homebrew am tx from drive in

[kd1nw]

Hi Folks,

I picked this up basically just for the tubes and maybe the iron. I was told it is a homebrew am tx that was used at a drive in. It's missing the output tube and I was wondering if any one knows what it might be out of curiosity? I just picked it up and was hoping maybe it was a typical hb and someone would recognize what the missing tube could be. Not much to go on, I know. It has

2x GE K53J205 10henry chokes
1x Thoradson T-70R62 pwr trans
2x 6SN7
2x 6v6
1x 5U4GB

Thanks
Kevin - KD1NW

[N1BCG]

Interesting! Since asking the builder doesn't seem like an option, can you determine which pins of the octal socket go to what tube elements? The heater leads are probably the easiest (6vac, right?). Knowing the pins for cathode, grid(s) and plate will help narrow down the selection.

Clark

[Opcom]

It seems the small rectifier 5Y3? is in a 6SN7 socket. Maybe that is one part of the answer.

one possible interpretation of the empty socket which looks like the oscillator:

socket at first looks like this, but what tubes match?
1 nc
2 fil
3 plate
4 screen
5 cathode
6 nc
7 fil
8 grid

would say another 6V6 or 6AG7 maybe?
could pin 8 be the cathode?
Pin 5 the grid?
With a 6000 Ohm cathode resistor?
1 nc
2 fil
3 plate
4 screen
5 grid
6 nc
7 fil
8 cathode



6SJ7/6SK7 - plate choke seems to argue against this tube working there.
1 nc
2 f
3 g3
4 g1
5 k
6 g2
7 f
8 p

[Pete, WA2CWA]

It seems the small rectifier 5Y3? is in a 6SN7 socket. Maybe that is one part of the answer.

one possible interpretation of the empty socket which looks like the oscillator:

socket at first looks like this, but what tubes match?
1 nc
2 fil
3 plate
4 screen
5 cathode
6 nc
7 fil
8 grid

would say another 6V6 or 6AG7 maybe?
could pin 8 be the cathode?
Pin 5 the grid?
With a 6000 Ohm cathode resistor?
1 nc
2 fil
3 plate
4 screen
5 grid
6 nc
7 fil
8 cathode



6SJ7/6SK7 - plate choke seems to argue against this tube working there.
1 nc
2 f
3 g3
4 g1
5 k
6 g2
7 f
8 p

Yep, looks like the far left tube (chassis label says 6SN7), but tube is 5Y3, should go where the taller fatter tube is plugged in between the two transformers. Odds are the tall fat tube probably goes into the empty socket. If its got a number on it, it should be easy to verify. In which case, the only tube you need to make this thing work is another 6SN7. I wouldn't be surprised it the tall fat tube is a 6L6 or something similar. 6L6's can be used for RF applications.

[Opcom]

Big tube looks like a 5U4 or 5R4, see the plate edge from the side.

[N1BCG]

I'm thinking this is a Hartley oscillator with the coil in the cathode circuit [8] and a cap coupling to the grid [5] that uses that 6k (?) resistor for bias. This version of the pinout opens up *a lot* more possibilities, such as Opcom's 6V6 suggestion.

I was motivated to take a second look at this after getting numerous paper cuts browsing through a GE tube manual, a Sylvania tube manual, and the 1976 Radio Amateur's Handbook and finding no octal tubes with G1 on pin 8.

[kd1nw]

Thanks everyone for taking an interest. I'll try and get the pin outs the best I can later today. I'm heading up to the framingham hamfest this morning. Also, the 6SN7 that was in question of course has no markings on it. I took a photo of it. It looks like a twin triode. The rectifier between the choke and trans is marked 5U4GB on the glass. the other 6sn7 is marked on the glass and the remiaing are 6v6 and 6v6gt. I also tried to get a close up of the scribbling right next to the V5 marking on the chassis. It looks like the builder has written the tube numbers in pencil. Its very hard to read, looks like he almost scratched it out, 6 something 6. Like maybe 6V6, 6 l 6, or 6H6 which it couldn't be.

[kd1nw]

this is the marking, its about the best i can get with a camera

[N1BCG]

The tube you posted looks like a twin diode and the light colored elements that loop up over the mica spacer at the top would be the filaments (no cathode). That's my guess. I can't think of a use for it in this circuit if you already have a rectifier properly positioned.

By the way, this post received *a lot* of interest on 3873kc last night and drew questions and suggestions from both familiar and new stations. In particular, I think the old schoolers really enjoyed the "What Could It Be?" mystery of V5 and discussion about various tube oscillators they built in days of yore.

If it turns out that the two 6V6 tubes are in push-pull modulator configuration, then a 6L6 would be a more likely candidate for V5 and it has the same pinout as a 6V6.

The schematic below came from the internet and seems to reflect your circuit.

Clark

[W3RSW]

fwiw, in the V5 pix posted, I see " 6L6" with scratches through it below the bright "V5" . The vertical part of the "L" has a jog in it. "L" I think is the only thing that middle character could be, scratched out later or not.

[kd1nw]

Thanks Rick, my wife agrees with you, the marking looks like 6L6 to her too. I asked her this morning before she left.
And Clark, I wish I hung in there longer. I started listening to 3.873 last night but switched over to listening to Tron on 5.110. I tried to get you guys on my S38 in the bedroom but the antenna is too short for 80m. Oh well, I really would have like to heard what you guys were saying! So, I guess its time to head down and see how the thing is wired. Probably head to VRCMCT to see if they have a 6L6 Saturday.

Thanks

[N1BCG]

It certainly would have been a surprise had you jumped in during the discussion! What you missed was hearing a myriad of checkins from all over and suggestions for tubes to check. Meanwhile, I was plowing through lists of base diagrams in several tube manuals. Lots of ideas from the group meant bookmarking the pages to explore each possibility.

On a mission!

I was surprised that the topic held such interest past midnight, and it turned out to be quite the "tube hunt". Plan on providing a bit of story about the rig (or at least what you know of it). Inquiring minds want to know!

[kd1nw]

Man I missed out, I would have jumped in for sure. Okay, my skills would be described a beginner at best so here's what I see that I know for sure.

1 - NC
2 - Heater
3 - Plate
4 -
5 -
6 - NC
7 - Heater
8 -

Sure looks like the 6V6 mentioned by OpCom or a 6L6. Considering the marking on the chassis looks like 6L6. It appears the mystery tube is a 6L6.

4, 5 and 8 all seem to be connected in some way as an oscillator as mentioned earlier in this thread. I think I found a small issue though, In Bottom2.jpg - Pin4 of the 6L6 goes through the 100ohm 10W resistor to a terminal strip, a red wire comes off there and runs  under the crumbly leaf all the way over to another terminal strip connecting to a black device labeled IRC TYPE DG RES 100. It looks like the other end of that black device is supposed to connect to Pin 8 of the 5U4GB but is broken off. I guess this would provide screen voltage for the 6L6 if reconnected?

[kd1nw]

I wish I had a great story for this tx but I don't know much about it. I went to a flea market at The Vintage Radio and Communication Museum of CT. It was the first thing I saw. Marked $40. The seller mentioned it was an AM TX from a drive in. I saw it had a couple of 6V6 and the 5U4 which I wouldn't mind as spares. So Im about to leave and he tempts me by asking if $25 would convince me. I said it would but I wanted to look around some.. anyway, I needed to buy some tubes for the S38A which developed a CK short and was humming and I needed to see how much I had left after that. It ended up being $20 which was great for the tubes and iron. If the seller happens to be reading thanks again. Anyway, if the TX ends up working it would be fantastic because I've been wanting to buy a little AM tx for playing some music to a couple of other AM consoles that I have. Hopefully, there will be more to come to the story!!

[Opcom]

The 6V6 seem to be push pull, but the audio transformer serving the audio output jack looks like it has no CT. Maybe the B+ is fed through one of the large chokes to a 6V6 plate. It looks like pin 3 plate of a 6V6 is capacitor coupled to the oscillator but it is hard to see for sure. Perhaps it is time to draw out the schematic from examination. That would be very interesting.

[W3RSW]

At this point It looks like your just about ready to rebuild the whole thing with new small parts and electrolytics since the iron's good, sockets are good, most tubes are good.  Wire it up properly , your schematic, and with no cutoff mystery wires. 
Look at all the fun had by squiring it by the ' hood' --and on the air too, before its time.

[kd1nw]

Yes, OpCom I agree I need to create the schematic. Unfornuately, I've never done it before so it will be a project for me in itself. I need to get experience doing this though so its good opportunity. And Rick, I made a mistake, the red wire I as talking about earlier actually goes to pin 3 of the top 6V6 closest to the edge of the chassis. The broken wire at Pin 8 of the 5U4 is connected through the black irc wirewound resistor to a 40mfd section of the multisection which then connects to one of the 10 henry chokes. There are 2 red wires that cross each other on their way and it tripped me up. Thanks again everyone in the 'hood for your help!

[N1BCG]

Reverse engineering a circuit to get a schematic can seem daunting, but when done, will be very rewarding. The key to getting started is to not worry about neatness. Start with the rectifier socket and work your way from there. You can even draw the sockets as a circle with pin numbers around it (leaving the actual tube details for later). The most common connection will be components connected to the chassis. These connections can be simply drawn as a "chassis" symbol (I use ground...bad habit) to reduce the number of lines to draw.

The two images below are an example using the oscillator from your transmitter...

[kd1nw]

Thanks Clark. I do it just like you showed starting with the rectifier. While I'm at it I'll try to number and list the parts i.e. R1, C4 etc maybe get a good photo of it and number the photo like the sams photofacts. I've already started looking at other schematics that have 5U4GB to see how they draw it guide me on how the schematic should look based on how I see it wired. Plus, I already have V5 done thanks to you!

[W1DAN]

Hi Kevin:

Looks like someone started rebuilding the transmitter in the 1970s or 1980s, as the caps are ploys and the pot on the front seems newer/foreign. Also the resistors look to be at least 1960s or newer, however the chassis and tube choices are from the 1940s. Interesting there does not seem to be an output circuit, lust a lead to the rear coax connector.

So I googled a bit and did not see and exact version of this transmitter. I did find an old Thordarson catalog:

http://www.ami-west.com/Thordarson.TX.Guide.344E.pdf

Pages 3 and 5 may give some ideas on what your transmitter may have been based on.

If I were you, I'd fuse the power transformer secondary and reduce the fuse rating on the primary.

It was good to see you yesterday at the flea. Interesting project.

Dan

[N0WEK]

Reverse engineering a circuit to get a schematic can seem daunting, but when done, will be very rewarding. The key to getting started is to not worry about neatness. Start with the rectifier socket and work your way from there. You can even draw the sockets as a circle with pin numbers around it (leaving the actual tube details for later). The most common connection will be components connected to the chassis. These connections can be simply drawn as a "chassis" symbol (I use ground...bad habit) to reduce the number of lines to draw.

The two images below are an example using the oscillator from your transmitter...

It's always fun to reverse engineer a schematic for a new piece of unknown gear. It usually takes me about three versions to get to a coherent schematic. The first iteration is just done up as I trace the circuit with wires everywhere. The second is trying to make a normal schematic out of the rats nest of the first attempt and the third one to clean up the flow of the second one. I'm still doing all three on paper.

[kd1nw]

Yes, Im sure it will take a few attemptsto get something decent. But, I agree it is a lot of fun. Bought it for parts but now I'll learn a bunch about transmitters. Im really glad I bought it now.

And, hi Dan it was great catching up with you at the flea market. Thanks for the info and tips, Im sure it will come in handy

[kd1nw]

well i finished the schematic   . its on 4 different pages though.. Here it is I redrew it on one page and attached it. I just noticed i forgot to mark a couple of resistors. The one off Pin 5 of the 6L6 is 5k Ohm. The one off Pin 8 of the 6V6 is 2k Ohm. I also messed up on Pin 2, 7 and 8 on the top 6SN7. I'll fix these and post the update 4/18 or 4/19.

[Opcom]

It's nice to see it taking shape. I noticed in the under-chassis picture the WHT and GN-WHT leads were reversed within red spaghetti near the 6L6. Maye the maker experimentally reversed the FIL leads to reduce hum. There are always interesting hackeries in home made stuff.

[kd1nw]

Yes, I was wondering why he split those wires. BTW, I think Pete was right when he questioned that 6SN7. I don't think it can be 6SN7 the way it is wired. I was thinking it should be another 6V6, probably metal because Pin 1 is grounded. But it still doesn't look quite right to me. I attached a sketch with the proposed 6V6 on the left and the original erroneous 6SN7 to the right for comparison. This is the first tube where the audio input comes in on the side with only the one choke. Do you guys think it should be a 6V6 or do I have another mystery tube on my hands..