This article will show a variation of the transformer coupled single ended amplifier concept. It is based on the 46, which was presented as tube of the month in the previous post.
The amp was built mostly with parts which I had available. The parts cost is moderate, even using a heavy duty power transformer, ASC oil caps and all Lundahl signal transformers and chokes, the parts add up to about 1100-1200 Euros, excluding chassis. It uses all the features of this amplifier concept which contribute to the overall sound quality: Multiple choke filtered power supply, high quality caps, interstage transformer coupling, ultrapath caps in each stage.
I still have some power transformers in stock with center tapped secondary and 3 independent 6.3V windings. To keep cost down, I used this one. Only this single power transformer is needed for B+ heater and filament supplies. The 2.5V AC voltage is obtained through dropping resistors from two of the 6.3V windings. The third 6.3V winding is used to supply the heaters of the rectifiers and the 6N7 driver tubes. This requires the heater winding to be referenced to ground. Two 6AX4s are used in a full wave configuration. Since it can withstand a large voltage between heater and cathode, the heater winding can be shared with the driver tubes. The PSU is common for both channels, only the filament windings of the output tubes are separate:
The B+ supply is the usual choke input PSU using two LC sections for smoothing. Heaters and filaments are fed with AC. Since the heater windings have no center taps, a hum buck potentiometer is wired across the filaments of each of the 46. Two resistors are wired from the wiper to the filament terminals to reduce the resistance of the 100 Ohm Pot which I had available.
The rest of the amplifier circuit is similar as presented in previous parts of this series of posts. my favorite indirectly heated driver for small output tubes, the 6N7 is used again, both halves wired in parallel. The output tube is driven by a Lundahl LL1660/10mA, wired almost 1:1 (actually it is 1.125:1). I had a pair of Lundahl LL1682/50mA in stock. These fit quite well to the 46. With the primary impedance of 5.5kOhm and a secondary of 5 Ohm they will provide a good damping factor when used with a 46 on 8 Ohm speakers. The 46 is wired for Class A use, with the second grid tied to the plate at the socket. See the 46 datasheet for details of the tube.
The B+ is around 300V. The 46 is biased at around 30V. With voltage drops across chokes and output transformer primaries this results in just under 250V across the 46.
Below, some photos of the construction of the amplifier. The first picture shows all capacitors, sockets, connectors, switches and resistors mounted and some initial wiring. The power transformer is placed on the top side of the plate and will be hidden under a cover.
The next photo shows the inside with all chokes, interstage and output transformers mounted and completely wired:
The top view of the completed amplifier:
The amp sounds very smooth and delicate. It shares a lot of qualities with the best 45 amps at a lower cost and size. The same circuit can be used with minor adaptions for 45 or 2A3 output tubes.