Bad tube? Blue glow

[Tym_ex]

Hey, question: One of the 2 power tubes of my Laney GH50L has a blue glow at the bottom of the tube. Also the sound is changing in volume and there's a weird hiss present. I can also produce the same hiss sound when turning the gain and master pots (poweramp pots). Do I have a bad tube? And what should I do?

Edit: I took a better look and read up on the topic. It seems you have 2 kinds of blue glow: one sharp light blue (good) and another fuzzy and almost purple (bad). I have the second. What's also interesting (or not, but for me tube noob it is ) is that the blue glow disappears when you turn the amp in standby.

Thnx!

 

[SoSomething]

It might be a bad tube. Have you changed tubes recently? If so, it might be a bias issue, in which case you need to take your amp to a tech to have them properly bias it.

If you're still using the same tubes you always have, it's time for some new ones.

 

[Hexer]

if you have "old" tubes in there, I'd guess this tube will die pretty soon. last time I saw a blue glow in my amp was about 3 seconds before one of the powertubes died a pretty spectacular death by shining blue, brighter and bright while producing louder and louder hum, then a loud pop and then..... well... silence....
so I'd suggest getting a new set of Powertubes

 

[sorceron]

mine are always blue...

 

[Hexer]

mine are always blue...

well, I guess it depends on the amp/tubes, but if it wasnt blue before...

 

[Tym_ex]

Well I just bought the amp, so the power tubes are all new. I'm just really a tube-noob, this is the first real tube amp I own so I thought I'd better ask.

It seems though there's nothing wrong with the tubes anyways, the problem is only there when I use the "lo" input, the "hi" is perfect. So maybe there's some dust or something in the lo input?

 

[SoSomething]

If the buzz is only present on your lo-gain input and the volume is moving up and down, you can almost guarantee it's a preamp tube going bad. A tube doesn't have to be old to die - they're like plants in that way - sometimes they just go bad for who knows what reason.

The weird noise coming from you turning your knobs could either be caused by a bad preamp tube or dirty pots. In either case I think your symptoms are pointing to a preamp tube issue which is very, very cheap and easy to solve. Sorry I didn't pick up on this when I replied before.

I did a pretty long-winded study of different preamp tubes in my amp for tonal reasons in this thread - if you decide to try something different in your preamp to tailor the tone more to your liking you might find it useful.

 

[Tym_ex]

Thnx Adam, isn't it weird there's no problem using the hi input then? Or does it use different preamp tubes or something (total noob to routing of a tube preamp too ). I did notice one of the preamp tubes is not completely vertical alligned. I guess I can press it in better? Can I just do that without flying through the room because of an electric shock? Including the metal thingie around them?

Thnx again for all replies!

 

[SoSomething]

Thnx Adam, isn't it weird there's no problem using the hi input then? Or does it use different preamp tubes or something (total noob to routing of a tube preamp too ). I did notice one of the preamp tubes is not completely vertical alligned. I guess I can press it in better? Can I just do that without flying through the room because of an electric shock? Including the metal thingie around them?

Thnx again for all replies!

Actually it depends on the amp, but based on what I know of preamp designs I would say it makes a lot of sense that only one input is affected if it's in fact a preamp tube problem. Each preamp tube in an amp serves a different purpose in a signal chain. Some control your preamp gain voicing, some run the EQ, others will run your reverb if your amp has it, etc.

You can pull tubes in and out without fear of electric shock. Most amps have all the really dangerous stuff enclosed inside the chassis so just reaching inside isn't necessarily a death sentence. Just make sure your amp has been powered off for a while because they do get pretty hot.

Also - those metal things around the tubes are called tube shields - they're there to protect the tubes and also to reduce interference which can cause unecessary noise. You'll need to remove them to change or push in a preamp tube. Be careful when you do this - they should push down slightly and then turn before popping off, kind of like the lid of a pill container. You don't just want to start twisting and pulling or you could damage the tube socket, which would require an amp tech to repair. Also, when you pull the tube you should notice the pattern of the pins in the bottom, because the tube fits in the socket in one particular way. It's pretty self-explanatory once you see it what I'm talking about.

EDIT: FYI the tubes just pull straight out once you get the shield off.