Dear people,
When I am driving a capacitive load ( cable), does this means that the reactance of the capacitor is in parallel with the input impedance of the stage it feeds?
If this is the case then driving 15 meters ( 49.2 feet) of for example Belden 8412 gives 2850 pF. At 20kHz this has 2.79K Ohm of reactance.
Driving a 470 K Ohm input this would be 2.77 K Ohm total load. This would be very difficult to drive for a tube wouldn't it?
Are there any tricks that help tubes deal with capacitive loads?
Thanks and kind regards
When I am driving a capacitive load ( cable), does this means that the reactance of the capacitor is in parallel with the input impedance of the stage it feeds?
If this is the case then driving 15 meters ( 49.2 feet) of for example Belden 8412 gives 2850 pF. At 20kHz this has 2.79K Ohm of reactance.
Driving a 470 K Ohm input this would be 2.77 K Ohm total load. This would be very difficult to drive for a tube wouldn't it?
Are there any tricks that help tubes deal with capacitive loads?
Thanks and kind regards
Yes, that's essentially correct. The current through the 470 kohm is 90 degrees out of phase with the current through the cable capacitance, so they add according to Pythagoras's theorem, but that only increases the impedance to 2792.142712 ohm. The inductance of the cable will also have a very small influence. Whether it is difficult to drive depends on the valve you use.
You could consider using a different kind of cable. A polyethylene-filled 75 ohm coaxial cable only has about 67 pF/m of capacitance, and a foam-filled one a bit less than that.
You could consider using a different kind of cable. A polyethylene-filled 75 ohm coaxial cable only has about 67 pF/m of capacitance, and a foam-filled one a bit less than that.
the lower capacitance cables are used for electric guitars. for example the CA-0446 at BPTA boasts 20pf/ft about 61pf/m.
the next step is 93 ohms coaxial cable, 13pf/ft, available at specialty stores..
the next step is 93 ohms coaxial cable, 13pf/ft, available at specialty stores..
Either a mosfet follower or a White cathode follower would help, but reduce the cable capacitance first.
When there was a business need to drive long cables with tubes, everybody used transformers. And specced the lines as nominal 150 Ohms to 900 Ohms (2 of 3 US networks called it 600 Ohms).
This is a 10:1 mis-match for a small tube (adequate for the actual power to the line) so transformerless solutions are compromises.
A line inside the building could be a 6J7 wired triode with a 15k:500r transformer. A line across town would usually be two 6F6/6V6 into a 10kCT load to 600r.
24.0 pF/Ft @ 1 MHz – COND - SHLD
A naive single-tube driver could look into 37pFd to common. 75 Ohm TV coax would be better. And probably incredibly cheaper.