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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 29th May 2014, 2:02 pm   #1
jonnybear
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Default Mullard flaking metallic shield again

The red/grey coatings on these valves is metallised to prevent oscillation and instability, when it flakes off problems start.
There are several ways of overcoming this, either paint with a conductive paint (expensive) or use a very thin foil (cigarette packet)(if you can find a smoker). there is a wire around the top of the valve base which needs to be connected, this can be done by scraping the original wire, wrapping foil around the valve after soaking off the backing paper, and winding a thin wire over the top of the foil see thumbnail, if you want you can always spray it red after.
john
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Old 29th May 2014, 3:54 pm   #2
Bazz4CQJ
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Default Re: Mullard flaking metalic shield again

Sometime ago, I bought some 'Granville Cylinder Black' paint http://www.granvilleoil.com/product_...hp?prod_id=252 to use on an exhaust heatshield. Quite by accident, I found out that this is surprisingly conductive paint, probably a mixture with colloidal graphite. I cannot compare it directly with whatever paint is used on the Mullard valves, but locating two pointed test prods about 4mm apart on the surface of a plastic object covered with it, it gives a reading of ~10kohm. It sells for about £5/100ml.

If anyone tries it on a valve, perhaps they could report back?

B
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Old 29th May 2014, 6:25 pm   #3
jonnybear
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Default Re: Mullard flaking metalic shield again

Hi B
Just connected test meter to pin 1 of NOS ECH35 which is connected to the shielding and it reads 0.8 ohm at the top of the valve, but Granvilles paint may well be OK and do the job, bet they're OPEN ALL HOURS, ?
John
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Old 29th May 2014, 6:35 pm   #4
mark pirate
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Default Re: Mullard flaking metalic shield again

I have repaired several valves that have lost their coating, I use the really thin 'wrapping' foil secured at the base by twisting 15A fuse wire that I either solder to the original screen wire, or if missing I connect it directly to the top of the relevant pin.

I have had no problems with this method, and end up with a usable valve.

Mark
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Old 29th May 2014, 10:00 pm   #5
Bazz4CQJ
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Default Re: Mullard flaking metalic shield again

Quote:
Originally Posted by jonnybear View Post
Just connected test meter to pin 1 of NOS ECH35 which is connected to the shielding and it reads 0.8 ohm at the top of the valve, but Granvilles paint may well be OK and do the job
Wow! I take it that the expensive paint your refered to in the OP is the silver loaded stuff? Obvioulsy, the foil method works, but if this fairly low-cost paint did the job it would be neat. Hadn't thought of the comedy connection; weren't they going to make a one-off update of that....off topic...

Bazz
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Old 29th May 2014, 10:31 pm   #6
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Default Re: Mullard flaking metalic shield again

I once made some conducive paint by mixing brass powder (as used in resin casting) with PVA glue. Unfortunately I never did anything with it and don't know if it would durable enough to stay on a working valve.

I also mixed brass dust with an oil based lacquer, but this did not work.

Regards
David
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Old 30th May 2014, 4:52 am   #7
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Default Re: Mullard flaking metallic shield again

CRC aerosol spray-on zinc rust-preventive 'galvanising' seems to work OK too. I add a little more length to the original grounding lead to enhance contact, but it is probably not necessary.

Cheers

Billy
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Old 30th May 2014, 6:48 pm   #8
G4XWDJim
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Default Re: Mullard flaking metallic shield again

Rapid on line have just started advertising what they call Bare-Conductive-Electric-Paint in a 50 ml Jar.

It seems the original idea of it is to enable you to draw printed circuits with a pen so presumably it will repair metalised valve coatings.

Jim
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Old 30th May 2014, 8:18 pm   #9
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Default Re: Mullard flaking metallic shield again

There's a conductive silver-loaded paint available from various classic-car-restoration places because it's used to bridge conductivity-debilitating scratches on surface-applied heated-rear-window tracks.

It's not cheap, alas. And I'd think you would need several bottles of the stuff to coat a single valve-envelope.

If your coated-valve is still otherwise OK my instinct would be to see if you could retro-fit a valve-base with the necessary extension and then put a screening-can over the whole thing. That's not entirely unusual - the frequency-changer stage of the Pye "PCR" series radios has an extended base and can/screening cap on top even though the official "ARTH2" gold-metallized valve [a.k.a the "Red" Mullard ECH35 or generic military CV1347 ] is a shielded item.
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Old 31st May 2014, 2:07 am   #10
marconi_pete
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Default Re: Mullard flaking metallic shield again

I do like the idea of using a screening can. I've often wondered if using old lead based paint on the glass valve envelope would be enough do the trick? It could certainly handle the heat.
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Old 31st May 2014, 1:24 pm   #11
G4XWDJim
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Default Re: Mullard flaking metallic shield again

I once spent some time filing a piece of steel to generate a little pile of filings which I then mixed with some aluminium paint. I painted a piece of wood and when dry measured the resistance. Megohms, how disappointing there was no conduction at all. It seemed as if each little speck of steel was completely insulated by the paint.

Jim
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Old 3rd Jun 2014, 10:45 am   #12
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Default Re: Mullard flaking metallic shield again

I wonder if there is a heat-resistant glue available into which the valve envelope could be dipped, followed by application of metal powder such as aluminium or red oxide (both readily available from online sellers via Amazon)?
Best wishes
Guy
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Old 3rd Jun 2014, 11:53 am   #13
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Default Re: Mullard flaking metallic shield again

Some web surfing threw up a number of options, the following are just two:-

Conductive paint

Conductive paint
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Old 4th Jun 2014, 10:45 am   #14
steviewonder
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Default Re: Mullard flaking metallic shield again

A very cheap method I came up with was this:
Break down a couple of 2b pencils,using some fine sandpaper,rub the graphite down to a dust,make a thick mix with pva glue,And you have very conductive paint!
Done three valves successfully,and some worn pads on a remote control so far!
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Old 4th Jun 2014, 1:09 pm   #15
ColinTheAmpMan1
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Default Re: Mullard flaking metallic shield again

What about this stuff?
http://uk.farnell.com/electrolube/en...0ml/dp/1675347

I have used it to spray-paint a screening shield inside the cavities of electric guitars. The metal is nickel. I used several coats and it has to be said that it flakes off wood to some extent, but it might well work on valves.
Colin.
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Old 4th Jun 2014, 2:08 pm   #16
G4XWDJim
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Default Re: Mullard flaking metallic shield again

Quote:
Originally Posted by steviewonder View Post
A very cheap method I came up with was this
What a good idea. Thanks for the thought.

Jim
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Old 6th Jun 2014, 9:24 pm   #17
Ian B
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Default Re: Mullard flaking metallic shield again

I've successfully used "weld through primer" (or weld thru ) many times.It's a conductive paint which is used on steel surfaces before spot-welding. I've never bothered to check the resistance of the coating ( I generally apply at least three coats), but as it works, I assume it's ok. Available from car body-shop suppliers. It gives a matt grey finish similar to the original metallic zinc. I've even sprayed red over it to reproduce the Mullard finish; although the colour was a good match it was rather too shiny.
It has to be admitted that the foil approach works, and is quick, easy and cheap - it all depends on how important you feel appearance is.

Ian Blackbourn
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Old 8th Jun 2014, 10:31 am   #18
davegsm82
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Default Re: Mullard flaking metallic shield again

Quote:
Originally Posted by ColinTheAmpMan1 View Post
This stuff does work, I have a can which was used to repair the coating inside a Tek LCD scope which took a really nasty blast and ended up with fractals all over the screening, I used it to repair the coating on an ECH35 which I then used in my wartime civilian receiver. It went from not functioning at all to relatively sensitive!

A word of warning though, it exits the nozzle at a high rate of knots, so it's easier to spray some into a cup and paint it on with a brush.

Of course I painted it bright red afterwards too.

Dave.
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