"Mirage" - 6H3N-EB preamp

There are many strange and wonderful tubes you can build preamps and power amps with. When repairing an Asian made 300B I thought the driver tubes (6H3B) may have been faulty. Accidently I ordered two triplets of 6H3 tubes one triplet 6H3N-EB Russian tubes. The 6H3Ns ended up in a headphone amp and the "EB" tubes in this two-stage preamp. Thankfully both builds perform beautifully and are as pleasing to the eye as to the ear.

"Mirage" is another example where a solid plastic enclosure looks like a metal one. This trick I have performed a few times in the past with good results. Also to add to the look this one is sprayed two-tone in Epoxy Enamel. Inside there is a mix of carbon film resistors, metal film and wire wound. Caps are a mix of polies, X2, and low ESR electrolytic. Further HT filtering is carried out at the front-end. This preamp is dead quiet. But the interesting component is the output cap. This is a vintage Australian PMG made NP 1uF cap. The PMG is where I started work in 1970 and was Australia's fledgling telecommunication government owned company. Evidently, they made a lot of their own components and that's where I picked-up the rare cap fifty years ago.

The PS is external and there is another external box with the stepdown tranni and power filter. The PS is choked in a double Pi configuration (300uf of filter caps) and there is DC on the heaters. The AC PS plugs into the PS and the PS plugs into the preamp. Two inputs are provided and hookup wire is fine silver plated copper wire. The knob is custom-made of American Oak on a black Al base. 

The topology is simple plate follower front-end followed by an SRPP backend. This combines good gain with very low output impedance. The sound is very clear almost SS like. Bass extends to 30hZ and mids upfront and very clearly and very cleanly presented. I've heard greater transparence in my other tube preamps but not the clarity this preamp presents. From cheap tubes and a plastic enclosure comes a true treasure of a beautiful simple preamp.