Circuit description:
Dirt simple... KISS concept. ECC81/12AT7 dual-triode. First section configured as a classic common-cathode amplifier. It simply provides voltage gain. Values are chosen for a low-frequency cutoff to avoid subsonic fluctuations (things like record warps, rumble, etc.). This is direct-coupled to the second section which is configured as a cathode follower. This drops the drive impedance considerably and is AC-coupled to the 45 grid. The current-flow in the driver is calculated against the input capacitance in the 45 triode, the voltage swing required for maximum output and the slew factor required for a wide frequency bandwidth. I've scoped some measurements out using square waves... input to the circuit and output to the 45 grid are virtually identical sans the phase inversion and voltage gain.
The output stage is the 45 triode feeding a RH-40 output. I also add my RH-factor with the split balancing technique I developed some time ago for my ultra-expensive SET amps. This uses the center-tap on the filament transformer with the bias resistor to ground which results in a fixed DC balance. I do this as the DC resistance in the filament transformer secondary is much lower than you can ever get with a balance pot and provides an even current flow across the directly-heated cathode. The 100-ohm balance pot is padded with a pair of 12-ohm WW resistors and the bypass capacitor to form the AC balance which can null-out the resultant hum from the AC-operated filament. With really good tubes, I've measured output noise as low as 125 micro-volts. Good tubes will usually null-out to 250 microvolts or less... marginal tubes can be upwards of 500 microvolts.... those should not be used.
The power supply is straight-forward.... classic FW-recitifer tube, pi filter using 2 caps and a filter choke and a simple RC decoupling to drive the input gain stage. With a zero-feedback design, specifications are very good and using good parts will certainly help.
HP 334A Analyzer used with the usual compliment of test equipment.
Measured specifications taken at 1-watt output:
20Hz - 40KHz within 1dB (James H3115... expect the RH-40 to do better)
THD less than 0.4% (less than 1% at 2-watts output)
Signal-to-Noise ratio: 80dB or better (referenced 1-watt/8-ohm)
Output noise ~ 250 micro-volts
Gain - approx. 11dB (775mv for 1-watt output - 2.83VRMS into 8-ohm)
Have attached the schematic, but it's a bit tough to get the image clear enough. Anyone can send me email and I'll send them a PDF of the scematic and a BOM list text file.
Regards, KM
