• New Sponsor: ShipNerd, Ship Your Gear with Us... for less! Click Here.

What's Happened to my Power Tubes?

Bluesful

Silver Supporting Member
Messages
42,031
Hi All,

Let me tell the story first, before I posted the pics here.

Last night I was playing my Two Rock when I started to smell a minor electrical burning smell. It wasn't a strong smell, and there was no smoke, but there was something there.

I leaned over to check the back of my amp, and I noticed black/dark spots on the 2 power tubes. So I then shut everything down as I didn't know if I'd be causing damage by continuing to play.

This morning I pulled the tubes out, and both of them have this black/dark spot in the exact same location. They also have a circular, key-ring shaped piece of metal between what I think is the plates and the glass. You'll see that hopefully in the pics. The black/dark spots are exactly where the key-ring shaped piece of metal is located.

I also this morning put in another pair of tubes, biased the amp and played through it for about an hour and there's been no issues that I can tell, and these new tubes in there haven't suffered any obvious, external damage.

So can anyone provide any insight into what's happened here?

The annoying this is that these tubes weren't cheap ones. They're 80s Jan Philips.

Here's the pics:






































 

metropolis74

Member
Messages
824
Your tubes look good to me. The key-ring looking thing is the getter. The dark spot on the glass is the flash that results when the getter is doing its job and removing gases within the vacuum. These JAN Philips happen to have a side getter and you're probably used to seeing other 6L6 with a top getter.

Sorry can't help with the electrical burning smell without more info. Hope you get it sorted out.
 

pdf64

Member
Messages
8,116
I can’t quite tell from the pics, but is the getter silvering becoming discoloured? A bit of a brown / rusty hue, perhaps with a rainbow effect around the edge?
What anode dissipation have you been idling them at?
 
Last edited:

HotBluePlates

Member
Messages
11,998
... I started to smell a minor electrical burning smell. ... So can anyone provide any insight into what's happened here? ...
I'm not confident anything happened to the tubes in the pictures.

... I noticed black/dark spots on the 2 power tubes. ...
I can’t quite tell from the pics, but is the getter silvering becoming discoloured? ...
IMO, the getter flashes look fabulously fine in the photos. The little bit of dark spot is barely visible, and probably means nothing.

A worn getter would look like the photo below:

 
Last edited:

Bluesful

Silver Supporting Member
Messages
42,031
The key-ring looking thing is the getter.
These JAN Philips happen to have a side getter and you're probably used to seeing other 6L6 with a top getter.
I thought it may be the getter, but then I figured it can't be because it's not at the top of the tube, so thanks for clearing that up.

I can’t quite tell from the pics, but is the getter silvering becoming discoloured? A bit of a brown / rusty hue, perhaps with a rainbow effect around the edge?
On one of the tubes there is no discolouring of the silvering.

On the other tube, there is a black spot in the middle of the silvering, and a faint rainbow at the bottom portion of the silvering.

I can't get the rainbow to show in a pic, but here's a pic which shows the dark spot in the middle of the silvering.








What anode dissipation have you been idling them at?
That I don't know.

I just bias to within the range that Two Rock recommends:




 

Bob Pollock

c'était des extraterrestres
Gold Supporting Member
Messages
5,699
You might have been smelling the paper label on the tube base, or its adhesive.
 

Bluesful

Silver Supporting Member
Messages
42,031
So I put the tubes back in and played through for 30 minutes or so, and there was no smell or anything.

So I think everything is ok.

Not sure what on earth I was smelling then yesterday?
 

Steppin' Wolfe

Silver Supporting Member
Messages
5,781
Phanrosmia…phantom smells??? various causes.
One might rather have a problem in the amp. If I smell an odor in an operating amp, I want to know what is going on….,IOW, it is tech time, imho.…just to be safe. Perhaps there was a bad connection in a tube socket and a power tube lost its proper bias, heated up, and you smelled the result. Pulling the tube may have cleared the oxidation in the socket???.
 

Bluesful

Silver Supporting Member
Messages
42,031
You may want to lower your bias setting to 0.050-0.055 as the 6L6WGB is really a 5881.
I actually got these tubes a couple of years back on Two Rock's advice.

When I was chatting to them about it, I asked if the bias was ok set between 0.055 and 0.060 and they said yes.
 

gldtp99

Member
Messages
4,058
Maybe there was a thin layer of dust on the output tubes and you smelled it burning off

And there's actually nothing wrong with anything and you're chasing phantom imaginary amp issues
 

Bluesful

Silver Supporting Member
Messages
42,031
And there's actually nothing wrong with anything and you're chasing phantom imaginary amp issues
This is highly likely.

The only other thing I've realised is that is was the first time using a new speaker cable that I had made.

Don't think that was on fire though.
 






Trending Topics

Top Bottom