"Li'l 7" AM Transmitter

[old_coot88]

First you need to address that intermittent between lugs 3 and 4. The break is most likely right where the coil wire joins the lug.

[Reece]

Lacking a tube tester, you can still do some basic tests. You know the tube lights up, so that's good. Pull the tube and using your ohmmeter, make sure that there are no shorts between tube elements that should not be touching. There probably aren't, but....you know how that goes. And then we can all rest more easily...

[old_coot88]

I wanted to double check that the tube basing (pinout) shown in the schematic is correct, and indeed it is, as verified here.. http://www.bunkerofdoom.com/tubes/sy...1949/117L7.GIF

After you've gotten that break in the coil fixed, there appears to be a couple of errors in the schematic. It shows pin 8 (cathode) of the pentode section going to ground. I don't believe this is correct. Try ungrounding pin 8.

Now connect lug#1 of the coil to pin 8 (after disconnecting lug#1 from the junction of C5/C6). Be sure lug# 2 connects to ground.

Connect the jct. of C5/C6 to lug# 4 of the coil (lug# 4 should already be connected to pin 3 {plate} of the tube, and to C1 which feeds the antenna).

If the circuit doesn't oscillate, try reversing polarity of either the primary or secondary of the coil (but not both).

[Winky Dink]

Thanks. I see six individual things to check out. I have to ask about the grounding of pin 8 first. That's something that actually didn't make sense to me, but I left it alone because it seemed to be consistent with source schematic:

So I'll break the circuit as above?
I'll be back later after I try to address the other issues.

- Winky

[old_coot88]

No, you want to leave the 47K resistor (R1) connected to ground, and break between pin 8 and ground.

[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

[old_coot88]

Hm. Possibly a bad tube. If the RF section of the tube is conducting, there should only be a very small fraction of a volt on pin 8, readable only on the lowest V scale on the meter. That's because the low resistance of the coil, only 10 ohms, will drop very little voltage at the current the tube pulls.

When you go to have the tube tested, be sure the tester will do both sections of the tube independantly (not a "drug store" type tester).

[wa2ise]

Originally Posted by Winky Dink

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

Oh, it can sometimes seem that way... And can get quite fustrating. Thing is, one small error can be the cause, and people can be their own worst proofreaders, as they become blind to their own errors. I would avoid extensive modifications to Phil's circuit, if it never worked for you. That usually leads to additional fustration. Maybe one of us who lives nearby can pay you a visit. Not me, though, I'm in NJ.

[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.

[old_coot88]

Originally Posted by wa2ise

Oh, it can sometimes seem that way... And can get quite fustrating. Thing is, one small error can be the cause, and people can be their own worst proofreaders, as they become blind to their own errors. I would avoid extensive modifications to Phil's circuit, if it never worked for you. That usually leads to additional fustration. Maybe one of us who lives nearby can pay you a visit. Not me, though, I'm in NJ.

Dang, you are right amigo. And i am wrong for coming up with an alternative circuit that obviously doesn't work. I need to apologize profusely for the wild goose chase. Upon visiting Phil's schematic, the circuit depicted is obviously the correct one, once the tube internals are added to the tube symbol. http://antiqueradio.org/transmitter.htm

Oh man. Sorry as hell.

The cathode (pin 8) should be grounded as it was originally.

The C5/C6 junction does go to lug# 1, not lug# 4.

All else remains as it was.

One thing that's still puzzling though, the resonant tank caps (C6a, b, and c) are now in parallel with the low impedance coil winding, whereas in seems like they should parallel the high impedance winding (as was the case in my "alternative" circuit).

[wa2ise]

Originally Posted by old_coot88

Dang, you are right amigo. And i am wrong for coming up with an alternative circuit that obviously doesn't work.
Oh man. Sorry as hell.

No problem, it's a learning process. I've been there.

One thing that's still puzzling though, the resonant tank caps (C6a, b, and c) are now in parallel with the low impedance coil winding, whereas in seems like they should parallel the high impedance winding (as was the case in my "alternative" circuit).

The high impedance winding should be connected to the resonant tank caps. And the low impedance winding should be in the tube plate circuit. Best to rewire to make it this way, and leave enough wire so you can swap the connections on one of the windings, if it doesn't oscillate.