Cause of purple flashing in a valve? - UK Vintage Radio Repair and Restoration Discussion Forum

Patrick Dixon · 2nd Sep 2008, 10:19 am

I'm restoring a RAP radio and removed a 6K7G valve to clean it. The valve top cap was loose, so I applied a little superglue before refitting the valve to the set. The radio worked before, but now doesn't, and after a short warm-up I get a nice display of purple fireworks inside the 6K7G.

Is this caused by the top cap connection being broken, or has the valve just died? I've tried the valve in a working set and I get the same effect, so I'm leaning towards it not being anything I've done to the original set!

murphyv310 · 2nd Sep 2008, 10:37 am

Hi.
Its sounds as if you were really unlucky, the top cap may have been pulled or bent enough to have cracked the seal around the glass, this of course would have made the valve go down to air causing the purple flashing. The silver coating inside the valve (getter residue) will gradually go white.
Trevor

Patrick Dixon · 2nd Sep 2008, 10:48 am

Thanks. Shame as it was an original with the 'RAP' stamp on it.

mickjjo · 2nd Sep 2008, 11:36 am

I think the problem could have been due to the use of superglue:-

https://www.vintage-radio.net/forum/...read.php?t=400

Two part epoxy glues like Araldite are safer for sticking loose topcaps.

Regards, Mick.

Patrick Dixon · 2nd Sep 2008, 11:44 am

I'll remember that in future, although in this case the valve didn't really get hot before the problem occurred, so I guess I just wasn't gentle enough with the top cap.

Lesson learnt!

Sideband · 2nd Sep 2008, 2:58 pm

I've always been cautious with glue when fixing top caps. I found Evo-Stik to be quite effective. It has a certain amount of 'give' to allow for expansion although as you say, a 6K7 should not get really hot. If there is not enough wire poking out of the top of the valve to resolder to the top-cap (and the seal is ok) it pays to clean the bit of wire and solder a short length of tinned wire to it. Then clean all the excess solder out of the top-cap (heating and shaking it will usually do this or use a solder sucker), apply a small amount of Evo-Stik to the base of the top-cap and also to the top of the valve, fit the top-cap with the bit of wire poking through and when it has dried, QUICKLY solder the wire to the top-cap. You can then trim the wire.

I've done this a number of times with both octal and Brit 4 and 7 pin types.

Rich.

2nd Sep 2008, 10:19 am	#1
Patrick Dixon Hexode Join Date: Aug 2007 Location: Bristol, UK. Posts: 480	Cause of purple flashing in a valve? I'm restoring a RAP radio and removed a 6K7G valve to clean it. The valve top cap was loose, so I applied a little superglue before refitting the valve to the set. The radio worked before, but now doesn't, and after a short warm-up I get a nice display of purple fireworks inside the 6K7G. Is this caused by the top cap connection being broken, or has the valve just died? I've tried the valve in a working set and I get the same effect, so I'm leaning towards it not being anything I've done to the original set!

2nd Sep 2008, 10:37 am	#2
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: Cause of purple flashing in a valve? Hi. Its sounds as if you were really unlucky, the top cap may have been pulled or bent enough to have cracked the seal around the glass, this of course would have made the valve go down to air causing the purple flashing. The silver coating inside the valve (getter residue) will gradually go white. Trevor __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

2nd Sep 2008, 10:48 am	#3
Patrick Dixon Hexode Join Date: Aug 2007 Location: Bristol, UK. Posts: 480	Re: Cause of purple flashing in a valve? Thanks. Shame as it was an original with the 'RAP' stamp on it.

2nd Sep 2008, 11:36 am	#4
mickjjo Rest in Peace Join Date: Jul 2004 Location: Dartford, Kent, UK. Posts: 1,661	Re: Cause of purple flashing in a valve? I think the problem could have been due to the use of superglue:- https://www.vintage-radio.net/forum/...read.php?t=400 Two part epoxy glues like Araldite are safer for sticking loose topcaps. Regards, Mick.

2nd Sep 2008, 11:44 am	#5
Patrick Dixon Hexode Join Date: Aug 2007 Location: Bristol, UK. Posts: 480	Re: Cause of purple flashing in a valve? I'll remember that in future, although in this case the valve didn't really get hot before the problem occurred, so I guess I just wasn't gentle enough with the top cap. Lesson learnt!

2nd Sep 2008, 2:58 pm	#6
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,578	Re: Cause of purple flashing in a valve? I've always been cautious with glue when fixing top caps. I found Evo-Stik to be quite effective. It has a certain amount of 'give' to allow for expansion although as you say, a 6K7 should not get really hot. If there is not enough wire poking out of the top of the valve to resolder to the top-cap (and the seal is ok) it pays to clean the bit of wire and solder a short length of tinned wire to it. Then clean all the excess solder out of the top-cap (heating and shaking it will usually do this or use a solder sucker), apply a small amount of Evo-Stik to the base of the top-cap and also to the top of the valve, fit the top-cap with the bit of wire poking through and when it has dried, QUICKLY solder the wire to the top-cap. You can then trim the wire. I've done this a number of times with both octal and Brit 4 and 7 pin types. Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....