About a year and a half ago I decided to convert the output stage of my 50-watt Marshall combo into a self-biasing cathode design. This method of biasing is normally found only in smaller amps because it dissipates a lot of heat in the cathode resistor and is considered to be inefficient. In other words it consumes a bit of more power. The old 20-watt Marshall heads built in the late 60's utilize this method, but all 50 and 100-watt models use fixed biasing. I was trying to reproduce the sound of my little Jaguar (that's my '69, 20-watt head that I totally redesigned and that been refinished in fake leopard skin).
The results were very impressive, the clean sound was now noticeably sweeter and had better sustain. Another benefit was that when the amp was driven to full power under hard clipping, crossover distortion was greatly reduced due to the self-biasing design With hard picking the amp was now less peaky and more compressed. I can't say that I noticed any loss of power. For good measure I installed a pull-switch on the amp in order to compare between the original fixed-biased circuit and my new cathode biased one. I played this amp constantly in all kinds of venues from bars to outdoor blues festivals and I can tell you now that I'm never going back to fixed-bias. To hear a good example of this, try out the Fender Prosonic. There is a switch in the back to select between solid state rectifier, tube rectifier Class AB, and tube rectifier Class A (Cathode-biasing).
Well, now all that remains to try is to see if I can do this safely on a 100-watt model..... can't wait!
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