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I'd like to announce the birth of twins. On friday the 23rd of Feb. weighing aproximately 50lbs. each was born a pair of parallel 6550 differential mono-blocks. Transformer coupled using Lundahl iron, forced class A biased using SS constant current sources suppling the bias current to each stage these pups sounded awesome right out of the gate! They continue to improve as the trannies have a chance to break in. Anyone interested in the design should check Kevin Carter's web site over on the K&K Audio forum.
Follow Ups:
Can you elaborate a bit more on how you implemented the CCS for biasing your output tubes? Thanks and enjoy!
The CCS's are Kevin Carter's design. They consist of a MOSFET and a single pot. with an additional gate stopper resistor. For the output stage there are four of these run in parallel. The cathodes of the output tubes are all connected together (acctually there are 1 ohm current sensing resistors in series with the cathodes); the plates of the paralleled tubes are also connected together. The current sources are set to deliver a total of 240ma to the output stage (does require matched quads). Hope this answers your question. The circuit for the amp, except for the output stage, is very similar to the Art Audio Vivo PP 300b amp designed and built by Kevin.
Enjoy !
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