C3G Single Ended Triode
This is the continuation of the C3G story. My first DIY headphone amplifier was based on this lovely pentode (and I also still have a decent supply of them), so I wanted to try this tube in a transformer coupled single ended schematic. As I don't have a lot of experience in circuit design I tried to keep things simple. After looking at the loadlines I decided to run the signal tubes in cathode bias, having them sitting at around 2V and 15mA with a supply voltage (B+) of around 150V. No parafeed this time or CCS in the anodes... just a classic single ended schematic. I could have gone for LED bias but choose to use matched carbon resistors with Black Gate N cathode bypass caps. With only a few high quality parts in the signal path you don't have to skimp on quality.
The power supply consists of a big custom wound toroid, two russian 6D22S half wave tube rectifiers followed by a LC prefilter section and a Salas SSHV regulator. Heaters are DC with another LC element (common mode choke and capacitor) following.
schematic, not final
On the breadboard I tested the circuit with different output transformers ranging from a pair of vintage Grundig transformers out of a console over SE-OPT's from german winder Reinhoefer to Lundahl LL1689 that I finally settled on. The vintage Grundigs actually sounded quite nice but were band width limited on top and bottom, rolling off treble slightly and being too soft in the bass.
The main idea for the case design was a kind off "slab look" with a simple massive board as a base, only exposing the tubes and the transformer on the top side. All four tubes are mounted on a semi floating and polished aluminum plate, just next to the volume pot knob that is matching the appearance of the signal tubes. All the other components and modules are hidden and mounted underneath on a CNC cut aluminum plate... and it all just barely fit! The case board is finished in ancient Walnut Burl over multiplex and is sitting on four massive aluminum feet.
Historically speaking this is my fourth tube amp, and it also is my best. It has been designed with the Sennheiser HD800 in mind and definitely has the qualities that make music come alive... it has really good detail and gives you the impression to be there, to be really close to the music. This is not one of the honey-sweet sounding amplifiers, in fact it is surprisingly fast. At the moment the C3G SET is my favourite amplifier for Classic and Jazz.