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MC275 - KT88 output tube not glowing

J

jacob.erik

New Member
Dear McIntosh experts.

I have had used my MC275 MKV for 1½ years and love it.
Some time ago the fuse blew. I replaced it and the amplifier was working and sounding fine afterwards.

But I noticed, that one of the (original) KT88 tubes (V11) was not having a purple glow in the surface of it, like the tree others. My question is:

  • Do I need to replace the tube immediately or can I still use the amplifier?
  • If it must be replaced, do I need to replace all four KT88 tubes?
  • I only one must be replaced, which kind is it? (I assume it must be of the same kind as the amplifier was delivered with)
I hope some of you can help me in this matter.

Best regards,
Jacob
 
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Not all tubes will have a blue glow all the time. I could be wrong but lack of blue glow doesn't mean the tube is bad.
 
Purple/blue glow on surface is stray electrons hitting the glass. Not all tubes exhibit this, and it can fluctuate over the lifespan of a tube. What matters is the orange glow of the filaments inside. Should be fairly faint/low level and similar to the other tubes. Sometimes tubes lose their vacuum, which shows when the silvery stuff sprayed on the inside of the bottle goes white or dirty gray.

If the tube is truly dark, then the answers to your questions are:
  • First and foremost, swap the tubes from one side to the other. You want to see if the tube is bad or there's an issue with the amp that needs service. Assuming the tube is the problem...
  • Recommend replacement. The amp won't get destroyed if you use with a dead tube (ask me how I know), but it will DEFINITELY not be performing at spec.
  • Some people would. However, McIntosh is quite adamant that the unity circuit means you can stick any good tube in there and it'll perform at spec. Despite this, there is no end to people claiming a given KT-88 manufacturer sounds different. Personally, I'd consider replacing in pairs rather than quads but to each their own - and it depends on the failure mode. Demystifying this requires a tube tester; those aren't common these days either, which is why a lot of people replace more than they truly need to.
  • If it's an original McIntosh KT-88 in the MKV, I believe those were Svetlana (Winged-C, the correct icon looks like =C=, there are two Svetlana tube plants). They aren't in production any longer and are getting less common. McIntosh is using a different OEM today, not sure which.
Hope that helps!
 
Last edited:
Had a similar experience recently with my MC2102. One bad tube after fuse blew, so replaced the KT88 pair with McIntosh tubes. Thanks AC/Ryan...:)
 
If a fuse blows in a Mc tube amp this usually means that a tube shorted out. At this pint you will need to of course replace the fuse but you also need to find the bad tube. Swapping tubes between channels is a great way to damage the amp in two channels instead of one. The way the amp protects itself is to open the 220 ohm resistors on the output tube sockets. Blowing a fuse does not mean the resistor will open but if it has then doing the same thing in the other channel is a bad idea.

With the amp powered down and UNPLUGGED you can check the 220 ohm resistors with an ohm meter. If any measure open you need to replace the resistor and you will also know which tube is bad. When the 220 ohm resistor is blown the amp will still make sound but one output tube will be doing the whole signal so you will only have 5 to 10 watts.

Thanks,
Ron-C
 
SilverTpt999 said:
Purple/blue glow on surface is stray electrons hitting the glass.
Click to expand...

I'm a little weak on atomic physics. How does the impact of electrons on the glass produce visible light? Are the electrons converted to photons upon impact? The glow wouldn't be due to ionized gas, would it, since there should not be any gas in a tube unless it's defective?
 
Rob-F said:
I'm a little weak on atomic physics. How does the impact of electrons on the glass produce visible light? Are the electrons converted to photons upon impact? The glow wouldn't be due to ionized gas, would it, since there should not be any gas in a tube unless it's defective?
Click to expand...

Indeed, it's not gas. It's fluorescence. The photons are being emitted secondarily from the glass.

Electrons carry quite a lot of energy with them, relatively speaking. That gets deposited when they change acceleration in any way, including just changing direction, and definitely when hitting the glass. When energy then gets deposited into the silicon dioxide of the tube, it excites electrons in those atoms raising them to higher energy levels. Then, when the excited electrons in the glass then return to ground state, photons are emitted. It happens that this is bluish-purple in the visible range for the glass in many vacuum tubes.
 
Thanks for the input.
I think I will get the 220 ohm resistors checked for its condition just to be certain.

Can any of you tell me how to measure this resistor?
I guess I need to remove the stainless steel cabinet and locate the resistor by the broken tube.
Any advise?

BR Jacob
 
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