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Preamp tubes are easily identified, in most cases, as the smaller tubes in your amp, and are usually positioned to correspond to your amp’s inputs and early gain and tone stages. They are responsible to progressively amplify the "small-signal" from your guitar or other instruments, and gain is added across each gain stage of the amplifier. Sometimes they are fitted with metal noise shields, which must be removed before you can access them. Since the mid-fifties, preamp tubes have mostly been of the smaller nine-pin “Noval” type, although some older amps will still have bigger eight-pin “Octal” tubes that fit the same size sockets as many types of output tubes. The most common preamp tube by far is the 12AX7. (also known in Europe as the ECC83, or the high-grade US alternative - 7025)
Some other types you will occasionally find in the Preamp look much the same, other than the numbers printed on them. These are the 12AT7, (or ECC81) often used in reverb-driver and phase-inverter stages; and the 12AY7. (or 6072) These are all known as “dual triodes” because they contain two independent tubes within the same bottle, and each is responsible to provide the gain within one gain stage of the amplifier. They are mostly differentiated by their gain factor. (i.e. the amount by which they amplify the signal they are given) The 12AX7 has the most gain, and the 12AY7 is a direct substitute but with less gain. The 12AT7 also has less gain but requires a slightly different bias voltage for optimal operation (it can be directly substituted in a pinch).
The most common pentode preamp tube used in guitar amps today is the EF86. (or 6267) This is found in early Vox amps, and more recently in models from Vox, Dr. Z, and a few others. Whilst having a similar Noval 9 pin base, it is not compatible with the dual triode and requires different circuitry.
Preamp tubes are normally self-biased and as a rule, do not require further bias adjustment when installed. Always refer to your original equipment manufacturer's specifications to confirm requirements for your specific amplifier.
MULLARD, JJ, SOVTEK, TESLOVAK, PHILIPS, SVETLANA, TESLA, E HARMONICS
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