Case Cleaning - Nagra CLASSIC PREAMP Bedienungsanleitung

Thus, the useful life of the tubes is somewhat unpredictable. Rather
than replacing the tubes arbitrarily after 5'000 hours, we suggest
that you identify the signs of ageing:
 The distortion gently increases to an audible level;
 Presence of clicks (dry and short noise, like breaking dead
wood);
 Presence of pops, short noise in the low frequencies;
 Presence of hiss, higher background noise.
As soon as one of these signs appears, please contact your Nagra
dealer to order a CLASSIC PREAMP replacement tube kit, reference
7055764000.

Case cleaning

Clean the CLASSIC PREAMP casing using a soft, non-fluffy, slightly
damp cloth. Do not use any cleaning products that could have a
corrosive effect.
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NAGRA – CLASSIC PREAMP