Presumably this is a 3-stage circuit: input stage, driver stage, output buffer? What is the overall circuit gain? How much negative feedback is applied and where - local, global?With the unusual output tetrodes being twin-tubes in one envelope, you're really running 16 tubes in the output stage. Any unusual configuration there? What is the effective output impedance? What are the S/N ratio and operational self noise figures? What type of output transformers (C core, double C core, EI, toroidal, other)? Which stage handles phase splitting and is this done by tube or transformer? Where in the circuit is the volume control embedded and what is it (presumably an Alps Blue or equivalent pot)? What is the power supply capacitance? What is the output power into 16Ω, 4Ω and 2 Ω?

Answers. "Our class A amp uses a 12AX7 cascode input with long-tail phase inverter to drive the push/pull power tubes. Phase splitting is thus by tube. See the multisim graph at right [enlarge by clicking on expansion icon - Ed.]. This nets 50dB of front-end voltage gain with max distortion of 0.1%THD. The ratio of the power stage, from grid 1 of the QQE to the output transformer speaker terminal, is nearly 1:1 to have no gain. 20dB NFB is a mix of local and global. By using 4 double tetrodes per channel, the sum total is 16 output tubes with really spectacular auto bias. This unique circuit has proven stable for more than 20 years. It supports our tubes under all static and dynamic conditions. An opamp on each QQE cathode sets the negative grid 1 voltage. This doesn't require costly matched tubes to save the customer a lot of money. On grid 2 and the power-tube cathodes we use reversible fuses for protection to never need service. The S/N ratio is -86dB. The output transformers are EI cores from Germany's Pikatron. An internal 4Ω tap allows us to rewire them on demand. 2Ω out equals a damping factor of 4. The pot is an Alps Blue RK with logarithmic curve. Power supply capacitance is just 330/450VAC per channel. We don't recommend this amp for 16Ω or 2Ω loads."

Here we see the Siemens QQE 03/12Y output bottles with their biasing opamps; and the 12AX/AU7 input/drivers at right. Bandwidth is 15Hz - 70kHz. Power consumption is 8 watts in 'remote' mode (red display); 80 watts in stand-by warm mode (blue display, no red LEDs on mother board); 220 watts in power mode (blue display, red LEDs on mother board on).

Here we see the relay-switched inputs and outputs in the left corner; the Alps pot tucked below the suspended red board on the right; and the compact EI-core output transformers strategically rotated for minimal stray radiation between them. Half the power supply can be seen at the lower left.

Finally the controller board.