I did some tests with 6.3VAC filaments on a 5U4 using GE NTC inrush current thermistors CL-50 and 80 to lower the 6.3 to 5V and lengthen the time to full B+.
Here they are:
I tested the 5U4 with a CL-80 and CL-50 (what I had on hand). Here's the results:
5U4 with 5 volts filament - 6.19 seconds to 95% (375VDC)
5U4 with 6.3 volts filament and CL-50 - 30.48 seconds to 95% (375VD)
5U4 with 6.3 volts filament and CL-80 - 1 minute 40 seconds to 95% (375VDC)
5AR4 with 5 volts filament - 32 seconds to 95% (395VDC)
To use a 5U4 and CL-50 or 80, you need a dedicated 6.3V filament; nothing else attached. You can also try a CL-70 or CL-60.
The voltage drop of each thermistor at 3 amps is:
CL-50 .93 volts
CL-60 1.01
CL-70 1.16
CL-80 1.47
The CL-50 and 60 you'll probably need to add a small resistor in series (0.1 to 0.15 ohm 3 watt) to keep the filament close to 5V.
The CL-70 and 80 will be OK, except for the long warm up time of the CL-80.
Here they are:
I tested the 5U4 with a CL-80 and CL-50 (what I had on hand). Here's the results:
5U4 with 5 volts filament - 6.19 seconds to 95% (375VDC)
5U4 with 6.3 volts filament and CL-50 - 30.48 seconds to 95% (375VD)
5U4 with 6.3 volts filament and CL-80 - 1 minute 40 seconds to 95% (375VDC)
5AR4 with 5 volts filament - 32 seconds to 95% (395VDC)
To use a 5U4 and CL-50 or 80, you need a dedicated 6.3V filament; nothing else attached. You can also try a CL-70 or CL-60.
The voltage drop of each thermistor at 3 amps is:
CL-50 .93 volts
CL-60 1.01
CL-70 1.16
CL-80 1.47
The CL-50 and 60 you'll probably need to add a small resistor in series (0.1 to 0.15 ohm 3 watt) to keep the filament close to 5V.
The CL-70 and 80 will be OK, except for the long warm up time of the CL-80.