freq - frequency in Hz
3.70 - excitation from one battery (3.7V)
7.40 - excitation from two batteries in series (7.40V)
In no case the current from excitation is allowed to go above 2A. In no experiment the current is observed to be above 1.5A.  
The results in tables are output voltages measured at 1000:1 HV probe, hence the output voltage is given in kV.
Push-pull configuration with two TIP122 NPN transistor with 330 Ohms resistor in base is used. 
Single cascade is used with 30 capacitors and 60 (30x two-in-series) diodes 1N4007.
Power supplied by RIGOL DP832 at frequency for max output DC voltage is measured, too.
 
 ETD49 core with 4 isolated layers as secondary, no gaps and 2x3-turn primary 
 freq   3.70   7.40
 ------------------
   1k   5.28   9.09
   2k   7.33  13.00
   3k   8.72  16.59
   4k   9.96  18.43
   5k  10.79  19.75
   6k  11.22  20.74
   7k  11.46  21.47
   8k  11.57  21.99
   9k  11.60  22.42
  10k  11.42  22.48
  15k  10.93  22.02
  20k  10.52  21.77
  25k  10.21  21.56
  30k   9.86  21.07
  35k   9.40  18.52
  
  Max power @ 9kHz / 3.7V = 1.71W
  Max power @10kHz / 7.4V = 7.30W

 ETD49 with one isolated layer as secondary, no gaps, 2x2-turn primary 
 freq   3.70   7.40
 ------------------
   1k   3.16   4.91
   2k   4.25   6.78
   3k   5.11   8.41
   4k   5.70   9.73
   5k   6.14  10.91
   6k   6.47  12.03
   7k   6.80  13.05
   8k   7.07  13.99
   9k   7.28  14.91
  10k   7.45  15.44
  15k   7.96  16.33
  20k   8.08  16.61
  25k   8.05  16.45
  30k   7.93  16.25
  35k   7.81  16.05
  
  Max power @20kHz / 3.7V = 2.72W
  Max power @20kHz / 7.4V =11.44W

 Trafo #3 (Git repository > Hardware development > 4-HV > Trafos)
 freq   3.70   7.40
 ------------------
   1k   6.16  12.87
   2k   8.34  15.55
   3k   9.60  17.33
   4k  10.32  18.42
   5k  10.98  19.48
   6k  11.22  19.85
   7k  11.20  20.37
   8k  10.66  21.07
   9k  11.94  21.82
  10k  11.19  22.12
  15k   9.65  22.17
  20k   9.35  21.68
  25k   7.64  21.23
  30k   7.56  17.17
  35k   8.86  18.22
  
  Max power @ 9kHz / 3.7V =1.06W
  Max power @15kHz / 7.4V =4.10W