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Flashing lights on preamp tubes on power up

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Active Member
Joined: 1 year ago
Posts: 4
Topic starter  

Hi. I have an Osprey 3.1. Can someone tell me why a light in the sockets of preamp tubes flashes on power up? I've found it can be one or two depending on the tubes installed. 

James Connell
Active Member Admin
Joined: 6 years ago
Posts: 7

It’s common on many nos tubes. It has to do with their construction. There is nothing wrong with the tube and will not have any ill effect on sound. Nothing to worry about. We have many at Raven that do it on the amps we use in our office and at home.

Estimable Member Admin
Joined: 6 years ago
Posts: 94

Yes I can try...   some of the older antique European tubes will flash brightly underneath the lower mica when they are cold - usually due to an inrush of current to the cold filament.  This doesn't mean the tube is going bad, just that the higher voltage tungsten filaments going to each side of the triode will glow brightly when first "lit".  Some of my absolute favorite tubes no longer flash upon startup, but they still sound glorious.  Why is this?  I am not positive about this, but some of the old tube guys long ago told me it was to "start-up" heat the filaments.  In other words, it was a purposeful design aspect of some European dual triodes.  This is believable because the engineers back in those days were quite brilliant.  The vacuum tube manufacturers back then had the best of the best engineers and technicians working for Brimar and Mullard, Philips, Amperex etc.  I also have many German dual triodes that do this also.  After WW2 these guys were working all over Europe, so that also makes sense. 

Mostly seen on Siemens, Brimar, Telefunken, Valvo, etc. and a few of the later German manufactured dual triodes like RFT did this, but I know of no USA or other country that manufactured these types of tubes that did this - that "I" know of anyway. 

There is a ton of bullsquat on the internet about this, a whole lot of which is completely inaccurate.  I also read that some of the Chinese tubes are purposely made to flash for no other reason than to sell tubes - so those flashers are/were only about "show" and trying to copy the originals.  Some had tiny little foil strips in them designed to flash upon startup.  I have not personally encountered this, but do believe that the Chinese are sometimes more interested in selling product than reproducing the absolute best metallurgy and coatings quality, which is the whole thing about the best vacuum tubes - the quality of the metals and especially the coatings.  But that is a whole other subject...