Phonoland

So, I posted on the forum a couple of weeks back about an issue I'd been having with the Playrak on my Wurlitzer. A couple of new springs later and it's registering the correct credits every time, so thanks for the help there. While working inside the machine, I noticed that one of the vacuum tubes on the amp has what looks like a burn mark. I thought it might just be a getter spot from the manufacturing process, but none of the other identical tubes have the same mark. I'm an electrical engineer but very much of the digital world, so would appreciate if someone could take a look at the below photos and tell me what they think. If the tube is on its way out then I'd rather change it sooner than later.

electronicsuk wrote:So, I posted on the forum a couple of weeks back about an issue I'd been having with the Playrak on my Wurlitzer. A couple of new springs later and it's registering the correct credits every time, so thanks for the help there. While working inside the machine, I noticed that one of the vacuum tubes on the amp has what looks like a burn mark. I thought it might just be a getter spot from the manufacturing process, but none of the other identical tubes have the same mark. I'm an electrical engineer but very much of the digital world, so would appreciate if someone could take a look at the below photos and tell me what they think. If the tube is on its way out then I'd rather change it sooner than later.

There are experts here; I'm not one of them.

However, I believe that's normal for an older tube.

That is "normal" for some tubes of any age-- Ron Rich

That is normal "getter".

The only real visual flaw that one can check for, other than breakage, is a portion of the plate structure that is discolored. This shows up on power or rectifier tubes and indicates gross over dissipation at some point.

It doesn't necessarily indicate a defective tube, but the tube may be weakened. However, there are cases where an internal G1 to G2 short caused the over conduction and that can also be dangerous for output transformers.

If a tube tester is not available to test for shorts, I would not use a tube with plate discoloration.

Rob/NYC

Thanks foxtrotxray/Ron/Rob, I wish there was a thank or upvote button on this forum. It's good to know that it isn't an immediate concern, but I may look at a complete amp rebuild some time before the year is out. I don't know anything of the history of the tubes, but I have noticed on rare occasion that I get some static/white noise from the amp until it has fully warmed up. It's had new tube sockets fitted as well as a lot of cosmetic TLC since I owned the machine, so I can rule those out. Looks like the previous owner had some of the caps replaced, but few of the resistors look particularly modern.