[AliG]

I have a McIntosh MC275 amp with four KT88 tubes, I noticed flashes of blue light in one of the KT88 whenever the amp is trying to pump out heavy bass.
Is this normal? Or it's time to replace that tube?

Thanks
barry

[Niteshade]

Hello-

Yes, this is normal. It sounds like the blue light might actually be dimming under high wattage demands. If you look closely, that should be what's happening. If the tube was arcing, you'd hear it and it would be loud.

[woodsyi]

I don't know about flash but blue glow is quite normal.  




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[AliG]

Thanks... now I can go back to sleep.  The more  I read about KT88 tube failures / reliability issue.. the more paranoid I got..

Last thing I want is to have the tube fails and damages other components in the amp.

[richidoo]

Barry, you're not the first to ask about the blue glow...

From "Sylvania Radio Tube Hints" - 1943 (courtesy this site)

Three Reason for Blue Glow

Many inquires are received relative to the blue glow which is present in a number of Sylvania Tubes. Most of these are based on the misunderstanding of the different types of glow that may be present in a tube. There are three different types of blue haze that may appear while tubes are in operation. The
are classed as: Fluorescent glow; Mercuty Vapor Haze; Gas.

The fluorescent glow is usually of violet color, and is noticable around the inside surface of the glass bulb. This glow is a phenomenon caused by electronic bombardment taking place within the tube. This glow changes with the intensity of the signal and may at time become quite brilliant. Fluorescent glow has absolutely no affect on the operation of a receiver. In fact, tubes with this characteristic are particularly good as regards gas content.

Mercury vapor haze is a blue glow which is noticeable between the plate and filament in Types 82 and 83 rectifier tubes. These are the only types of Sylvania receiving tubes in which this type of haze appears. The perfect operation of Types 82 and 83 is dependent upon a mercury vapor which comes from free mercury that has been placed in the bulb during the exhaust period. Therefore this type of blue haze is in no way detrimental to the operation of these tubes.

Gas is a blue haze which is usually confined to the vicinity of the plate and filament structure. Its presence, when of large content, affects the operation of a receiver to the extent that erratic performance is noticeable. Gassy tubes should always be replaced with new tubes.

Testing for the above conditions can be best accomplished by actual operation in a receiver. It is not necessary to test for the blue glow evident in Type 82 and 83, since this is characteristic of these two tubes.

When in doubt as to the blue content of other types of tubes a sure test can be made by using a strong magnet next to the blub. A gassy tube will not be affect in any way by the presence of the magnet, while the fluorescent glow, which has no affect on the performance of the tube, will shift about as the magnetic filed is shifted.

The blue usually means the tubes are nice and fresh. It usually fades with use even though the tubes are still strong. I love the look of the blue glowing tubes with orange filament inside.

A well designed amp like your Mac has fuses to protect itself from just about any normal failure. Just keep a couple extra tubes and a couple extra fuses (B+ and mains) and you can fix normal failures yourself. Read the manual for basic repair info like tube replace, fuse locations, bias routine.

You have a great amp!!
Rich

[Niteshade]

Just don't run the tubes too hot- bias them as cool as possible before the sound quality degrades. It's no secret that the 6L6 is my all time favorite tube. I have had great luck with JJ/Tesla 6L6's, KT88's and El34's.

[RodMCV]

Niteshade
,
Why is the 6L6 your fave tube?

I have read a designer who said he could get better sound out of a KT88 then the classic 300b or 2a3.
Your thoughts?

Then there is the 6c33 tube.
What are your thoughts on that tube?

Rod

[AliG]

Niteshade,
   When you said biasing them as 'cool' as possible, What voltage will that be??

Thanks
barry

[JoshK]

Richidoo's three reasons post is good.  It is obviously not mercury vapor because those aren't mecury tubes.  It is likely the flourescent glow given the description.  The gas glow is common too, but it would be more constant.

I think you should bias tube where they are most linear, but choose a linear operating point at 20-25% lower than max dissipation to lengthen tube life.

[Niteshade]

You will have to experiment with the voltage. Don't worry, you won't hurt anything by increasing the bias voltage. Reducing it too much so the tubes run hot is how damage can be sustained.

I do not know what your B+ (High Voltage) is, so I'll assume it's around 400 volts. My guess is that your bias is around -35 volts. Whatever it is, try increasing it one volt at a time (-36,-37, etc...). When the bias is too high, the sound will become dirty and/or your volume will decrease. Keep a log of which bias voltage sounds the best with the least amount of heat and don't forget to write down the voltage you started with. Do not decrease the bias, or go below your starting point with this experiment.

In a nutshell: When I tune up amplifiers, I always start from the highest bias voltage (-50 or so) and work my way down. The goal is to find a 'sweet spot' that produces great sound (at full output wattage) with as little heat output as possible.

BTW: You may already be at your 'sweet spot' if the tubes don't seem to get overly hot. The amp should not feel like a toaster oven, but be moderately warm.

Niteshade,
   When you said biasing them as 'cool' as possible, What voltage will that be??

Thanks
barry