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In Reply to: RE: I'm Confused posted by Grainger49 on June 30, 2009 at 04:26:31
"Tube rush" is a phrase that covers a multitude of sins, leading to some confusion :^)There are many sources of random noise in low-level circuits, and books have been written on the subject. So far as I know, none of them are what you might call an easy read! Here are some basic ideas though:
Resistors make noise due to random vibration of the molecules, the noise power being proportional to absolute temperature. This often sets the limit on noise. The noise is "white noise", equal energy per Hz, which is perceived as being tilted to the high frequencies.
Tubes also generate noise by this thermal agitation mechanism. Triodes have an equivalent noise resistor whose resistance is the inverse of the transconductance; this resistor is at an elevated temperature of course so it's a few dB louder that that resistance at room temperature. For this noise mechanism, high transconductance tubes like the 6DJ8 will naturally be quieter than lower transconductance tubes like the EF86. Pentodes have an additional source of noise connected to the partition of electron flow between the screen grid and the plate, which adds 3 to 5dB of white noise.
There are several sources of noise with a low-frequency bias; most of them are lumped together as "1/f noise", also called pink noise and perceived as having a fairly flat spectrum. This noise usually dominates at low frequencies, with (to the best of my knowledge) the crossover being somewhere in the 1kHz to 100kHz range. This is not a problem for VHF radio applications, and many high-transconductance tubes were designed for such applications with little consideration given to the low audio frequency noise - so high transconductance does not always mean low noise in audio applications. Unfortunately this information is rarely specified on data sheets for specific tubes and can be quite variable between manufacturers. The very few tubes (such as the EF86) that were designed for small-signal audio applications would usually have taken this into consideration, though some of the mechanisms were only discovered in the late 50s/early sixties. Incidentally, one of those mechanisms also causes a problem in digital circuits, and a few tubes (such as the 5965) developed for early computer applications avoid that particular mechanism, making them more suitable for audio.
I don't know whether that will increase or decrease confusion!
Edits: 06/30/09
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