I am currently building a crystal controlled AM transmitter. The circuit that I'm using call for a NTE – 613 varactor diode (which will come in a few days). I was wondering if there was an alternative to the director diode that I could use for the meantime to test the circuit? couldn't I use an NPN transistor as a varactor diode? Any help would be appreciated thank you so much! I will attach the schematic link below that I am using below:
Yes, you can use the BE or CE junction of an NPN (or PNP) transistor as a varactor diode, however it may not have the capacitance or capacitance range you want. The BE junction may not have the breakdown voltage required.
The NTE613 has a 22pF capacitance at -4V and a 2.9:1 range from -2V to -30V. That seems on the low side to start with for an AM-band radio- I would expect several hundred pF. Perhaps it is not operating in the usual ~530 to ~1600kHz MW band.
Anyway, a couple 1N4004's in parallel will be fairly close to the NTE part.
As you can see, the higher voltage parts use a different (PIN) construction so they have less capacitance.