CT100 Convergence Transformer Solid State Replacement Update

[Penthode]

Hi All. I had taken a year long hiatus from this project as there had been a lot of things going on. I began revisiting this last month and spent time repairing my old Tektronix FG504 Function generator to provide an experimental test sinal for this project. It generates 30v p-p which matches roughly the signal required to feed the convergence transformer.

Here is the link to the last posts a year ago.
https://videokarma.org/showthread.php?t=276331

I left off a year ago with partial success. The circuit improved convergence but the parabola swing was not of sufficient amplitude. The limiting factor was the 400v B+ supply voltage which I was using. I figured if I could find convenient way to obtain a higher B+ to run this circuit I could make it work.

The Power MOSFET I was using had only a maximum Drain Source voltage of 800vdc. I needed to obtain something which would work at a higher voltage.

This past week I cascaded two HP high voltage B supplies to obtain 800vdc. I also obtained an IGBT (Insulated Gate Bipolar Transistor) which is an odd hybrid between a mosfet and a standard junction transistor. The one I have been experimenting with is a IGW15T120CR. You may easily find the specs with a Google search. It has a Vce maximum voltage of 1200v and a phenomenal transconductance of 10S or 10,000,000 umho! (To me a transconductance that high is creepy, especially when dealing with early vacuum tubes like the UX201A which has a transconductance of about 650umho on a good day).

Anyhow, I have been testing the past two evenings on the bench. I was previously obtaining a linear amplification of about 400v p-p which was just barely insufficient to obtain the vertical convergent at the top of picture. The past two days with the 800vdc supply, I am able to obtain linear amplification to 725vp-p. I am tusting this will leave more than ample headroom to prevent distortion of the parabola convergence waveform.

The last item was where to obtain the 800 volts. I had previously considered a separate power supply or tapping off the boost B+. Neither solution appealed to me, Then I turned to the CT100 power supply itself. The Voltage doubler used in the CT-100 can with the addition of two diodes and two capacitors can and to it a voltage quadrupler circuit. I tested it and it works fine with output topping 800 volts. The IGBT collector current is only about 2mA so the transistor only gets mildly warm without need of a heatsink.

Please find my attachments of the circuits. Also the scope output in two screenshots because the lowest sensitivity of the scope was 50 volts per div for a full vertical screen of an only 400v span. The 727vp-p is covered in the two screen shots of the upper and lower half of the waveform.