Another 2A3 SE amp !!!! oh no !!!!
What in the added value in my implementation?
What I present in this blog is the result of many, many….. hours of listening sessions to various configurations and components about a SE class A 2A3 Amplifier. My target was the best design to reproduce the body, sonic texture and fine gradation characteristic of natural sound. This is what, in my opinion, is the best result in terms of listening pleasure, that I obtained. First of all, I think is useful adopt the monoblock concept for optimize power supply and internal wiring ( layout), furthermore ensures short cable between amp and speakers. The starting point for any DHT-SE tube amp in the couple audio output transformer- DHT power tube. In this critical stage I choose the well reputed excellent Tamura F-2007 SE output transformer: 3Kohms primary impedance ( 100mA ), 4-8-16 ohms in the secondary side and Shuguang Treasure 2A3-Z as power tube.
About the topologies I test various type starting with the classic Loftin-White direct coupled, RC coupled with various tube front end. I try out pentode ( 6SJ7 ) or triode ( 6J5 and various other ) in the first stage. In the RC capacitor coupled schema I test several type of capacitors : paper in oil NOS or new production, polystyrene, polypropylene and so on. At last I choose the IT interstage transformer coupled topology with fixed bias for DHT power tube.
As input/driver I found that the old pentode EF6 in triode mode strapped goes very well with Lundahl LL 1660 10mA interstage transformer. The high primary inductance of 130 H ensure good low frequency response. To get out the best performance from couple EF6/Lundahl 1660 I thought to supply it with a fine tube CCS ( Constant Current Source)/voltage regulator.
Being very simple in circuit design, every part is important when building quality DHT SE amp. You should know the tonal characteristic of the part you are using. For my ears carbon composite resistors ( Allen Bradley 2W ) and paper in oil ( or polypropylene in oil ) in PS give the best audio natural performance. In power supply, I want to try TV damper vacuum diode as rectifier for their excellent specs about voltage drop, huge peak currents, and slow warm-ups (about 30 seconds). Furthermore TV damper diodes are the quietest from the viewpoint of switching noise. Power supply for HT is a fine CLCLC schema. The fist capacitor ( 4 uF) MUST be paper in oil the other polypropylene in oil or MKP. For wiring in the signal path use thin solid core copper ( max 0.4mm in diameter ) teflon or cotton insulated. Only for filament supply thicker stranded wiring can be used. In the EF 6 cathode R-C group I think the best choice is Black-Gate or Elna Silmic II and a Allen Bradley Carbon composite 2 W resistor ( or Mills wirewound ). About the schema layout It’s good thing also providing separated ground returns for both low-level ( SG ) and high-level ( PG ) as in the schema below. To reduce hum I adopted a LM317 based regulated slow turn on power supply for EF6 filament. The N ch mosfet souce follower in the negative bias circuit keep constant the 2A3 grid voltage even if a little grid current flow.