"Li'l 7" AM Transmitter

[old_coot88]

Yeah, just gotta be sure C5 (the feedback cap) remains tied to its junction with C6 when swapping things.

[Winky Dink]

At Murri's Electronics, the man says, "I put my tube tester in storage. Only needed to use it a couple of times a year. You should try A-Gem." A-Gem Supply is the place with the bad tube tester.

On reading Old Coot88's comments about very low voltage at pin 8--well, it does consistently read either 8 mV or -8 mV.

Questions:
From the data--voltages, resistance--is there any indication that either the tube or the coil is bad?
Please identify for me high impedance/low impedance windings and which is primary/secondary.

Fortunately I followed Phil's advice to not solder the coil connections until the thing actually works. In fact, I've gone one step beyond:



I'm inclined to buy another tube and coil, revert to Phil's original design, and rewire it as if starting from scratch. That will wait until I open the Atwater Kent 37 and find out what I need for it.

Until next time, thanks much, and as my old professor said, "When you hear hoofbeats, don't look for zebras."

- Winky

[old_coot88]

Originally Posted by Winky Dink

...very low voltage at pin 8--well, it does consistently read either 8 mV or -8 mV.

Questions:
From the data--voltages, resistance--is there any indication that either the tube or the coil is bad?

The voltage drop across the 10K filter resistor (R3)indicates the tube is conducting. R3 is dropping 8V, which works out to .0008 amp, which squares just about perfectly with the 8mv you were seeing across the 10 ohm coil. Offhand i would say the tube is good, but will wait and see if Robert concurs.
If the coil shows good solid continuity on the ohmeter with no intermittency it is probably good.

Please identify for me high impedance/low impedance windings and which is primary/secondary.

The primary would be the 10 ohm coil and carries B+ to the tube's plate (pin 3), and the secondary would be the 78 ohm coil which forms the tank circuit with C6.
According to Phil's schematic, the primary is shown as lugs 3 and 4, and the secondary is shown as lugs 1 and 2. This appears to be backwards from the way it's been getting hooked up. Maybe Phil could have an eyeball at it and see if a little switchamaroo in the labeling mighta snuk in.

In truth, i was trying to devise a circuit that'd conform to the coil lugs' labeling shown in the schematic, while keeping the high impedance winding in the tank circuit.