CT-100 B1001599 Failed Convergence Transformer

[Penthode]

Here is the results on my first test with the single MOSFET. Yesterday I bought an IRF830-1 500v 1A MOSFET locally. It has a forward transconductance of minimum 2.5 Siemans which is 2,500,000 umho. That is a whopping lot of gain available espectially when you are used to dealing with only a few thousand umhos with vacuum tubes!

With the single FET, there was too much gain. I added a 330k resistor on the input side to brring the gain closer to 11 times. I was able to achieve 350 v p-p on the output with the input only 20 v p-p. (My HP 204 Sine Generator maximum output).

I found non-linearity (no clipping) became noticable at 250 v p-p. You will see the output trace slight distortion in the photo.

I found I could slightly reduce the non linear distortion by increasing the drain current slightly, hence the reduction of the source resistor from 5.6 kohm to 4.7 kohm.

Note the phase shift. Testing the circuit at 600Hz the phase inversion is exactly 180 degrees. At 60Hz as depicted in the photo there is a shift. I tried bridging the coupling capacitors which did not make much difference. I believe the shift may be due to one of my scope probes. I am investigating this.

the circuit was tested as per the diagram including the 27k 12AU7 plate load and the 560k and 330k load resistors.

Next step is to put the CTC2 chassis on the bench and perform some measurements.