Class-A 6SN7 Stereo Amplifier
Class-A 6SN7 Stereo Amplifier
project progress
This project is a four-watt-per-channel stereo amplifier using two 6SN7s in each channel as output tubes. The 6SN7 is known for its full-bodied, rich sound. Two 6SN7s paired in parallel deliver four watts of Class-A sound comparable to more expensive triode output tubes.
A BUD manufactured chassis is used for this build. It is a traditional amplifier layout with a front panel and rear connections. Speaker output posts are on top. The bare aluminum chassis, as shown, has been lightly sanded using 400-grit sandpaper. This removes sharp edges around chassis holes and provides better primer paint adhesion to the chassis.
The chassis is shown after painting. Spray cans of Rust-Oleum oil-based paint were used. It was first given a coat of primer paint. The primer coat is applied just enough to completely coat the chassis. The primer was allowed six hours for drying. After the primer dried, the finish coat was applied.
Painting was done in a shed outside. The finish coat was allowed to dry in the shed overnight. The chassis sat on the work table for a couple of days to allow the paint to completely cure before mounting components.
The majority of the components that mount onto the chassis are mounted. Each socket has a plate with the tube type engraved on it. Since this is a design-as-you-go project, there will be some additional drilling for tie point terminal strips as needed.
Finding transformers for a project like this can be a challenge. The power and output transformers were purchased from Musical Power Supplies in Columbia, Tennessee. The PT275.2 power transformer has an unusual configuration. It has two 6.3-volt windings, one with a 5-volt tap. The high-voltage secondary has a 55-volt bias tap.
The output transformers have dual primary impedance taps of 5,000 ohms and 7,000 ohms. The secondary taps are 0, 4, 8, and 16 ohms. Using different output taps, the primary impedance can be set to different values. Amplifier output would be considered 8 ohms regardless of which output taps are used.
A 6-Hy choke is mounted inside the chassis under the power transformer. Two large filter capacitors will be mounted in clamps inside the chassis at the back side. Terminal tie point strips are mounted with the same screws that mount the tube sockets.
There is a bottom plate for the BUD chassis. BUD provides four holes for mounting the bottom plate with self-tapping screws. Rather than use self-tapping screws, the original four holes were drilled out and tapped for 10-32 screws. Four additional holes for 10-32 screws were added for a total of eight mounting screws. Aluminum is a soft metal. The holes need to be tapped carefully. When the bottom plate is secured, the screws cannot be over-tightened; the screws are tightened until they are just snug.
A ground wire for the input jacks was pre-wired before mounting the input jack plate. The input jack ground wire connects to the speaker ground posts. All the ground circuits in this amplifier will be connected together with wire.
The circuit above is what will be used for the power supply. The filament load is divided between the two transformer filament windings. The filament winding rated at 3 amps has V1, V2, V3, and V4, the four output tubes with a total load of 2.4 amps, 4-6SN7 at .6 amps each. The filament winding rated at 2 amps has V5, V6, V7, and V8; 2-6SN7 at .6 amps each, and 2-12AY7 at .3 amps each, with a total load of 1.8 amps.
I have most of the power supply parts. I seem to have run out of 1N4007 rectifiers and will need to order more. I also need to order the .01 uF 250 VAC AC power inlet noise filter capacitor.