"Li'l 7" AM Transmitter

[Winky Dink]

Big "oops." That should have been obvious to me. I'll have to retreat and repeat that last six steps later tonight.
- Winky

[old_coot88]

You want to double-check that C5 connects from pin 4 (control grid) to the jct. with C6 (which also connects to lug# 4 of the coil and pin 3 of the tube.

C5 provides the feedback to sustain oscillation so it has to be in place. Also double check that the 47K resistor R1 goes from pin 4 to ground.

Maybe post the schematic of the most recent iteration when you get it done.

[wa2ise]

Something else that bothers me a little is that I would not have though a pentode would work with its G2 at ground voltage. It's usually around 70 to 100% of the plate voltage. Being at 9V might cut the tube off? Or it's a way to keep the amount of RF power low?

[old_coot88]

Yeah, i thought about that too. But figured that the cathode being at `near` ground (except for the coil resistance) probably mitigates for the absent G2 voltage a little. Then with both grids at the same potential, the tube behaves electrically more like it's a triode running at lower power than a full-up pentode. Or at least that sounds sorta plausible.

Got to looking at the 117L7's spec sheet posted earlier, and it's classed as a beam power amplifier. If that's correct, it's a tetrode with beam-forming plates and there would be no suppressor grid (G3) between G2 and the plate. So with G1 and G2 "strapped" at the same electrical potential, the tube would be running as a "beam triode".

[Winky Dink]

The last two posts are way over my head, but I'll point out that G1 and G2 are -.8V and -.9V.

In addition to curing cancer and establishing world peace, this is what I've done this evening.

Tube:
- Pin layout is consistent with Tung-Sol, RCA, bunkerofdoom, radiomuseum diagrams.
- No continuity between any pins except the heater, 2 & 7.

Coil:
- I don't think there's a break in the coil. Lugs 1-2 read a consistent 10 ohms, lugs 3-4 read a consistent 78 ohms. Checked the continuity tester through a couple of resistors, and it reads "open" at 75 ohms and above. Visually all connections appear solid.

Changes in circuit:
- Ungrounded pin 8.
- Disconnected junction C5/C6 from lug 1.
- Connected pin 8 to lug 1.
- Connected junction C5/C6 to lug 4.
- Tested the unit.
- Reversed connections on lugs 1 and 2.
- Tested the unit.

Here are the last two versions that tested negative. Voltages are updated. Don't understand why there's 0V at pin 8. I'm going to let it sit for now and try to get the tube tested tomorrow morning.



Is this actually a hazing? Are you guys going to come back with, "Gotcha! Nothing was wrong except you forgot to plug it in!"

- Winky

[dieseljeep]

I have one of those units. It was factory made by, IIRC Bud radio products. The oscillator coil was several turns of scc wire wrapped around a chassis mount compression trimmer. Don't remember if the coil was tapped or not. I was going to clone it, but the performance wasn't that great. My "radio dj " works a lot better.