When to fit valve screening cans?

[Alan_G3XAQ]

I'm designing and building a simple valve receiver as a Corvid-19 lockdown project. I'm not sure whether screening cans on the valves are useful, necessary or detrimental to performance and service life. When I look at commercial equipment there seems to be no rhyme nor reason to it.

Can anyone offer reasons for fitting screening cans, or not?

Thanks,

Alan G3XAQ

[Skywave]

Some B7G and B9A valves are designed to run hot, sometimes very hot. Fitting screening cans painted matt black to those valves increases the radiation of heat from those valves - and that, in turn, increases the valve's life.

Al.

[Radio Wrangler]

Fitting the ordinary bayonet-fit screening cans that go onto skirted B7G and B9A valve holders make valves run hotter.

The things sometimes called 'screening cans' that are a blackened roll of springy metal that actively grip the outer of the glass tube DO reduce temperatures and improve reliability. There was a military-funded research paper on this back in the fifties. Racal cite it in some of their handbooks where they strongly recommend this type.

However, the thermal thingies rarely include an earthing wire, so the one thing they don't do is screen.

With small, low power valves and not living in a desert or jungle, the ordinary loose-fitting cans are useful screens.

How do you decide?

Well the outermost structure of valves that don't have internal screens is the anode.

If the anode is either going to be an aggressive radiator of E field, or if it is a fainting violet of hypersinsitivity, then screening seems sensible.

Manufacturers tried them out and found the least combination they could get away with.

Think of large gain amplifier chains, all running the same frequency. You don't want any output getting to the input.

David!

[paulsherwin]

In the octal era, most British and European valves had an external screening coating applied, so didn't need screening cans. Cans were mainly found with American designs.

Used in mil. spec. stuff to stop the valves dropping out under vibration too.

[turretslug]

As a very broad rule (no doubt subject to exceptions in either direction), B7g signal valves had outer, unscreened anodes and B8a and B9a signal valves had an outer mesh screen surrounding the anode structure. Note the emphasis on "signal", as well as the emphasis on "exception"!- the ECH42, EF89 etc. have the screen, whereas EL42, EL84 etc. don't.

As merlinmaxwell points out, military kit frequently applied screening cans on a blanket basis, taking advantage of the common bayonet-lock type's feature of keeping the valve securely in place. Posher screening cans also existed- many here will recall the TF144H's chunky brass screening cans with their fine-threaded base and an inner stretchy copper-braid "girdle", fulfilling both tightly-screening and cooling functions.

[Alan_G3XAQ]

OK guys, thanks for the variety of input. My impression is that screening cans are unlikely in our climate to cause excessive overheating. The EM screening might be beneficial, perhaps more with amplifiers and oscillators than with mixers.

In my specific case I am looking at 12AT7, 12AX7 and E88CC valves which, although B9A, don't seem to have internal screens outside the anodes. In a domestic setting there doesn't seem to be much need for protection against vibration.

So I'm inclined to fit bayonet type screening cans as a routine.

Thanks, Alan G3XAQ

[G6Tanuki]

What are the valves doing?

If the anodes are at significant RF-potential [they're being used as RF/IF amplifiers, frequency-multipliers, VFOs, mixers] then the 'can' isn't really there for heat-dissipation issues, it's there to provide a consistent, predictable and long-term-repeatable capacitance between the anode and ground.

I remember a "distributed line" six-6AK5 RF amp that someone tried to align without the cans. He spent an afternoon on it before I pointed out that the few picofarads of anode-to-can capacitance was part of the design equations and he couldn't possibly get it to be both stable and have low 'ripple' across the necessary 50MHz bandwidth without the cans fitted.

[Alan_G3XAQ]

>What are the valves doing?

I have 4 valves as a superhet: E88CC double triode 3.5MHz/455KHz mixer, E88CC double triode 455KHz product detector, 12AX7 two stage AF amplifier, and 12AT7 oscillators near 4MHz and 455KHz.

It's all narrow band with trimmers to absorb circuit strays. At these frequencies small changes in anode/earth capacitance may not be significant but I do take your point.

All in all it looks like it's better to use screening cans as the default.

73, Alan G3XAQ

[Radio Wrangler]

What did 12AU7 do wrong to get left out?

David

[turretslug]

I'd clean overlooked the B9a double-triode gang! Yes, a very significant exception to the ready-screened 9-pin list.

[Alan_G3XAQ]

>What did 12AU7 do wrong to get left out?

The 12AT7 and its swanky cousin the E88CC has the high slope needed for an oscillator used with modern sissy crystals and ceramic resonators. The 12AX7 claims an especially low heater/grid capacitance which reduces its susceptibility to hum with an AC heater supply. So the 12AU7 falls between the two stools.

But really it's because there isn't a 12AU7 in my junk box.

Alan

[emeritus]

A number of the valves listed in the 1938-9 GEC-Osram valve catalogue were available in both screened and unscreened versions, both versions having the same type number.

[Diabolical Artificer]

A lot of guitar amps and reel to reels had screening cans on the front end valves especially , maybe to damp mechanical vibration. The only time I've used them on an amp is because I had some very nice screw thread B7A bases and cans and the valve in question was sitting right near a big OPT, not really needed but why not.

Andy.

[AdrianH]

Originally Posted by Alan_G3XAQ

I'm designing and building a simple valve receiver as a Corvid-19 lockdown project. I'm not sure whether screening cans on the valves are useful, necessary or detrimental to performance and service life. When I look at commercial equipment there seems to be no rhyme nor reason to it.

Can anyone offer reasons for fitting screening cans, or not?

Thanks,

Alan G3XAQ

Alan I will be most interested and will be following your progress. Are you following a set design or doing your own?
This idea is on my wish list. I will be looking at ideas for AM demod and if possible SSB demod as well. I have only stripped down some ex military VHF receivers and little else I am planing for a double superhet with odd ball IF's, but that is for the future.

Wish you well and keep safe.

Adrian

[Radio Wrangler]

There was the format of a TV cookery show. Someone turned up with an eclectic mix of ingredients and a celebrity chef had to turn them into a decent meal.

Only, Alan's trying to work such magic on the contents of his junk box

In all probability no two people can come up with the same result.

David

[Heatercathodeshort]

Originally Posted by emeritus

A number of the valves listed in the 1938-9 GEC-Osram valve catalogue were available in both screened and unscreened versions, both versions having the same type number.

The MSP4 employed in the first generation EMI television receivers was supplied without metalizing to reduce the internal capacity between the anode and what would have been earth.

Screening cans were fitted to each RF section but were a safe distance from the actual valve.

The B7G EF91 is an unscreened RF pentode and will be discovered in pre ITA television RF and IF amplifier stages complete with screening cans to prevent instability.

The later replacement, the B9A EF80, was fully screened internally and did not require an external screening can. Pye continued to fit square screening cans to their receivers maybe as a belt and braces exercise but soon realized they were not necessary.

EMI painted their screening cans matt black in the notorious 1807 in a hope to reduce operating temperature but the cause of poor stability was eventually traced to nasty decoupling capacitors and leaky paxolin valve holders.

We all know what happens when the octal EF39 used as an I.F. amplifier in 1940's radios looses it's earth connection to the metalizing..

Much later tuner valves such as the PCC84 and PCF80 were not internally screened probably to prevent high interelctrode capacities but were screened externally with cans at a safe distance from the valves electrode assembly Regards, John.