Buck converter for high impedance source

[Les Jones]

The maximum amount of power that can be obtained from the end of the 1780 ohm resistor is when the load is also 1780 ohms. this will be when be when the voltage at the end of the resistor is 52.5 volts (105/2) So the maximum power will be 52.5^2/1780 = 2756.25/1780 = 1.548 watts As the required heater power is 1.4 watts the efficiency would need to be 1.4/1.548 = 90.4% Expecting to achieve this is being unrealistic.

 

[Les Jones]

While looking to see if there were any nuvistors with higher voltage / lower current heaters I came across the 13CW4 which has a heater of 15.5 volts at 60 mA If you just soldered a resistor across R4 to increase the current slightly this might solve your problem. I found one of these on ebay
**broken link removed**
This is some data on it https://www.drtube.com/datasheets/13cw4-rca1964.pdf

Les.

 

[Marcus4audio]

What is the AC Output Voltage on the Transformer TR1?

I never measure that value but unloaded DC voltage is approx 265V

 

[Marcus4audio]

The maximum amount of power that can be obtained from the end of the 1780 ohm resistor is when the load is also 1780 ohms. this will be when be when the voltage at the end of the resistor is 52.5 volts (105/2) So the maximum power will be 52.5^2/1780 = 2756.25/1780 = 1.548 watts As the required heater power is 1.4 watts the efficiency would need to be 1.4/1.548 = 90.4% Expecting to achieve this is being unrealistic.

Sorry I don't understand your calculation. At the end of buck is 7V@0.125ma (with current limit). How it can be 1.4W? I just mention that at powering current goes to 200ma but limiting that to 125ma tube just take some more time to burn. Great deal BTW regarding saving valve life.

 

[Marcus4audio]

While looking to see if there were any nuvistors with higher voltage / lower current heaters I came across the 13CW4 which has a heater of 15.5 volts at 60 mA If you just soldered a resistor across R4 to increase the current slightly this might solve your problem. I found one of these on ebay
**broken link removed**
This is some data on it https://www.drtube.com/datasheets/13cw4-rca1964.pdf

Les.

Not Imposible. People did some tricks with 13CW4 but hey, why not play a bit with some other tubes My project is not commercial purpose at all, more like a challenge. I just wanted some more ideas to play with.

 

[chemelec]

Here is the Full Data on this 13CW4