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Old 13th Jun 2014, 7:09 pm   #1
Paul Mathew
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Default Coating on EF39 valves??

Hi,

I'm busy with a Philips 171A, and there are 2 x EF39 valves with a thickish, red rubbery coating of sorts, which is peeling off. The other valves seem to be coated with regular paint, and are in reasonable condition. Is this coating critical to their operation? Any advice on the way forward would be appreciated.
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Old 13th Jun 2014, 7:25 pm   #2
mark pirate
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Default Re: Coating on EF39 valves??

The paint is electrically conductive, and provides screening for the valve.
There should be a thin copper wire coming out of the top of the base to connect to it, you can wrap the valve in silver foil, making sure it is making contact with the wire. Or fit a screening can.
There are conductive paints available, but these tend to be quite expensive.

Mark
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Old 13th Jun 2014, 7:29 pm   #3
m0cemdave
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Default Re: Coating on EF39 valves??

The paint is metallic screening, and there is a high probability that the radio will not work properly without it.

In an old Philips set that I used to have (170A I think - it was about 45 years ago), the IF stage oscillated until I repaired the connection to the EF39 screening.

You can get conductive metallic paint for EMC screening and mending circuit boards. It should make an effective repair. Self-adhesive copper tape will also work, if cosmetics don't matter.

(message written concurrently with Mark's !)
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Old 14th Jun 2014, 12:29 am   #4
AC/HL
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Default Re: Coating on EF39 valves??

The Forum search came up with this: https://www.vintage-radio.net/forum/...ad.php?t=42531
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Old 14th Jun 2014, 10:38 am   #5
Paul Mathew
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Default Re: Coating on EF39 valves??

Thanks for that helpful info. I did spot a thin wire coming out from the base of one of the valves and wondered. I think I have some cans from another set, alternatively I'll use aluminium foil. I presume the wire connected to the foil can be connected to anywhere on the chassis?
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Old 14th Jun 2014, 10:52 am   #6
Paul Mathew
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Default Re: Coating on EF39 valves??

Thanks Bill. Just looked at the link on recoating metallic valves. The pictures make it seem easy enough! I'll just have to get creative about getting the "EF 39" back on the new coating!!
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Old 16th Jun 2014, 5:37 pm   #7
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Default Re: Coating on EF39 valves??

Hi. Just a thought, on my Mullard MAS281 (wood cabinet version of the Philips 170A) receiver, one of my EF39 valves was missing. I found that you could use a 6K7 octal valve (mentioned as an equivalent to the EF39 in a book). It worked quite well indeed. The "new" EF39 that I had in stock had not lost its conductive paint, but there was no connection between pin one and the screening. I carefully cleaned up the connection to pin one, added some tin copper wire and soldered them together, and wound it around the glass (not all the way up the valve).
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Old 3rd Jul 2014, 9:31 am   #8
Paul Mathew
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Default Re: Coating on EF39 valves??

Hi Telstar, thanks for the input. I've posted a new thread for the next phase of the saga, regarding the ouput transformer, which now appears to be the final barrier to success!!
Regards, Paul
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Old 4th Jul 2014, 12:49 pm   #9
telstar
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Default Re: Coating on EF39 valves??

Hi Paul. Glad that you found my input useful. It is a shame that the red screening was poor on these valves. You may be better obtaining the C.V type valve equivalent used in military equipment.
Cheers
Mike
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Old 4th Jul 2014, 8:37 pm   #10
Neil Purling
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Default Re: Coating on EF39 valves??

Mullard seem to have a problem with their metalised coatings. I came across a unused ARP34: The entire coating was seperated from the envelope in what looked like one peice!
I daren't unwrap the valve fully in case the coating fragmented.
There is little 'key' for the paint to stick to a smooth valve envelope & then damp gets in.......
Wrap the valve in tin foil. There has been a thread about this problem of re-screening valves with flaked coating. I believe you have to somehow solder a copper wire to pin 1 and tie the other end around the foil.
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Old 5th Jul 2014, 5:56 pm   #11
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Default Re: Coating on EF39 valves??

The North American versions of British-type metallized valves seem to have a far more tenacious coating- don't know whether these were made because of war-time production requirements for British equipment or whether there were existing licenses anyway. They seemed to have devised a much more successful process than UK makers- perhaps the glass was pre-etched with HF or something else that would make CoSHH folk turn blue nowadays!
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Old 6th Jul 2014, 7:04 am   #12
Neil Purling
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Default Re: Coating on EF39 valves??

Was the metalised coating specific to British or European practice?
I have a WW2 period American 6K7 which has no coating, not sure about Canadian normal practice.
I believe that the Hytron VR53 I have is an American component.
I also have a Rogers (Canada) VR56 that also has perfect coating.
I don't know if it is some specific ingredient in the coating or the valve envelope is etched, but it means the coating sticks. I do have a green coated EF39 type which has had the coating lacquered, so the issue of flaking coating was known about back in the day. A component for Tropical use where humidity could be excessive?
I believe this valve is a Army component, ARP34, but that it is a Mullard item.
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