Saturday, November 24, 2012

"Heavy Metal"- 6N7 metal valve preamp

In a box of very old valves, I was given recently, was a new looking pair of black metal can RCA 6N7s. These are dual triode with a common cathode. I could find no schematic for a preamp but a DIY buddy came up with a design. Construction is in a timber box. There are three switchable inputs and power is supplied by an external 18V 2.2A AC power pack I was also given. Internally 255V is developed with a linear PS and regulated 12V for the heaters in series.

An Alps blue velvet pot is fitted to the output for attenuation with a fine solid brass knob to compliment the gold right-angle RCAs, name plate and ventilation holes. Wire Warp wire is used in the signal path with PS and amp on separate brds. No PCBs or shielded wire is used. Bitumanised heavy Al foil lines section of the preamp for resonance control.

The preamp is dead silent at idle with stunning drama and dynamics. It's fast, crisp, clean and makes every track sparkle. Best preamp I have ever built, SS or valve based. Nothing comes close to this one. Nothing!

Monday, November 12, 2012

4S Universal valve preamp in a Tea box (4SUTea)

All housed in a Twinnings timber Tea Box: Bitumanised Al foil controls enclosure resonance and Russian paper in oil caps link the modulated HT to the real world. Silver solder is used throughout. Heavy duty high grade gold plated RCAs grace the back and all is powered by a 24V 1A plug pack. There are two switchable inputs, Alps blue velvet pot and black chromed solid brass knob.

Power supply and amp reside on a single piece of vero (strip) board. No PCB or shielded wire is used. Any 12A?? tube can be inserted with no components changes. Sound is big, bold and fast with a Mullard 12AX7 in place.