Long Plate Structure
Moderator: jingle_jangle
Long Plate Structure
Charlie: I have read that tubes with long plate structure are not recommended in combo amplifiers. Can you describe what is meant by a long plate structure and why it might be a problem in a combo amp?
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In the early 1950s to the mid 1950s, RCA, GE, Raytheon, etc made 12AX7 and 12AU7s with long plates. The actual triode plate is a few millimeters longer than the later types.
The problem was that they became microphonic very quickly due to production defects, poor grid to cathode alignment, and other issues. Many manufacturers complained about this. Fender went to Tung-Sol for most of their small signal tubes in Tweed amps, because they were not as microphonic as the RCAs.
In the late 1950s RCA came up with the quieter and low microphonic 7025, with medium length plates, specifically designed for Fender combo amps and designed to be installed upside-down and near transformers and speakers. These also had spiral filaments to reduce hum. They also introduced the medium plate 12AX7A in that approximate time frame.
The problem with long plate NOS tubes, is that most of the ones I test are microphonic. Telefunken eventually came out with some long plate 12AX7s that were the best. RCAs long plates from the late 1940-50s tend to be microphonic. However, Raytheon and Tung-Sol long plate 12AX7s are much higher quality and less noisy or microphonic. GEs large plates from the 1950s are also good.
The problem was that they became microphonic very quickly due to production defects, poor grid to cathode alignment, and other issues. Many manufacturers complained about this. Fender went to Tung-Sol for most of their small signal tubes in Tweed amps, because they were not as microphonic as the RCAs.
In the late 1950s RCA came up with the quieter and low microphonic 7025, with medium length plates, specifically designed for Fender combo amps and designed to be installed upside-down and near transformers and speakers. These also had spiral filaments to reduce hum. They also introduced the medium plate 12AX7A in that approximate time frame.
The problem with long plate NOS tubes, is that most of the ones I test are microphonic. Telefunken eventually came out with some long plate 12AX7s that were the best. RCAs long plates from the late 1940-50s tend to be microphonic. However, Raytheon and Tung-Sol long plate 12AX7s are much higher quality and less noisy or microphonic. GEs large plates from the 1950s are also good.