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How difficult is it to 're-tube' a guitar amplifier?
I have an all tube guitar amplifier that is losing some of it's clarity. I've been told that re-tubing will clear that up. How hard is it to do this? On the surface it looks like you pull the old tubes out and stick new ones in. Is it that simple? What is 'biasing'?
Thanks!
Pull the old ones out and stick the new ones in. That's all there is to it. Tubes can deteriorate over time. There are a number of reasons for this, but the most common is the loss of the cathode oxide coating due to bombardment by the small amount of ions that the getter has failed to remove during manufacture. Not all tubes will deteriorate at the same rate. My point is that it is proably unnecessary to change all the tubes. Try and find someone who will test them for you.
If you can't find someone with a tube tester. Try changing them one at a time to see which one restores the sound. It is usually the power output ones that go first. Try looking at the tubes in the dark. If they glow blue, that suggests that there is gas inside. Even new tubes will glow blue, but a tube that has been glowing blue for a long time will probably have lost some of its output and will be starting to operate outside the desired region.
Biasing is the application of a small negative voltage to the grid to bring the tube into the linear region. In the old days, (and I am talking about a very long time ago here, when valve equipment was usually powered by batteries because very few people had mains electricity) this was done by a separate battery. Then someone realised that by simply connecting a large value resistor between grid and ground, the grid spontaneously adopted a negative voltage. Selection of the resistor value, known as the grid leak, sets the voltage. In a properly designed amplifier with in-spec tubes, the grid bias will be automatically correct.
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