Wow! Lucky guy! THD makes fine equipment!
No matter how good the amp design or manufacture, tubes wear out. That's why you have to replace them. They are similar to light bulbs in that respect. Now for the tech-talk...
Tube bias for guitar amplifiers?
It’s easy to understand tube bias without an electronic-engineering background.
Imagine that electrons are a bunch of energetic little guys, who run around when you put pressure on them. The pressure gives them energy, so they want to move. The pressure is called VOLTAGE. (Voltage is to electrons what Starbucks is to humans.) The number of electrons moving thru a wire per-second is called CURRENT.
Think of electronic current exactly as you would of a stream or river. Some streams are narrow, but flow down a steep incline. Gravity is akin to voltage in the sense that it provides the impetus to move downhill. If a stream is narrow, it has less “current” than a mighty river. Here, electrons are analogous to the number of water molecules. The big river could have much more potential energy, because it has more water, even if it flows down less-steep incline. Of course, you could have a BIG river that flows straight down, like Niagara Falls. That is a LOT of CURRENT!
In a vacuum tube, electrons emit from a hot thing called a CATHODE. They pop off the cathode and float around in the empty space around the cathode. They do this because the cathode is made of a material that emits electrons when it is heated.
Remember that electricity is like magnetism; it has a positive and a negative component. With electrons, as with magnets, opposites attract, while like-charges repel.
Since electrons all have a NEGATIVE charge they tend to REPEL each other. So a cloud of them in a tube tends to want to disperse OUTWARD. (“negative and positive” are both part of an arbitrary nomenclature standard that was established hundreds of years ago by Benjamin Franklin.)
Now we have the cathode with a cloud of electrons all jostling to get away from each other, but there isn't anyplace for them to go.
So there's another metal thing in the tube. It's called either a PLATE or an ANODE. Now, we put a POSITIVE charge on the plate.
The electrons, which are negative, are ATTRACTED to it, because of its OPPOSITE POSITIVE CHARGE.
Electrons pop out of the cathode and zip straight to the plate. They hit the plate, penetrate, and become moving electric current in a wire. This current can do useful things, like push a speaker-cone to make sound.
That is (basically) how any amplifier works.
OK, but are there a LOT of electrons moving through the tube? Yes, there are lots of them. So many that if we left this tube operating, it would destroy itself! The electrons hitting the plate cause it to heat up. If there are enough electrons heating it up, it will turn red-hot, melt the tube's glass envelope and the tube would die a dramatic death. (hissing, sparking and other dangerous fireworks.)
So, we apply a BIAS voltage to a grid (of little wires) that acts like a sieve. This is a steady NEGATIVE voltage, more negative than the cathode. Since like REPELS like, the negative voltage will REDUCE the number of electrons passing through the grid.
You can think of this bias voltage as a shutter or venetian blind on your window. You control how much light gets through by adjusting the blind. In a very real sense, this is exactly what we do with bias on the grid. It acts like a shutter to repel some of the electrons, just as a shutter repels photons... (light.)
We set this voltage so the current to the plate is a tolerable IDLE current. The current is determined by the tube's design and ability to handle it, plus by the circuit outside the tube. Each kind of tube has its own optimal operating point.
By making this bias voltage adjustable, we can set it as desired. However, only certain amplifiers have an adjustment for bias, and those that have bias adjustments usually don't make them accessible to the user.
With so many litigious lawyers around, manufacturers are understandably squeamish about letting unauthorized people play with deadly voltages.
I am NOT joking here! I have seen idiots get knocked down to the floor while diddling with amplifiers.
It is kind of funny in a Darwinian sense... maybe electricity is meant to weed out the weak.
However, if you like your musician "buddy" enough to buy him that expensive boutique amp, you probably don't want him to catch a jolt from the large capacitors in that tube amp while he has it open on a bench.
Professional technicians have strict procedures for evacuating capacitors and grounding the amp-chassis before they putz in the amp's guts.
I am not kidding at all. If you don't have the know-how, and you poke around inside an amplifier, even when it is off and unplugged, it can KILL!
THAT is why you pay a technician!
So why do you have to re-bias when you replace tubes?
You have to understand; tubes VARY from sample to sample. Applying a given bias voltage to a tube is OK, if it's adjusted the tube will idle properly (and work properly). But if you CHANGE your tube or tubes, how do you know they'll bias properly without adjusting the bias? The new tube might be different in bias requirement.
It would be GREAT if tubes were rated by their individual bias voltage. Actually most companies that sell "matched" tubes usually print the bias current the tube operates at, right on the box. Unfortunately, each company has their OWN STANDARDS for measuring bias. Plate voltages, plate currents, and other parameters are DIFFERENT from one dealer to another. So they may or MAY NOT be interchangeable.
BIAS POINT RATINGS split up the Gaussian distribution into a series of ZONES. This scheme is very similar to the 1-10 rating scheme used by one of the major sellers of pre-tested tubes. (I won’t mention the name, but the initials are “GT” and name rhymes with “move boobs.”)
Basically, that company says that tubes with a lower number on that 1-10 scale "BREAK UP SOONER" than usual. And tubes on the high end of the scale "BREAK UP LATER".
Even though their rating system is based on the individual bias variances of the tubes, the other company likes to claim their system is "secret" and "proprietary". That’s BS, …a marketing lie. Their "5" rated tubes are smack in the middle of the Gaussian bell curve. That's all it means!
The tubes that are on the LOW side of the bell curve are low BIAS POINT tubes. They are usually prone to "breaking up" or going into distortion at lower than the proper volume (for that kind of tube).
So they have LOWER HEADROOM. For some musical-instrument applications, the user may want this effect. (distortion mongering Rock-guitarists for instance)
That is undesirable for high-end audio, because lower idle current means higher distortion, which most people dislike hearing in their music playback.
Tubes in the MIDDLE zones fit the average profile for that tube type. They are most commonly recommended for the average amplifier user, especially for high-end audio amplifiers. These tubes will give the PROPER amount of headroom for their operating point. So the amplifier will perform as the manufacturer intended.
Tubes with a high BIAS POINT will idle at a higher current than specified, and give more headroom than the amplifier designer intended. With high BIAS POINT tubes, your amplifier volume / gain will need to be turned up much higher to achieve "break up". Some instrument amplifier users might wish for this sound effect. ( i.e. Jazz guitarists)
If you want more information about this subject, go to the TUBE DEPOT website. There you will find some nifty animated graphics that help people understand tubes and tube-amp operation.
Tube Depot is a good company in my opinion. Tube Depot does not “sell” in the pejorative “lying” sense… (you know, like used-car salesmen.) Instead, they inform customers so that we can make appropriate choices.