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Old 27th April 2005, 08:55 PM   #1
jogas is offline jogas  Italy
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Default Bad tube?

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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Old 27th April 2005, 09:35 PM   #2
dhaen is offline dhaen  Europe
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Default No problem

This is a frequently asked question. There are various explanations given, but this type of glow is not a fault or a problem.
Poor vacuum or gassing can cause coloured glows, but they tend top be more diffuse, and not confined to the glass envelope.
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Old 27th April 2005, 09:55 PM   #3
jogas is offline jogas  Italy
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Thanks John,

I will continue my testing, anyway, attached there is a complete picture of the amp, I am very proud of it )
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Old 27th April 2005, 09:59 PM   #4
dhaen is offline dhaen  Europe
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Looks very nice
Is that a copper chassis?
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Old 27th April 2005, 10:01 PM   #5
jogas is offline jogas  Italy
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Yes,
a very heavy chassis 100% copper made by a local hobbyst for 130 Euro
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Old 27th April 2005, 11:03 PM   #6
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oooh.. I'm a big fan of the copper chassis...

I have a bass amp project shoved in the back of my mind, and Copper is likely my chassis choice... I'm thinking of doing like a THD-like design so the copper and the tubes are visable.

Anyway.. nice amp!!
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Old 28th April 2005, 09:24 PM   #7
PRR is offline PRR  United States
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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.
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Old 28th April 2005, 09:28 PM   #8
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If I ever build a valve amp, I would want all my tubes to glow like that.
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Old 3rd May 2005, 02:59 PM   #9
n0plb is offline n0plb  United States
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Very nice looking amp! did you build it yourself? If so I sure would like to see the underside and a copy of the schematic.
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Old 3rd May 2005, 07:32 PM   #10
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Quote:
Originally posted by PRR
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
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