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.