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

Output Transformers

The output transformer is arguably one of the weakest links in a vacuum tube amplifier, hence the rise of output transformerless (OTL) vacuum tube amplifiers. There is a problem with this arrangement, however. Driving a speaker directly with a tube results in a terrible damping factor and very high distortion levels. This is why OTLs … More Output Transformers

Tube Rolling for Sound: JJ KT88, Genalex KT88, EH KT90, Tung-Sol KT120

  I must admit that in my previous article on tube rolling, I didn’t go too much into the actual sounds of the tubes used. I long delayed in adding that detail. However, it was all for the best — for I’ve added the JJ KT88 to the list of tubes in this comparison. If … More Tube Rolling for Sound: JJ KT88, Genalex KT88, EH KT90, Tung-Sol KT120