. Ludwell Sibley is currently editing my article for publication in the Tube Collector's Association newsletter. In the article I share voltage sweep curves showing impressive voltage gain linearity and also faster step response than comparable western subminiature battery tubes.

The pictures you posted of various grid structures show structures that I had not seen. I  was already familiar with sqirrel cage grids in transmitting tubes, but the use of individual rods for all the tube elements is quite unique, and has unique benefits.

The following photo shows some detail of 1J29B filamentary Russian Subminiature Pentode that I sacrificed to remove the glass.

 

The following photo shows the rod arrangement to implement G1, G2, G3, Anode, and shield rods S. The filament is represented by the two smallest white dots at the center and consists of two segments wire beyond the micas to the rods labeled F1-F3. The dashed lines represent electron flow.

These tubes were used exclusively in Russian Military radios. The original Russian data sheet indicates that these tubes were intended for 60MHz service, however my step response measurements indicate that their 1.8ns delay time will provide useful service into the VHF range. Aside from the very good gain linearity and very fast transit time of these tubes, they perform similarly in power and gain to the 5678 and their brethren.

Your comments about the importance of cathode emission for fast pulse response help me understand the sub-ns rise time I was seeing after the 1.8ns delay in the 2V step response of the 1J17B. The filamentary cathode is a thin capilary, so electrons leave the cathode synchronously when G1 steps from -2V to 0V. This is also the case with a planar tube with very flat cathode and grid construction and very good parallel allignment.

The parallel G1 rod alignment also favors a more direct control of electron flow because there are no grid wires in the way for the electrons to move around, and no problems of "inselbildung" that dispose the electron cloud at various distances from the grid.

Another characteristic worthy of note in the step response was the very well matched rise and fall delays and the very well matched rise and fall times in the 1J17B step response. Most other VHF unipotental cathode tubes I tested (6HQ5, 6HA5, 6EV5, 6EA5, among others) showed noticeable assimmetry in the rise and fall times of their step response, with the turn-on response to a rising grid step being slower. This is to be expected from the cylindrical shape of the cathode, with electrons departing the cathode at various distances from the grid. This further illustrates the speed benefit of planar structures.

I just made a quick calculated estimate of the current density at the surface of the 1J17B cathode, and came up with 150mA/cm²; very much in line with the receiving tube density you mention above. I estimated the filament at 0.025mm thickness from a close up photo, and multiplied the circumference by 25mm length. I used 3mA for the current, which is what flows with zero G1 bias and 45V at G2 and Anode.

I have a lot more material in the article about the Russian subminiature rod tubes, but this discussion has advanced my understanding of how these tubes work with regard to the importance of the filamentary cathode and rod control grid.

Amazing to hear about 30A/cm² in high power pulsed transmitting tubes. This is also the first time I have ever heard of a 1S (mho) tube. Was this a monster transmitting tube several feet long, or somthing not much bigger than a fist?

I knew about the TEK519, but had not looked closely into it's circuitry. I found the 4CX250F 2ns/div generator tube on page 98 of the DJVU file available at BAMA.

http://bama.edebris.com/manuals/tek/519/

Surprising to see the 4CX250F followed by 6DJ8 follower a 6DJ8 bootstrap and another 6DJ8 driving the 100 Ohm deflecton plates.

Which of my planar tubes do you suggest I measure for step response? Would the WE416B be a good choice? I own three of these. I also own two of the pencil types.

It would be trully amazing if the 200mW 1J17B was comparable in step response to tubes dissipating more than 10X more power.

Best regards,

-Joe

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Dear Joe,

thank you for the pictures of the 1J29B tube and for the detailed description. It has very unusual electrode structure indeed and, as you pointed out, its apparent simple geometry may explain the excellent pulse response you found. Its designers had clear ideas about the electrostatic field distribution inside. And the simple rods add very little parasitic parameters. I do not remember anything similar.

It would be interesting to know if the WE 416B or the GE smaller 7588 have step response roughly comparable with the one you found or considerably better, as one would expect from tubes designed for UHF operation.

Just few notes about the current density of cathodes for microwave tubes. The GE article talks of the performance and of the internal design of planar tubes in general. I will scan the GE article and send you a copy by e-mail. I gave a look to the density of some planar tubes, when the cathode surface was given. Small signal amplifiers, as the 7077, can operate from 0.2A/cm². Power triodes intended for pulsed operation, as 7815, 7910 or GL51025 reach values from 6 to 8.5 A / sq. cm. A common UHF planar triode, the 6442, has 0.32 sq. cm. cathode surface and a peak emission of 2.5A, with a density near to 8 A / sq. cm. These figures are roughly similar to the values obtained for filamentary thoria cathodes of big transmitting tubes.

Even higher density figures can be reached with barium-tungsten dispenser cathodes. Sylvania reports of experimental cathodes pulsed to 150 A/sq.cm. after having applied a small d-c current to activate the interface layer.

Best regards, Emilio

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Hello Emilio,

I have a new post with updated internal dimensions of the Russian Subminiature Tubes.

I just noticed that I never got around to measuring step response of the UHF planar tubes, as you suggested. Perhaps I will find some time soon.

Regards,

-Joe

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