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Manley Reference 350 Mono
Block Amplifier Repair
This customer was
unfortunate enough to have killed his entire set of
replacement tubes for his Manley Reference 350
amplifiers. Despite having turned down all
16 biasing trim pots as explained in the owner's manual,
all the tubes were burnt out within seconds of powering
on. The customer was afraid to experiment
with another set of tubes.
We diagnosed the problem and
performed a simple biasing resistor change from 1000
ohms to 100 ohms. This change increased the negative biasing
voltage which then allowed the use of tubes with
different biasing points. We also changed
the slow-blow B+ fuse from 20A to 10A.
There are 4 different types of KT90 tubes
on the market and each batch bias differently than the
other. The bias resistor and trimpot does
not provide enough of a "range" to accomodate KT90 tubes
which bias differently than the original. In
fact, this is a common problem among Jadia, VTL, Manley,
Raymond Lumley and Audio Research amplifiers.
Has the customer ordered
replacement tubes directly from the factory, the problem
could have been avoided. We have seen a
similar occurrences with VTL and Jadis amplifiers.
This type of biasing problem can be easily avoided if
customers were simply aware of the biasing requirement
of these great amplifiers.
Total Cost of Repair: $ 50
dollars including $ 10 in parts.
Turnaround Time: 30 mins,
repaired on spot. |
