Ampeg B15S

[bsco]

quick question....just had to replace output tubes n this one....I used a couple of used sites that I had here....6L6's and I noticed one of the tubes has that bluish/.purpleish glow and gets more pronounced as you hit the notes harder...is that normal as these things do have a pretty high voltage on them...also noticed that the tube with the bluish glow also seems to be hotter than the other one.....any thoughts ???
Cheers
Bernie

[mozz]

It's just a gas inside. I have seen 6v6's (7408's) do this of a particular maker.

[The Dude]

Blue = OK
Red = Bad


There was a company many years ago (can't remember who) that sold glowing blue or purple tubes as a special feature.

[bsco]

ok. Thanks guys...so I guess the bluish tube will also run hotter than the other tube that does not have that gas inside??

[The Dude]

Do both tubes have the same bias current? In other words, are they closely matched?

[Helmholtz]

It's just a gas inside.

Just a gas?
That would be a severe issue as it means a poor vacuum.
A gassy tube is a candidate for thermal runaway.

But mostly the blue glow is just fluorescence of the glass envelope when hit by stray electrons.

[Mick Bailey]

I've seen this a lot, sometimes around where the electrodes leave the glass base. I had some early Chinese 6l6 tubes that had a glass pinch construction (like 1930s era tubes) that put on a spectacular light show. These would have made a nice hifi amp with them on display.

Some tubes have a particular spot or area on the envelope that glows blue or purple. The very first time I ever saw this I thought something was wrong, but it doesn't seem to correlate with a bad tube or reduced lifespan.

[Helmholtz]

Some tubes have a particular spot or area on the envelope that glows blue or purple.

Probably in the vicinity of plate windows/openings where fast electrons can escape and bombard the glass.

[mozz]

"The very first time I ever saw this I thought something was wrong, but it doesn't seem to correlate with a bad tube or reduced lifespan"

Yes. For as long as i owned the amp it never gave me any problems. I think they were Westinghouse tubes. Both tubes did it. I do not think it was a vacuum problem, more likely a impurity.

[Mars Amp Repair]

'Funny you don't Blueish' (Yellow Submarine movie quote).

[bsco]

Do both tubes have the same bias current? In other words, are they closely matched?

They are supposed to be...I will have to double check that later today...somebody had repaired this before it was sold on Reverb and they had removed the main filter caps...instead of properly replacing them with multi sectional ones they strung about 20 radial caps and epoxied them upside down with a shit load of resistors...and then wired up an adjustable bias pot which is also epoxied onto the inside of the chassis.....what a mess......I will have to probably cut the cathode traces and install 1 ohm resistors there in order to measure the bias current.....

[bsco]

'Funny you don't Blueish' (Yellow Submarine movie quote).

Ha...I have yet to see the movie....

[bsco]

Do both tubes have the same bias current? In other words, are they closely matched?

Ok. I just installed a couple of 1 ohm/1W /1% resistors in the cathode circuits for both tubes....I ran the amp through a dim bulb limiter.....I am not really interested in the specific value...just wanted to see if both cathode resistors showed the same voltage drop...they are not...one is reading about 15 mV while the other reads about 9.5 mV..so they are not matched after all......I will have to go through the spare 6L6's I have here to get a matched set and then run the amp for awhile before I install new tubes in there....they are getting pretty pricey up here.......if somebody has anything else I should try, please let me know......

[J M Fahey]

ok. Thanks guys...so I guess the bluish tube will also run hotter than the other tube that does not have that gas inside??

Not necessarily.

[bsco]

Not necessarily.

just found that out....the tubes are not matched........I cut the traces and soldered in some 1 ohm/1W/1% resistors at the cathodes....the reading s are pretty different....