Restoring the shielding on 'metalised' valves - some thoughts.

[David G4EBT]

Occasionally on the forum the topic crops up of shielding of valves on which the metalised coating has flaked off or is in a poor state, which - apart from possibly leading to instability - spoils the appearance of the valve. I know that some will resort to sticking cooking foil on, which might provide adequate shielding but wouldn’t win any beauty contests. The appearance can be improved by spraying the valve in the original colour, but of course that does nothing to restore the shielding.

RF shielding paint is available for commercial and industrial use but is eye-wateringly expensive. Also, 'DIY' kits are widely available on internet for shielding the wiring and electronics on electric guitars, some of which are, it seems, especially prone to background hum and as the guitar may have a rat’s nest of unshielded wiring and multiple pick-ups whose job is to do just that – pick up – and any hum is then amplified. (Not relevant to this thread).

On occasions for restoring the metalising on valves I’ve used ‘KEMO 1100’ silver electrically conductive paint, primarily intended for repairing PCB tracks and heater rear windows, to restore the shielding. It’s highly conductive, though expensive at about £10.00 for a 3 gramme phial, though it goes quite a long way and does do the job. That said, we don’t really need the paint to be highly conductive – we just need it to adhere well to glass and to provide a metallic coating for shielding purposes:

https://www.rapidonline.com/kemo-l10...ber-3g-87-0781

I’ve been having a poke around on internet looking for something that might be more suited to our needs, and ‘Bare’ conductive paint looks promising at first sight. It’s available from Rapid Electronics and others, is water soluble, is quite thick so will give a decent coating, and can be applied by brush. It costs £17.95 + £2.95 P&P for a 50 gramme jar which doesn’t seem outrageously expensive. (50 grammes of KEMO 1100’ silver paint would cost £166).

http://www.ebay.co.uk/itm/BARE-Condu...-/131288723902

The technical info states that it’s ‘highly resistive’ which for a product described as electrically conductive paint seems a bit of an oxymoron. The description of the product states thus:

Quote:

Electric Paint is a nontoxic, water based, water soluble, electrically conductive paint. It can be used in circuits as a painted resistor element, a capacitive electrode or can function as a conductor in designs that can tolerate high resistivity. It is intended for applications with circuits using low DC voltages at low currents. Electric Paint adheres to a wide variety of substrates and can be applied using screen printing equipment. Its major benefits include low cost, solubility in water and good screen life. It is black in colour and can be over-painted with any material compatible with a water-based paint.

End quote.

https://static.rapidonline.com/img/b...hDataSheet.pdf

There’s an interesting video demo of the paint here:

https://www.rapidonline.com/Bare-Conductive/paint

The formula for calculating the resistance is: 19.77 (length/width) + 12.

Hence, if we want to apply shielding to a valve to a height of 50mm, and which is 120mm in circumference, that would be 19.77 x (120/50) + 12 = 59.44 Ohms.

So, no big deal for shielding a valve.

Easily applied by brush and we can wash the brush in water. On the face of things this seems like it could be ideal for our purposes, and for appearance sake, could be over-sprayed using water-based acrylic aerosol in the desired colour – silver, grey, red, gold etc. I’ve attached a pic of a VP4 valve which tested fine in my Taylor 45D, though the shielding was in a poor state and the lettering had flaked off. I painted it with’ KEMO 1100’, sprayed it gold and made and applied a water-slide transfer to restore the shielding and appearance as best I could. I think that might have been an ideal candidate for ‘Bare’ conductive paint.

I'd be interested to hear any thoughts on this and if anyone found a suitable alternative product at a realistic price, available retail.

[McMurdo]

I always wondered if aquadag would work, maybe over-sprayed with something more attractive and protective afterwards. A can of that isn't too dear.

[ms660]

Gold leaf ?

Lawrence.

[emeritus]

You can get Copper and Brass leaf from craft shops. I paid about £8 for a pack of 11 leaves of Copper foil, 140mm x 140mm, in a local stationers last year (they didn't sell gold leaf). My test meter measures pretty well zero ohms over a diagonal. I bought it to practice lettering books after I had attended a bookbinding course, but it is really intended for covering large areas and didn't work for me. Being so thin, it ought to be made to conform the the complex shape of shouldered valves without too much trouble. There are plenty of videos on how to cover things with metal leaf on the web, but I haven't tried any of them (I didn't find anything useful about lettering).

[Herald1360]

Originally Posted by pwdrive

Gold leaf ?

Not that expensive for enough to do a valve!

[kalee20]

It's just a pain to handle, though, gold leaf.

Sainsbury's sell it, no need to go to your art shop!

[Mark James]

Electrolube nickel screening compound has been recommended in one of our magazines in the past - cannot recall which. Available from RS and CPC ... (http://uk.rs-online.com/web/p/produc...FRAz0wodT5kP0g)

Not cheap but one can should do quite a few valves.

Mark

[David G4EBT]

Thanks to everyone for chipping in with their thoughts and suggestions.

Gary Tempest, who specialises in full restorations of pre-war British, American and Continental radios, will be well known to BVWS members as his articles on the total restoration of complex sets have featured regularly in the BVWS Bulletin for many years. The winter 2011 issue of the Bulletin featured an 8-page article by Gary on a full restoration of a French Philips 638A 'Superinductance' set. (Basically rather like the 634A 'Ovaltiney' but with a rather more stylish and smaller cabinet).

On page 9 of that Bulletin Gary outlined how he re-metalised a VP4 valve to cure instability in the set caused by deterioration of the metalising on what was otherwise and excellent and quite rare valve. Gary solved the problem by using Electrolube Nickel Screening compound, which comes in a 400 ML aerosol can. There are three excellent pictures in the article covering how Gary restored both the screening and the appearance of the valve, over-spraying the screening coat with gold paint and applying a waterslide transfer.

I've looked into the Electrolube nickel spray and it seems as good as any product we're likely to find. The spec is described thus:

Quote:

8-<

An electrically-conductive coating based on nickel powder in a thermoplastic resin. It is an extremely efficient EMI/RFI screening coating. It will adhere to a wide variety of substrates and can be used to coat cabinets for a variety of electronic and electrical housings.

• 100% Ozone Friendly
• Excellent surface resistivity (0.3-0.7R per sq. at 50 microns)
• Enhanced adhesion to substrates incl. ABS and other plastics, superior scratch resistance
• Excellent attenuation levels
• Specially formulated to avoid problems of settlement, therefore providing even surface coating
• Fast drying at room temperature
• Coats complex shapes
• Prevents static build-up
• 400ml

8-<

End quote.

http://www.farnell.com/datasheets/301435.pdf

Given that it is claimed to have 'enhance adhesion' to substrates such as ABS and other plastics, it's reasonable to assume that it will also adhere well to glass, but if not, by experimentation, maybe a primer of some sort that will adhere to glass could be applied? (Although aquadag was used on glass TV tubes, I doubt that the adherence would be as good as the Electrolube product because Aquadag is basically colloidal graphite in suspension with water, rather than say a resin base lacquer).

True, the Electrolube product is not especially cheap at £29.19 + VAT (= £35.00) from Farnell, but that's for a 400mL can so would coat a good many valves. For rare valves in otherwise good working order, even if just a couple of valves were rehabilitated a can would pay for itself. For lower value valves, the cost per valve soon comes down to a sensible figure. I guess that if only used intermittently, the shelf life may become an issue. I've lost count of how many cans of paint such as hammered finish and crackle, that I've binned over time.

http://uk.farnell.com/electrolube/en...und/dp/1675347

One thing I'm still not clear on is the extent to which shielding needs to have good conductivity. We're not trying to use it as a conductor to pass current through it, so does it matter if we use a product with higher resistance - 800 Ohms per square as compared to Electrolube nickel spray at 0.3 - 0.7 Ohms per square? We know that Electrolube is highly conductive. But that said, one of its stated applicationsof Aquadag is electrostatic screening:

This is from the Aquadag spec sheet:

Quote:

8-<

ELECTRICALLY CONDUCTING COATINGS

“Aquadag”, the original Acheson conducting coating, is a very useful general purpose material for this group of applications. Coatings formed with “Aquadag” are suitable for electrostatic screening, as electrodes in many types of electronic and allied equipment such as dose meters, ionisation chambers and Geiger counters and for the prevention of corona discharge at sir gaps adjacent to stressed dielectric materials.

The electrical characteristics of the coating vary according to the degree of dilution of the concentrate, the method of application and the heat treatment given to the coating. The following figures are quoted as a guide:

“Aquadag” diluted 1 : 1 with water and applied by brush

Air dried coating approx. 800 ohms per square

Heated 200C approx. 500 ohms per square Heated 300C approx. 20-30 ohms per square

The resistance of an applied coating may be reduced up to one third by polishing. The addition of a wetting agent will also noticeably reduce the resistance of the “Aquadag” coating. The maximum addition recommended is 2% by weight of the diluted product.

8-<

End quote.

http://www.agarscientific.com/media/...G303_Jan14.pdf

I'd quite forgotten that there was a thread on this topic back in 2011 to which I chipped in, but that thread - now closed, was inconclusive, so we never did hear if Aquadag was a success.

https://www.vintage-radio.net/forum/...ad.php?t=67245

[David G4EBT]

Originally Posted by Mark James

Electrolube nickel screening compound has been recommended in one of our magazines in the past - cannot recall which. Available from RS and CPC ... (http://uk.rs-online.com/web/p/produc...FRAz0wodT5kP0g)

Not cheap but one can should do quite a few valves.

Mark

Ooops - our posts crossed Mark, but we're on the same lines!

I notice that they all charge the same price - like the old days when RPM (Retail Price Maintenance) was the order of the day.

That said, they're cheaper than Amazon who'll sell you a can for £55.00.

[ex seismic]

How about Halfords "Weld through" paint? Or even their zinc based primer?

Gordon

[Boater Sam]

Colloidal graphite in a water base is sold for blacking stoves at a much more sensible price. Has anyone tried it as a screening material?

[McMurdo]

Kontakt-Chemie Graphit spray is about 6 quid a can, it is sold for repairing aquadag tubes and the datasheet says it sticks to glass

[David G4EBT]

Originally Posted by ex seismic

How about Halfords "Weld through" paint? Or even their zinc based primer?

Gordon

You'd think so, in reading the spec, which states:

U-POL Weld #2 Weld Through Zinc or Copper Rich Primer
U-POL Weld #2 Weld Through Zinc or Copper Rich Primer is a high adhesion, super conductive primer, which offers the best rust protection for SPOT, MIG & TIG welding applications. It features a super high concentration of zinc or copper in the dry film and the fast drying formulation speeds up the welding process.

http://www.halfords.com/motoring/pai...-weld-2-primer

I guess it depends on how you define 'superconductive', which from our perspective would mean a fraction of an ohm - as good as metal would be, as inferred by the term 'super high concentration of zinc or copper in the dry film'. Maybe it is 'superconductive' when 140 amps from a MIG welder is passing through it, but I sprayed some onto a piece of glass, let it dry, and even with the prods 1 cm apart it was infinite resistance, so it's certainly not 'superconductive' at very low/no voltage. Rather a let down, but then in fairness, it's not meant for our application. The video below explains what it's for.

Basically, when metal has been prepped for welding, weld-thru primer stops the metal from oxidising before it's welded:

https://www.youtube.com/watch?v=FKyu3MWuPR8

[Stylo N M]

Hi David,

can i just say, that VP4 looks fantastic WELL DONE

paul.

[David G4EBT]

Thanks Paul! The valve tested good, but the metalising was poor, so I repaired the flaked off areas using 'KEMO 1100' silver conductive paint, then primer, then gold spray paint. I made the waterslide transfer in Photoshop from an image of another valve.