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Bad tube?

jogas

Member
2004-06-11 10:51 am
Milan
Hi,

I am testing a new PP 300B and I have one tube showing an intermittent blue light as shown in the attached picture.
Does this mean it is a bad tube or any gas/vacuum leakage?
Thanks for your help
 

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PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
Blue/violet glow attached to the glass is harmless and pretty. Stray electrons miss the Plate and hit the glass. Various trace elements in glass will fluoresce. Enjoy the glow.

Gas inside the tube will glow inside the plate structure. This is bad. Usually by the time you notice the glow, grid current (normally nearly zero) has become so large that the tube bias shifts, usually toward melt-down. Check the voltage across the grid resistor.

I recently worked with a well-used (and abused) 1939-vintage 6L6G: it was gassy enough to cause 5 volts of grid shift, but there was no visible gas-glow. By letting it cook a week I was able to getter some of the gas, but bias was still non-zero and erratic. People talk of gassy tubes, but in 40+ years experience, I never saw a "slightly gassy" tube, only complete failures (dead TV or amp) caused by a gassy tube.
 
PRR said:
By letting it cook a week I was able to getter some of the gas, but bias was still non-zero and erratic. People talk of gassy tubes, but in 40+ years experience, I never saw a "slightly gassy" tube, only complete failures (dead TV or amp) caused by a gassy tube.

I agree with every bit you mentioned.........often the fault with weird displays inside is that the tubes don't run hot enough! The notion of heatsinks fitted to glass serves no purpose. A bonus of the 6550B (transparent bulb) with zirconium getter fitted to the anode basically gets stewed each time the tube was run. I've never had a contamination problem with these tubes however hard run.

I get blue glow on warm up between interelectrodes on a cool tube but vanishes once quiescent current is stable.

richj
 
jogas said:
Hi,

I am testing a new PP 300B and I have one tube showing an intermittent blue light as shown in the attached picture.
Does this mean it is a bad tube or any gas/vacuum leakage?
Thanks for your help


Hi Jogas,

Intermitent blue glow is bad for me. Sometimes is for oscillating circuit, and one time it was for failure tube (320B-XLS) that inject an horrendous noise when blue glow appears.

Permanent blue glow is normal.

Do you have large and intermitent noise?

Cheers.

Luis.
 
Re: Re: Bad tube?

LuisMCP said:
,

Intermitent blue glow is bad for me.
Do you have large and intermitent noise?

Cheers.

Luis.


Hi there.........I had this intermit blue glow problem with a batch of EI KT90's.....only to find when <I tapped the envelope it disappeared.......internals mechanically unsound....intermittent int contact somewhere. The blue glow disappeared simply because the tube s/c and blew B+ fuse....

richj