Standby Switch?

In the tube amplifier world, there has been quite the debate over whether a standby switch is needed. The standby switch’s job is to keep the high voltage off of the plates of the power tubes in an amplifier until the tubes are warmed up to prevent a phenomenon known as “cathode stripping” from occurring. … More Standby Switch?

High Frequency Response

High frequency roll-off in vacuum tube amplifiers can frequently be a problem. Fortunately, there are several tricks that can help. The first thing to note is that with more stages, there will be more capacitive attenuation. The reason for this is simple: every vacuum tube amplifier stage is also a filter. After all, the tubes … More High Frequency Response

A Tube Amplifier: Biasing

A tube amplifies by creating a changing voltage drop across a resistor in response to a small signal coming in at the grid. The grid is almost always kept at a negative potential relative to the cathode to prevent it from pulling current. There are several ways of achieving this bias, the most obvious being … More A Tube Amplifier: Biasing

Pentodes vs. Triodes

Triodes always have relatively low amplification factors, representing the maximum possible voltage gain. The maximum amplification factor number commonly seen is 100. A common tube with this number is the 12AX7. Pentodes, on the other hand, have significantly higher amplification factors — sometimes as high as 9000, as in the 6AC7, a metal tube originally … More Pentodes vs. Triodes

Tubes in Parallel (Part 2)

Pictured is a schematic of a Freed-Eisemann NR-7 radio receiver from the 1920s. Besides being an interesting example of the primitive circuitry in use throughout the ‘20s, this receiver also illustrates using output tubes in parallel — without any kind of balancing circuitry! All of the leads are tied straight together, which would cause some … More Tubes in Parallel (Part 2)

Tubes in Parallel

In hi-fi tube amplifier building, a frequent temptation is to use two power tubes in parallel. After all, it is a simple way to double the output power – provided, of course, that you change the audio transformer’s impedance to one half of the value you would use for one power tube. However, in practice, … More Tubes in Parallel