Can I do this?

[crhyner]

I am also the proud owner of a '66 Vibrolux Reverb. I have a hunch that the problem you encounter has nothing to do with a potentially faulty tube. However, I could be totally wrong AND it's still worthwhile learning how to change and bias your power tubes.

With that said, you could theoretically change the power tubes and BEFORE biasing run the amp for a few minutes to see if you encounter the same problem. If you do, it isn't the tubes and you don't need to change/bias them.

What do you think might be the cause?

 

[AlbertaGriff]

What do you think might be the cause?

It sounds similar to a problem I had with mine when I first obtained it, in that when playing at louder volumes all of a sudden the bass would cut out, making a lower volume thin sounding amp. In the end, I reflowed more solder joints than I care to have counted, but it fixed the issue.

So something was slightly loose, a cold solder joint, and the vibration of the amp would essentially separate it..

Just a thought. It certainly could be a tube, but as I said you may be able to confirm that without rebiasing first.

 

[loopfinding]

Excuse my ignorance, but how is the Eurotubes bias probe safer? What other safety precautions do you suggest I take when doing this?

because you're going to need to measure the plate voltage, which if you don't have the bias probe, involves sticking hands inside the amp and poking at high voltage with your meter.

with the bias probe, you either don't have to reach into the chassis at all (the bias pot is accessible from the outside on BF/SF fender amps), or if you do, you're only turning the bias pot with a screwdriver.

 

[crhyner]

Alright, cool. I've got to say, I'm extremely grateful for this thread. I now know of a gizmo that will let me bias my amp for $100 + shipping, while making it unnecessary to open up my chassis and poke around with high voltage. Also, saving myself the energy, money, and time of going to a tech whenever I want to swap out my tubes and rebias. Cool, thank you all!

 

[crhyner]

Follow up - I retensioned my tube sockets and checked/adjusted my bias. How are these readings for the two 6L6GCs? I used the Amplified Parts calc and they look good, maybe a little low? I can't seem to get my amp to go any hotter than this with the bias? Should I be concerned that I can't dial it up any more than this?

thanks