Uber Parts Quality and then some (Part 2)
The rest of the Tube Amplifier Circuit of the Retro Stereo 50 is as classic as they come, though the use of modern SMD components including MELF resistors...
and C0G capacitors...
...take the performance of an old friend to new heights.
How good are these parts?
Put it this way, for the revised AMR Signature Series, the C0G has superceded the Sanyo OSCONs capacitor which was used in the digital section of AMR machines. A case of trickle-up technology. Yes, these have also been used in the micro iDSD as covered in the Crowd-Design thread.
The tube circuit itself is ultra-simple, yet also highly refined.
The ECF82 driver tube is what is called a "compound tube", it contains a small signal pentode and a fairly high power triode.
The pentode is employed as the only voltage gain stage, in this it is very similar to the design of the classic Western Electric type 91A amplifier, which introduced the 300B tube in single ended operation to the world and which is still held in highest esteem among the tube cognoscenti.
The triode is directly coupled to the pentode gain stage and operates as what is called a "split load phase splitter". It provides the two absolutely equal, but opposite drive signals to the power stage. As this triode is quite powerful we use this power to create a powerful driver stage for the power tubes, with much better drive than the ECC83 tubes often found as drivers in EL84 Amplifiers.
The only coupling Capacitors (German Wima MKP-04 types) in the Amplifier link the driver stage to the EL84 power tubes, which operate in cathode biased mode for maintenance and adjustment-free use. The output stage uses a configuration named "Ultra-linear" which gives a good mix between the high efficiency of pentode operation and the smooth and musical sound of triode operation. Around 15dB of negative feedback are used to lower output impedance and distortion, just enough to improve performance but not so much to create the dead, high-feedback sound many solid state amplifiers suffer.
Next time....Power is everything (Part 3)
