Building a Modified Mullard 5-20 Amplifier – by Dave Kimber

Dave Kimber writes .....

The Mullard 5-20 design is about 50 years old. It uses a pair of EL34 in almost Class A push-pull ultralinear configuration to give 20 Watts with very low distortion and up to about 30-35 watts with some distortion. The phase splitter uses a cathode coupled ECC83, driven by a directly connected EF86. There is lots of information about the 5-20 on the web.

My modifications

Various modified versions of the 5-20 have been built by lots of different people. One of the most common mods is to use a solid-state HT rectifier. I have done this, using BYV26E soft-recovery diodes. Another common change is to use triode wiring for the input EF86, in order to reduce noise. Instead of doing this my triode input uses half an ECC81. This is mainly because of the cost of decent EF86s.

I retain the direct coupling to the phase splitter; this is more or less the original Mullard ECC83 circuit. For the output stage I use a combination of fixed and cathode bias. I increased the value of some coupling/decoupling capacitors as this seems (according to Spice) to improve the LF stability, which was rather marginal in the original circuit - notice the hump right on the LF edge of the passband in the Mullard frequency response plot. When I get the time I will put together a more detailed description of the circuit I used.

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