Bugera 1990: Why is the first not glowing when amp is turned on.

[station5343]

I have a bugera 1990 that I just got out of the shop for a power tube change and some other thing's. I brought it home and turned it on and noticed that the first of the 4 power tubes was not glowing. I unscrewed the back and put on a surgical glove as to not touch the tube with my finger's and just lightly jiggled it a little bit and got it to light up but I am now wondering why this is happening. And now each time I turn the amp on I have to do the same thing because the tube will not light up on its own. The funny thing is and what I don't understand is that the amp will work with just 3 of the tubes lit so what is happening? If anyone reads this and has any suggestions as what could be going on please feel free to post a reply...thank you.

[Jazz P Bass]

You have either a bad solder joint on that tube socket or the contacts inside the socket are loose.
The amp will function (term is used loosely) with only one output tube.
It takes all four to develop the full 100 watt output.

[52 Bill]

Jazz is right, it could be a loose socket or a cold solder joint or it could be a bad tube. Try switching positions of two of the tubes and see if the problem stays with the socket or with the tube, that will help narrow down the problem.

The amp is set up with two sets of paralleled pairs of output tubes. If one of the two paralleled tubes stops working, the other tube continues to work, so the amp still makes sound. It just doesn't make full specifications.

[Enzo]

And it will not harm the tubes in any way to touch them with your fingers. It is projector lamps and stage lighting lamps that get so hot, not vacuum tubes.

[David Schwab]

First thing I would do is to take it back to the shop that worked on it.

Do you know that they actually put working tubes in the amp?