Last week I wrote an article on vacuum tubes in guitar amplifiers. If you haven't read it, you can check it out right here! This article briefly went over the basics behind what a tube is, what some of the popular models of tubes are, and what some of the differences are between tube amps and solid state.
This week I wanted to go into a little more detail in discussing how to change the tubes in your amplifier. First, how would you even know if your tubes need to be changed? Second I'd like to talk about buying "matched" sets of tubes and what that means, and finally I want to talk about what it means to "bias" your tubes and whether or not that's something you can do at your home with some simple tools.
When To Change Your TubesIf you own a tube amp, it is inevitable that eventually you will need to change the tubes inside as the lifespan of a tube is much shorter than that of an amplifier. That said, there is no consistent amount of time that any tube is good for and in fact even two tubes of the same make and model can function properly for very different lengths of time depending on several factors.
Crackling sounds, unpredictable and sudden changes in volume, complete cutting out of signal, or hissing and high pitched squealing sounds can all be symptoms of a bad tube that needs to be replaced. To determine which tube is causing the problem, try removing any plates or anything preventing you from accessing the tubes and then turning the amp on and gently tapping on each tube with a pencil one by one. If the issue you have been having (whether it's crackling, hissing, etc.) is exacerbated by tapping on a particular tube, it is likely that this tube is causing the problems. This diagnostic procedure is not foolproof however, and tapping on tubes can hypothetically create new problematic sounds, so this is not an exact science.
If you're able to see your tubes, a tube that is glowing more red and bright can be an indication that the tube is red plating and may need to be changed. In the past, I have blown a tube causing my signal to completely cut out. In this case I was able to determine which tube needed to be replaced by looking at it because the tube itself had a black mark on the inside, similar to some blown lightbulbs.
Often once I have determined that I need to replace the tubes in my amp, I will decide to replace all the tubes at the same time. This can be expensive depending on the amp, but I do this for two reasons. The first reason is that it keeps me from having to open up my amp and diagnosing which tube is causing a problem, and the second is that replacing all the tubes allows me to ensure that I have a matched set of tubes.
What Are "Matched" Tubes?
As I mentioned in last week's article, vacuum tubes are pretty old technology. Two tubes of the same make and model are liable to have very different performance characteristics. It is important to make sure when you're replacing the tubes in your amp that each new tube you're putting in to the amp has consistent amplification characteristics. Many vacuum tube manufacturers will test the plate current draw as well as other amplification characteristics of each of their vacuum tubes and then group together ones that have the same measurements for sale as "matched" sets.
All of the tubes in the above links are sold as matched sets, and it is common to walk into a guitar shop and see tubes for sale in bundles sometimes bound together with rubber bands to indicate a matched set. It's important to have matched sets of tubes in your amplifier so that when you "bias" your amp once the new tubes are put in, each tube will behave the same way. If the tubes are unmatched, settings that are great for one tube may be terrible for another.
What Does It Mean To "Bias" An Amp?Often when replacing your tubes it is important to "bias" the amp. This process involves setting the idle current that a tube draws with no input signal. While the scientific principles behind this is a little outside of the scope of this article, I've found a really great article here that you may want to check out if you're looking to get into the technical side of what that means. To put it simply, because vacuum tubes vary in their specifications, if you were to just remove an old tube from the tube socket, place a new one in there and turn your amp on and begin to play, there is a chance that everything will work out fine. However there is a high chance that your new tube has different properties than the old one, and putting it into the same circuit under the same conditions as the old one may shorten the life of the tube or negatively affect the tone of your amp. Worst case scenario you could cause significant damage to the amp. By biasing the amp, you are setting the bias current (the electrons flowing from the cathode to the anode with no audio signal) at its optimal level. If the current is set too high, this is called biasing the tubes "hot" and will shorten the life of the tubes. In extreme cases this could cause damage to the amplifier. If the current is set too low, this is called biasing the tubes "cold" and while this will lengthen the life of the tubes, it will negatively affect the tone of the amp causing it to sound thin.
It is important to note here that preamp tubes (the smaller sized tubes, often 12ax7) do not need to be biased. When talking about biasing tubes, we are only ever talking about power tubes. Preamp tubes can just be replaced without having to bias, but it is still a good idea to buy matched preamp tubes as well.
How exactly do you go about finding and setting the optimal level for the bias current of your matched set of power tubes? If you're new to working with electronics the truth is I really wouldn't recommend attempting this. The inner workings of a guitar amplifier carry enough current to kill a person, even if the amp has been turned off and unplugged for days in some cases. Capacitors inside the amp can store charge for days and if you don't know what you're doing, messing around with this kind of thing can be very dangerous. If you're a little more skilled with this kind of thing than the average Joe, I've found a website right here that sells manuals on how to bias some popular amplifier models.
Also, YouTube is a great resource for learning how to bias your amp. Here's a well done video on how to bias a Fender Hot Rod Deluxe:
Thank you once again as always for checking out my blog today, and for letting me be a super small part of your journey with the guitar. It's a privilege that I do not take for granted.
If you have any questions or concerns, please feel free to reach out!