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Old 5th October 2010, 07:59 PM   #11
tofts is offline tofts  United Kingdom
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Fixed bias. But dead easy to bias as Randall conveniently added biasing pots on the rear with test points for each valve!
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Old 5th October 2010, 08:03 PM   #12
M Gregg is offline M Gregg  United Kingdom
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I would check the bias and reset if it is out of spec!

Things can drift over a period of time!
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Old 5th October 2010, 08:05 PM   #13
tofts is offline tofts  United Kingdom
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I shall bias now. Let you know....
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Old 5th October 2010, 08:07 PM   #14
M Gregg is offline M Gregg  United Kingdom
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There are no red plates in the photo so it can't be to far out, if at all.

It's just something that is easy to do!

Make sure the amp has been on for a while to warm up before you bias it!
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Old 5th October 2010, 08:22 PM   #15
tofts is offline tofts  United Kingdom
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It must have been on for about an hour so far, just checked it and they were only about 2mv out, but they all seem to be a little on the "warm" side, at 42,41,40,42 (L to R in photo) respectivly. So I have set them all at 40mv (yes i know mv, but there is a 1ohm resistor in circuit path according to manual, thus mv=ma in this case)
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Old 5th October 2010, 08:32 PM   #16
M Gregg is offline M Gregg  United Kingdom
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How long has the amp been running these tubes?

Except for the colour is the sound still clean?

The voltages you are measuring show that all should be O.K. The tubes are only running at the manual settings so I would just run it and keep an eye on the colour if you get any sudden changes address it then.

Last edited by M Gregg; 5th October 2010 at 08:43 PM.
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Old 5th October 2010, 09:38 PM   #17
TheGimp is offline TheGimp  United States
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I've seen quite a few NOS tubes that exhibit blue glow on the inner surface of the glass. Latest ones I've seen it in are 12AB5s. Out of 20+ tubes from several manufacturers that I've collected over the past two months, every one does it to some degree.
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Old 5th October 2010, 10:01 PM   #18
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Even some of the new issues are doing this. My EH and Genelex kt88's have a blue glow emitting from the glass envelope. It is more pronounced as the B+ increases.

Looks kinda similar to the above picture.
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Old 5th October 2010, 10:15 PM   #19
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It looks like plasma. Here it is:

http://en.wikipedia.org/wiki/Plasma_(physics)
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Old 5th October 2010, 10:29 PM   #20
kruesi is offline kruesi  United States
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It's fine.

Seeing a blue glow on the inside of the envelope is a fairly normal phenomenon. A bright violet/purple/pink fuzzy glow between the cathode and the plate (not appearing to be on the surface of the glass) indicates a gassy tube- usu. an older tube that has lost its getter effectiveness or one that has a poor vacuum.
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