• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Is this ok

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Noticed a blue glow, I think this is ok, but I thought I would double check.
A little blue glow on the inside of the glass envelope is quite normal for output tubes. It's caused by residual outgas from the metal elements being bombarded. Nothing much to be concerned with. A blue haze or glow inside the plate structure would be cause for concern. But this is not the case here.
Just another quick query, I have been told that I could use other types of valves when I come to replace these ones. I know the KT66 has the same pins as the 6L6, but what other valves would offer similar specs?

Presumably I could drop them in, rebias and off I go. So long as the replacement valve are similar in specification. I know heater current draw can be vastly different, but judging by the shear size of the power transformer I don't think it should be an issue.
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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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.
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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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