Summit Audio MPC-100A Mic Preamp Compressor - Manual (Page 9)

A Word About Tubes
The tubes that are used in the MPC-100A are selected to give the best possible
performance in the position that they are in the circuit. Switching them to
different positions might cause performance deterioration of the audio path.
Replacing them with “gold” or high end tube types may not be desirable. In cases
that we have measured, these tubes have shown higher distortion and noise
compared to the tubes we have selected. In some cases, the so called “gold” or
high end tubes have made the unit unusable. The reason for this is some of the
“gold type” tubes are selected for high distortion in guitar amplifiers. Using gold
tubes is no guarantee of better performance. All of these so called “Brand X”
types are selected with 6.3 volts AC on the filaments, whereas Summit Audio
uses 5 volts DC on the heaters for longer tube life and lower noise. The reduced
gain may raise the noise floor, increase distortion, and reduce headroom.
For proper performance from a tube, the replacements must be selected using 5
volts DC on the filaments. The tubes we use are selected in the circuits that they
are used in to ensure proper operation, long life, and low distortion and noise.
In at least 50% of the cases we have tested, gear that has been used for several
years actually has lower noise and distortion levels than when new. This makes
the question of when to replace tubes difficult to answer. If a tube turns micro-
phonic, the distortion will be obvious and the tube must be replaced. However,
tube life will most likely be greater than 10,000 hours of operation. Tubes are
generally very reliable; don’t replace them just because they are old. In the MPC-
100A there are gain adjustments that will need to be checked when the tubes are
replaced, or else the metering could become inaccurate and the noise floor could
change. Replacement should be done on the bench with a distortion analyzer
attached to ensure that the distortion levels are proper and it is comprised of
second order harmonics.
Before replacing the tubes in your MPC-100A, please talk to your dealer, call
Summit Audio, or find a technician who has experience working with tubes and
high end audio equipment.