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Old 8th Aug 2018, 4:38 pm   #1
Diabolical Artificer
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Default Valve construction.

I've had a look at various double triodes over the years, EG ECC81, 2 & 3 but apart from difference in grid spacing can't see much difference. So what dictates the parameters, IE U, ra/slope etc? I can't find any noticeable difference.

The same goes for OP valves too, I taken apart various beam tetrodes who's anodes are all about the same, what is funny is that both the small triode and big OP valve have about the same size cathode.

While I'm at it the size difference between cathode and anode is enormous. Obviously the cathode is more efficiant at chucking out electrons than the anode is at catching them, per sq mm or whatever.

Just curious. Andy.
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Old 8th Aug 2018, 4:48 pm   #2
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Default Re: Valve construction.

Grid spacing, anode spacing, cathode surface area are the main issues for a triode.

The anode gets heated by electrons arriving, so it needs to be able to radiate the heat. Some small-signal RF valves have quite small anodes, to reduce capacitance.
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Old 9th Aug 2018, 6:56 am   #3
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Default Re: Valve construction.

"Grid spacing, anode spacing, cathode surface area are the main issues for a triode" Are you refering to inter electrode capacitance Dave?

Maybe the physical differences are subtle, IE a few thou difference between k and a affects the gain. I guess I'd have to mic up a few valves to get an answer which wouldn't be easy and go into much finer detail.

I'd just wondered if anyone knew of rule of thumbs like more grid windings = less gain, thicker electrodes = more current capabillity.

Andy.
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Old 9th Aug 2018, 7:31 am   #4
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Default Re: Valve construction.

Gain/gm is strongly affected by the g-k distance and the pitch of the grid wires. Then come things like alignment of grid wire positions between g1 and g2.

Variable-mu is done with grid pitch which varies across the active area.

Inter-electrode capacitance is a side effect of these things rather than their method of effect. It comes down to electron ballistics/optics.

David
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Old 9th Aug 2018, 8:53 am   #5
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Default Re: Valve construction.

Given the many thousands of valve types, even after discounting known equivalents, I have always found it difficult to believe that there could be so many variations in dimensions of the electrode structures!

Another question, did the manufacturers design a valve to give certain characteristics, or did they build a prototype and measure the characteristics?

Andy
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Old 9th Aug 2018, 8:57 am   #6
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Default Re: Valve construction.

Hi Andy, Google for Philips Technical Library, loads of books on valve design there. Similarly for RCA.
There is also the famous website (Pocnet?) and some others that have scans of Gb of tech books.

Ed
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Old 9th Aug 2018, 9:06 am   #7
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Default Re: Valve construction.

All of the above.

Valves were not developed in isolation. As you said, many types were true equivalents, many were the same thing but with a different heater voltage, many were the same thing on a different base or crammed into a smaller envelope. Some were variants with and without top caps. Some were the same electrode designs mounted in a bottle with a second device - some similar, some different.

Progress in the parameters of successive devices for the same application was often a matter of small tweaks. Better process control would allow closer g-k spacing and still get a good yield.

Occasionally some new application came along and new devices were designed for it... EG 7360 beam deflection valve for early colour TV, uhf triodes for TV tuners etc

Occasionally some new ideas of construction came along and allowed changes, so they experimented to see what they could now build. Disc-seal triodes for UHF, Nuvistors

You could reduce the total number of types by just lumping together those that worked sufficiently similarly at any job - ignoring bases, heater volts and pairing. And you'll come down to a manageable number of archetypal valves.

Same sort of thing happened with transistors, opamps, and most other areas of technology.

David
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Old 9th Aug 2018, 10:56 am   #8
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Default Re: Valve construction.

Andy, the construction & functions of a radio(Thermionic) valve is covered by a wealth of written & internet knowledge. Too vast for any of us on the Forum to bang-on about. However, if you want to zero in on basic info - look up "Bright Emitters" & "Dull Emitters" - early valves from the end of WW1 & up into the 1920's. Particularly the use of Thorium. Its an element which richly releases electrons when subjected to heat. Hence its initial use with early "Filaments"(combined heater & cathode). A study of basic physics/chemistry relating to the ability of various basic metals(elements) to release electrons, perhaps would be advisable. Then you can understand why a positively charged Anode can attract negatively charged electrons.
I suspect that a check through the Amazon listings will reveal heaps of cheap 2nd hand radio books. As for the internet - just google & google. You could ask your local Amateur Radio Club if they minded you sitting in on some of their Foundation or Intermediate training classes. Those classes which cover electron theory and valve & transistor theory.
Perhaps there is knowledgeable Forum chap living near you who might tutor you in exchange for a full coffee pot & heaps of hobnobs ? (or a bottle of malt).

Regards, David
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Old 9th Aug 2018, 12:11 pm   #9
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Default Re: Valve construction.

Thoriated tungsten is still used for its high temperature stability and electron emitting properties as TIG welding electrodes. It really is dramatically good.

David
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Old 9th Aug 2018, 12:17 pm   #10
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Default Re: Valve construction.

You can probably get a very rough first approximation of triode mu by comparing g-k distance with a-k distance. Mu ~ Da-k/Dg-k. This is because what actually moves the electrons away from the cathode is the electric field they see there, and this is a linear combination of that created by the grid and that created by the anode. Assuming simple geometry (an approximation, a gross approximation for some valves!) the electric field in V/m depends on the 'V' of the electrode and the 'm' of the distance.

This assumes that the grid spacing is fine enough to make an equipotential plane but wide enough to let the cathode 'see' the anode. In reality it is not so simple, as the grid field is stronger near the grid wires so the parts of the cathode immediately 'under' a grid wire have lower mu (but higher gm) than parts between grid wires. The strength of this effect depends on how the g-k spacing compares with the grid wire spacing.

Assuming you have made a valve, you can double the gm (and halve the anode impedance) by wiring two in parallel. Or just make it twice as big (e.g. double the length of the cathode etc.).

Or you can make a valve to handle more power but with similar characteristics by simply doubling all dimensions (spacings, thicknesses etc.). The larger cathode can emit more current and the larger anode can dissipate more heat.

In the early days they had to do design graphically or by trial and error. Later they could use computer models of the electric fields but these would probably still have been fairly crude even towards the end of the domestic valve era. Only the residual areas of valve use (e.g. CRTs, high power transmitter valves) would have benefited from really good computer models.
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Old 9th Aug 2018, 4:47 pm   #11
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Default Re: Valve construction.

Andy, once you feel comfortable with the basic electron theory, & metallurgy of valve's electrodes - then the placement & sizing of the different electrodes will become apparent. To delve further into the tabulations of the AVO Valve Data Manual, or an Illiffe or Barnards Valve Data Book, you'll need to study the maths relating to Mu, Gm, ra, Ra, Gain & Amplification Factor & such.
In the early 1920-ish times those old 2V Filament BE & DE Valves drew very little anode current (Ia), needed very little grid bias or non at all, and decimal values of mA/V(Gm). Since then, dozens of valve companies & come & gone. Thousands of valve designations/ref.nos are with us, along with dozens of valve types. That's confusing enough as it is. Then Prof. Barkhausen's formulae awaits you.
At times in the past, folk have questioned why just about every valve in different radio's circuit diagrams have different anode, grid, & cathode resistors. Very confusing for some novice &/or non-technically minded folk. Your points raised about anode & grid sizes & shapes are the reason. Each different shaped electrode needs different values & potential(negative or positive) of voltages to either slow down the negatively charged electrons, or speed them up. Talking of ECC type valves, of which I've tested hundreds, & standardised loads, different brands give a variety of results for the same NOS valve number on a valve tester.
Then one has to consider the tolerances of resistors over the years. In bygone times, many valve companies tested their valves under calibrated DC conditions, but were obviously aware that many radio companies just put cheapo 20% resistors in the circuitry. So many bog standard valves were constructed to account for this.

Regards, David
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Old 10th Aug 2018, 5:42 am   #12
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Default Re: Valve construction.

Thanks chaps, lots of good ideas for further reading. One thing occurs... of the few valves I have and have built circuits with, datasheet specified u/gain has always been pretty bang on, odd that transistor manufacturers couldn't do the same; tranny current gain specs being a bit like broadband ISP spiel, EG up to X hfe.

A gross comparison probably as the two are different in construction and operation.

A.
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Old 10th Aug 2018, 10:22 am   #13
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Default Re: Valve construction.

Newbie question - does the size of the evacuated envelope have any bearing on the parameters of a valve?
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Old 10th Aug 2018, 10:30 am   #14
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Default Re: Valve construction.

Valve characteristics largely depend on physical and chemical characteristics which are easier to control precisely than those involved in the manufacture of transistors.

In fiscal terms it's likely cheaper to grade by selection than to attempt to make them all exactly the same.
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Old 10th Aug 2018, 1:05 pm   #15
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Default Re: Valve construction.

Bipolar junction transistor current gain comes from the base current, which is almost an error term. This means that it is unavoidably poorly controlled. It is actually the base-emitter voltage whch sets the collector current, and this is fairly predictable. Hence precision circuits design from this rather than base current.

Triode valve mu is set largely by geometry, so will vary little from sample to sample. It is anode current and transconductance which is less well controlled, and changes with age.
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Old 10th Aug 2018, 2:17 pm   #16
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Default Re: Valve construction.

Quote:
Originally Posted by kellys_eye View Post
Newbie question - does the size of the evacuated envelope have any bearing on the parameters of a valve?
As far as I'm aware the only effects are a) thermal ones in power valves and b) electrical breakdown, leakage and pickup ones related not directly to the size but indirectly via the constraints on the pin location and spacing.

An example of b) would be in very high voltage valves where it makes sense to bring the anode connection out on a top cap to keep the HV conductors away from the rest, both inside and outside the valve.

Turning to a) power valves have to dissipate heat from the anode very largely by infrared radiation through the vacuum and then out through the glass envelope. The glass is not completely transparent to the infrared, so it gets hot and has to be cooled itself by a combination of more radiation and conduction into the surrounding air, which may be moving either through convection or because there's a fan driving it. The bottom line is that in power valves with small envelopes the glass tends to run very hot indeed and some of this heat can be radiated back to the anode which makes the anode run hotter. This is usually A Bad Thing. One good thing about hot glass is that if the getter is deposited on it then that runs hotter too and getters tend to work better when they're hotter.

For a while it was common to coat the inside of glass envelopes with a (weakly ?) conducting grey material one of whose functions was to discharge any static which might build up on the glass. In some cases (e.g. the M-OV KT66s) manufacturers stopped doing that. I seem to recall hearing that in fact static charging was either uncommon or unimportant and that applying the grey coating therefore did little good.

Cheers,

GJ
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Old 10th Aug 2018, 2:25 pm   #17
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Default Re: Valve construction.

Quote:
Originally Posted by kellys_eye View Post
Newbie question - does the size of the evacuated envelope have any bearing on the parameters of a valve?
I've wondered if some '30s valves had quite large envelopes simply to keep the glass temperature down for a given anode/heater dissipation- later, smaller envelope types appeared (6V6G to 6V6GT being a classic case, also similar development with the 6L6G and many others), I'd assumed that this had something to do with higher-temperature rated glasses. It would take an exacting trawl through data sheets to find out if there were any cases of significant anode dissipation changes but I think it was generally accepted that more up-to-date small bulb types could be substituted for original large bulb types of the same type.

There were certainly '50s series regulator valves with slim tubular bulbs of specific high-temperature glass for this arduous application that were expected to run brutally hot, I have a suspicion that a few late-era tetrodes of this nature were re-packaged and re-branded '30s power valves. I'm not giving specific examples in an era when anything with a significant anode area is seized upon by the silly and biddable with money to burn....

There's also an argument to be made that, for a given gas leakage whether externally from the atmosphere or internally from poorly-processed or over-heated electrodes, that a bigger envelope gives proportionately more vacuum to weaken!
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Old 11th Aug 2018, 11:31 am   #18
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Default Re: Valve construction.

It confused me as a young teenager starting out in the RAF, then years later as an instructor was often asked by young trainee apprentices - why does the pictorial representation of a valve(and its current flow) appear to have vertically mounted electrodes when the beam of electrons within a valve actually(in most cases) flows horizontally? I.e. from the inner vertical tubular cathode out to the bigger tubular anode. Now, many years later - I suspect that this confusion still exists amongst a few non-technically minded vintage radio collectors & novices.
Electrons :- an analogy - If one could fire a stream of small steel ball bearings out of a steel tube & into bell-mouthed orifice of another steel tube then the steel tubes would get jolly warm from friction, & the orifice itself would eventually get really hot - because of the concentrated friction. Electrons, even though they are infinitesibly smaller than wee ball bearings, they are already jolly warm from being pulled out of the molecules of heated thoriated tungsten(cathode or filament) then battered into the molecules of metal which forms the anode. The infra-red heat from the anode thus heats the glass envelope. In extreme cases, such as 807's in old AM Tx's, or KT88's in old over worked guitar amps - valves would glow like an electric fire, due to their anodes not their heaters.

Regards, David
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Old 11th Aug 2018, 1:38 pm   #19
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Default Re: Valve construction.

Quote:
Originally Posted by David Simpson View Post
... Electrons ... are already jolly warm from being pulled out of the molecules of heated thoriated tungsten (cathode or filament) then battered into the molecules of metal which forms the anode ...
Actually the electrons at the cathode aren't (usually) nearly as 'warm' as they are when they arrive at the anode. Cathode materials are chosen for having a low work-function (a measure of how hard it is to pull electrons out of them). I seem to recall that the electrons in the space-charge cloud which blankets the cathode have a typical energy below 1eV or so. By the time they've been accelerated by the anode-cathode potential they will have picked up perhaps 300-400eV of energy which is then lost, as you say, by inelastic scattering at the metal anode, heating the anode up as a consequence.

Cheers,

GJ
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Old 11th Aug 2018, 1:50 pm   #20
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Default Re: Valve construction.

The extra kinetic energy coming from the attracting electrode (anode or screen grid) will be dependent on the voltage.

One ampere is basically some fixed number of electrons per second. One watt is that number of electrons per second accelerated through a PD of one volt. And one joule is the amount of work done by one watt in one second -- i.e., by accelerating some particular total number of electrons through one volt. So an electron-volt is a (tiny) unit of energy.
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