50 Watt, KT88-based Monoblock Power Amplifiers

     
. . . . .These are a pair of hand built, point-to-point wired, Class AB1 pentode power amplifiers.
. . . .The output power is 50 watts from 20Hz to 20KHz at approx 0.1% distortion plus noise. The small-signal output power extends within 1dB to 100KHz while the signal-to-noise ratio wrt to a 5V output is >95dB.
. . . .Featured are individual voltage regulators for all the stages except the main output's B+. The differential input stage, the capacitively-coupled driver stages, the negative rail for the servo-controlled bias for the power output tubes and their screen grids are all regulated, as are the voltages for the filament supplies in the four small-signal tubes.
. . . .First class capacitors and resistors are used throughout while all the wiring has been carefully routed for best SNR and lowest inter- and intra-stage crosstalk.
. . . .All tubes are PEARL CryoValves with a 100%, 90-day warranty and a 10 month sliding scale warranty. See details on PEARL CryoValves here.
. . . .Not shown are life-extending PEARL Tube Coolers on all tubes, details of which are seen here.
. . . .In all, these are beautiful sounding power amplifiers with superb top and bottom extension and an open, and effortlessly detailed presentation across the entire range and at powers from almost whisper quiet to full-on.
       
       
       

     
  Fig. 1 - The paintwork is hand polished, two-part epoxy.    
       
 

     
  Fig. 2 - Although Chinese made KT-88s are shown here the amplifiers will be supplied with Russian-made KT-120s in addition to the '88s.    
       
       
 

   
  Fig. 3 - The leftmost cylinder houses the input transformr while the center and rightmost house the B+ chokes. The round black 'bump', near the front on the side is the switch between singl-ended and balanced inputs.   The left rectangular can is the output transformer and the right is the power transformer.
       
       
 

  Fig. 4 - View of all connections to the amplifier.    
       
       
   
  Fig. 5 - Input jacks closeup. On the left is an XLR by which means one can connect a balanced input driving the primary of a transformer. On the right is the standard, single-ended RCA. The input is switchable from balanced to single-ended.   Fig. 6 - Rear connections from the inside.
       
       
   
  Fig. 7 - Left: filament regulator; Center: screen regulator; Right: main B+, optimized for step response.   Fig. 8 - Fast recovery rectifier bridges can be seen in this shot. They are the flat, black, four-legged parts with holes thru them seen left and right.
       
       
 

 

  Fig. 9 - Diff amp input, note the pair of metal can current regs in parallel, these will be replaced with a proper current source tied back to a negative rail once the Tent Labs/Menno van der Veen auto-bias board is installed.   Fig. 10 - Half of the two tube, parallel-connected 12AU7 driver stage.
       
       
 

 

  Fig. 11 - The other half of the parallel-connected 12AU7 driver stage.   Fig. 12 - Driver stage shunt reg.