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 broadband, unweighted 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, an open, effortlessly detailed presentation across the entire range at powers from 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 and PEARL tube coolers throughout.    
       
       
 

   
  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 single-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 seen 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.