16-07-2020, 10:35 PM
The 1j18 is poorer than 1j24b or 1j29b. It's an older type.
The 1j42a is only better than 1j24b or 1j29b at low HT. In a proper circuit, the 1j24b is far better RF than 1j42a and better audio than 1j24b. The 1j24b replaced the 1j18b. The 1j29b is higher gain, higher power etc, but about x4 filament current of 1j42a or 1j24b (which are the same). The 1j18b is about twice the 1j42a or 1j24b filament current.
The 1j24b and 1j29b gain/performance depends on an optimal "stiff" g2 voltage. You can only tie it to HT, if the HT is optimal for g2, else you need a potential divider, not a single resistor. Look at g2 current to see why. Or else a tap on the HT battery pack at about 54V or 63V for 1j24b's g2, and higher for 1j29b's g2.
You can use 1/2 the filament of a 1j29b for same filament current as a 1j18b but better performance. The only penalty is 1/2 the gain (which is still high) and 1/2 the power which is still higher. The 1j24b is OK for 27v (poor) to 90V HT designs, the 63V or 72V is good. The 1j29b is about 54V minimum, good at 72 to 90V and excellent at 120V.
A pair of 1j24b is about the same filament current as a 1j18b and gives about x4 power in Class AB, and then can use a small centre tap mains transformer without saturating. I used a VFD indicator as phase splitter, but another 1j24b or a transformer is lower power. The only advantage at all for the 1j42a is the 9V HT or the unusual mixer action, which is kind of opposite to how a dual gate FET works.
The 1j42a is only better than 1j24b or 1j29b at low HT. In a proper circuit, the 1j24b is far better RF than 1j42a and better audio than 1j24b. The 1j24b replaced the 1j18b. The 1j29b is higher gain, higher power etc, but about x4 filament current of 1j42a or 1j24b (which are the same). The 1j18b is about twice the 1j42a or 1j24b filament current.
The 1j24b and 1j29b gain/performance depends on an optimal "stiff" g2 voltage. You can only tie it to HT, if the HT is optimal for g2, else you need a potential divider, not a single resistor. Look at g2 current to see why. Or else a tap on the HT battery pack at about 54V or 63V for 1j24b's g2, and higher for 1j29b's g2.
You can use 1/2 the filament of a 1j29b for same filament current as a 1j18b but better performance. The only penalty is 1/2 the gain (which is still high) and 1/2 the power which is still higher. The 1j24b is OK for 27v (poor) to 90V HT designs, the 63V or 72V is good. The 1j29b is about 54V minimum, good at 72 to 90V and excellent at 120V.
A pair of 1j24b is about the same filament current as a 1j18b and gives about x4 power in Class AB, and then can use a small centre tap mains transformer without saturating. I used a VFD indicator as phase splitter, but another 1j24b or a transformer is lower power. The only advantage at all for the 1j42a is the 9V HT or the unusual mixer action, which is kind of opposite to how a dual gate FET works.
