Case No. 5) DIY 6BX7 Mono Block Amplifiers

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A customer brought in a pair of DIY 6BX7 Mono Block Amplifiers which were suffering from power loss, premature clipping, excessive hum and an overheating transformer.   The customer explained that he had already paid hundreds of dollars to several technicians, who replaced some insignificant parts but didn't solve the specific problems.  In his last attempt, a local technician charged him $ 200 dollars just for changing one resistor to no avail, but was told by the technician the amplifier was modified and "tuned" to perfection. 

These amplifiers failed our bench test miserably, registering an output of 5 watts when it should be delivering 15 watts or more based upon its configuration.  The distortion test were equally unimpressive.  The circuit was properly designed but incorrectly implemented in many areas.  We decided to completely rebuild the circuit from the ground up using point to point wiring:

• Added a choke stage

• Changed the circuit to auto bias

• Reconfigured the power tubes to parallel arrangement

• Added a soft start stage

• Added NFB

• Added voltage regulation

Afterwards, we bench tested the amplifier and it gave the following readings:

• Output: 18 Watts

• Freq. Range: 20hz-20khz +/- 0.01 % distortion

• S/N Ratio: 100 db

These reading are very good for a DIY design   Over the course of the next several months, we worked with the customer to fine tune the sound of the amplifier by making small changes to the circuitry.   The customer was extremely satisfied with the results.

Total Cost of Modification:  $ 700 dollars

Turnaround time:  5 days