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 Post subject: Tim's AM tube transmitter build
PostPosted: Feb Fri 23, 2018 4:57 am 
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Hi all! I have been reading and trying to piece things together. I have found a few schematics and mostly complete parts lists. I have little experience with transmitters other than a mike oscillator that is so-so at best. Range is good, sound quality less than desirable. I know not to expect FM sound quality, but I do hope for something decent. I would like to build a version of Norm's tried and true circuit. I haven't decided if I am going to do the simple 1 toober with the output transformer or use a 6AV6, 6C4 version yet. It looks like it is pretty versatile.

So far, I have a sleeve of 6888 tubes and a BTL 'project' box. I have been checking out a LP-427 transformer that may be perfect. I will try to post the schematic that I want to use and hopefully get a little coaching along the way of this build. I find it interesting that the 6888 is actually a "computer" tube. :D It is going to be interesting since there is almost zero clearance between the chassis and top of the cabinet. Going to have to get clever on this one. I need to run this around the 950 khz area to have a clear channel.

I do have a question about possibly running a pair of output tubes in push pull. Anyone tried this? May be complicating things unnecessarily tho.

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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Fri 23, 2018 6:52 am 
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I'm glad to see that you've decided to build Norm's 6888 transmitter. Excellent choice. Lately there's been a lamentable undertone around here that if it was designed by one of our forum members, then it can't be any good. So, let's put that myth to rest.

I'm not familiar with an LP-427 transformer, but it will have to supply 800 mA at 6.3V to the 6888 heater, plus the heater current of any other tubes that you decide to incorporate. In my experience, I found it unnecessary to have an audio preamp stage on the 6888 transmitter. I had plenty of audio drive feeding the transmitter from any of my iPhone, my computer, or my CD deck, but others have found an extra preamp stage to be useful.

Forget about the push-pull idea. A single 6888 will give you gobs of RF power. Trying to design a push-pull RF output transformer that will match a short antenna will be an immensely difficult job.

You mention that you want to operate at 950 kHz. Are you able to find a crystal or crystal oscillator that operates at that frequency? Or, do you intend to have an adjustable frequency oscillator?


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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Fri 23, 2018 8:13 am 
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You notice that tight fit too. :oops:
I moved the socket under the chassis.

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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Fri 23, 2018 3:41 pm 
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The osc we used are 1Mc so that the transmitted freq, unfortunately it is not adjustable... There are older, now obsolete osc of various frequencies but are getting difficult to locate... It's not that difficult to add a VFO to use whatever freq optimum in your area...

I don't have any experience with the LP-427 xformer, but for it's rated 1A @ 6v output, the primaries are supposed to be connected in parallel... I take that as if only one is used output is limited to 500ma @ 6v... To supply B+ the second primary would also be placing demand on the first... For low voltage transformers generally primary windings fail far before the secondary... You may be able to get away with using the 6888 but no doubt would have voltage sag... Two transformers, one for tube heater & other for B+ supply should be OK...

BobWeaver wrote:
I'm glad to see that you've decided to build Norm's 6888 transmitter. Excellent choice. Lately there's been a lamentable undertone around here that if it was designed by one of our forum members, then it can't be any good. So, let's put that myth to rest.

LOL, you know how it is today, we have 100s of I-net ex-spirts that are yet to build the first one... Out of all the dual control pentodes I've tried(12-15), only the 9KC6 requires additional circuitry to overcome severe non linearity issues... With approx -6v bias applied to G3, it's a great performer...

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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Fri 23, 2018 5:06 pm 
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LP-427: If the 1A version doesn't have enough juice, here's a similar one that's 2A for $8.95. http://www.mpja.com/12V-2A-Center-Tappe ... /27839+TR/

They have a 4A version for $15.95.


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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Sat 24, 2018 2:10 am 
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Thanks for the support. I really like the pics of your project Mike! This is kinda what I am going after. I will do some more sniffing about frequencies and see if I can get something that the crystal will work on. If I cant find a good clear channel, then maybe I could build an oscillator that I can adjust across the band. In reading some more, I understand that I can use the osc part of a tuner from an AA5? I am hunting some parts now and thinking on things.

Ok, the push pull idea is out (for now lol). I am surprised at the amount of filament current the 6888 takes. From what I have read, they are a very dependable tube and can run 24-7. For now, I am kind of wanting to use it to re-broadcast a local FM station that I like, which used to be on AM also, but went dry. I may set it up so I can switch inputs from a CD or MP3 player.

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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Sat 24, 2018 2:52 am 
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I'd think most freq would be open in your area, there's also a 1228 xtal available if you're worried about 1000 from Chi Town...

Map of southern IL is great, Oblong is just south of Orange, Moonshine, Bellair & Yale but north of Oil Center, Chauncey, Birds & Pinkstaff.. Of course I didn't mention Hazel Dell, Advance, Sainte Marie, Chauncey, Amity or Palestine...

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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Sat 24, 2018 7:33 pm 
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WKZI 800 from Casey is our strongest station, Robinson (WTAY) used to have a strong station on something like 1600, but has gone pretty silent. I cannot even receive it-go figure. In our particular location, actually rural Annapolis, I can receive a lot more stations due to skip. I mentioned 950 because it is completely clear, even at night. I will do some more listening and checking and try to see of one of those 2 frequencies are clear enough. Trying to avoid that "beat" whistle. I imagine tho that this transmitter would overcome about anything except for a close local. I may just order both crystals so I can see which one works the best.

EDIT: I think the 1228 crystal would set me in a fairly clear station. Is there any adjustment on the frequency with Norms circuit?

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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Sun 25, 2018 12:34 am 
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This is what I get from my database showing the stations within 100 miles of you. It's probably not up to date though. (Distances are shown in kilometers.)

Code:
Freq Call   Dist     Day    Night    City
             km     Watts   Watts
---- ----   ----    -----   -----    ----
580  WILL    123     5000    100     URBANA  IL
740  WVLN    37      250     7       OLNEY   IL
800  WKZI    34      250     250     CASEY   IL
810  WDDD    153     250     250     JOHNSTON CITY   IL
820  WSWI    117     250             EVANSVILLE      IN
860  WSON    132     500     500     HENDERSON       KY
870  WINU    105     500             SHELBYVILLE     IL
880  WIJR    154     1700    160     HIGHLAND        IL
910  WAKO    38      500     50      LAWRENCEVILLE   IL
940  WMIX    113     5000    1500    MOUNT VERNON    IL
980  WITY    122     1000    1000    DANVILLE        IL
990  WITZ    111     1000    6       JASPER  IN
1050 WDZ     130     1000    250     DECATUR IL
1060 WMCL    115     2500    2       MCLEANSBORO     IL
1090 WCRA    58      1000            EFFINGHAM       IL
1130 WSDX    87      500     20      BRAZIL  IN
1160 WKCM    158     2500    1000    HAWESVILLE      KY
1170 WLBH    70      5000            MATTOON IL
1180 WGAB    124     670     1       NEWBURGH        IN
1210 WILY    122     10000   3       CENTRALIA       IL
1220 WSLM    164     5000    82      SALEM   IN
1230 WTCJ    158     850     850     TELL CITY       IN
1240 WEBQ    153     1000    1000    HARRISBURG      IL
1250 WRAY    77      1000    59      PRINCETON       IN
1270 WEIC    62      1000    500     CHARLESTON      IL
1280 WGBF    118     5000    1000    EVANSVILLE      IN
1300 WBOW    66      500     75      TERRE HAUTE     IN
1300 WFRX    153     1000    60      WEST FRANKFORT  IL
1330 WVHI    109     5000    1000    EVANSVILLE      IN
1340 WBIW    125     1000    1000    BEDFORD IN
1340 WSOY    130     1000    1000    DECATUR IL
1340 WJPF    162     770     770     HERRIN  IL
1350 WJBD    97      430     59      SALEM   IL
1360 WVMC    63      500     20      MOUNT CARMEL    IL
1370 WGCL    112     5000    500     BLOOMINGTON     IN
1390 WFIW    78      710     58      FAIRFIELD       IL
1400 WEOA    123     1000    1000    EVANSVILLE      IN
1400 WDWS    124     1000    1000    CHAMPAIGN       IL
1410 WIHM    132     1000    63      TAYLORVILLE     IL
1420 WVJS    151     5000    1000    OWENSBORO       KY
1440 WPRS    69      1000    250     PARIS   IL
1450 WAOV    49      1000    1000    VINCENNES       IN
1460 WROY    105     1000    85      CARMI   IL
1460 WKJR    148     500     65      RANTOUL IL
1480 WPFR    71      5000    1000    TERRE HAUTE     IN
1490 WDAN    130     1000    1000    DANVILLE        IL
1490 WOMI    156     830     830     OWENSBORO       KY
1500 WPMB    105     250     2       VANDALIA        IL
1510 WXOZ    147     5000            HIGHLAND        IL
1520 WHOW    152     5000            CLINTON IL
1540 WBNL    118     250     1       BOONVILLE       IN
1540 WMYJ    136     500             MARTINSVILLE    IN
1540 WSMI    145     1000    1       LITCHFIELD      IL
1550 WNDI    45      250     7       SULLIVAN        IN
1550 WCVL    145     250     5       CRAWFORDSVILLE  IN
1550 WMSK    148     145     6       MORGANFIELD     KY
1560 WSEZ    134     250             PAOLI   IN
1570 WTAY    11      250     188     ROBINSON        IL
1580 WAMW    75      500     5       WASHINGTON      IN
1580 WBCP    129     135     6       URBANA  IL
1580 WDQN    158     170     7       DUQUOIN IL
1590 WRCY    119     500     35      MT. VERNON      IN
1600 WBTO    63      500     32      LINTON  IN



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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Sun 25, 2018 1:59 am 
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Madmurdok wrote:
EDIT: I think the 1228 crystal would set me in a fairly clear station. Is there any adjustment on the frequency with Norms circuit?

The crystals are not tunable...

I have a 1230 approx four miles away, within 15-20ft the 6888 on 1228 would completely cover it...

I do have some 1554Kc that work nicely... Being in between a std frequency there is no interference using those... Of course today's digital tune receivers won't receive the signal but analog radios could care less... Deviating from 1000 will require a different coil in plate circuit for proper antenna tuning...

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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Sun 25, 2018 3:38 am 
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Is 1,000kc quiet? Build it to that freq and change your coil slightly if not.
Your var cap will tune to it.


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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Sun 25, 2018 4:31 am 
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WMVP is a 50kW station in Chicago on 1000 kHz, but is outside the 100 mile radius of the stations I posted above. Probably enough power to cause interference. Here are the high powered stations within a 300 mile radius:


Code:
Freq   Call   Dist     Day    Night    City
               km     Watts   Watts
----   ----   ----    -----   -----    ----
620    WTMJ    412     50000   10000   MILWAUKEE       WI
650    WSM     348     50000   50000   NASHVILLE       TN
670    WSCR    326     50000   50000   CHICAGO         IL
700    WLW     311     50000   50000   CINCINNATI      OH
720    WGN     335     50000   50000   CHICAGO         IL
780    WBBM    333     50000   50000   CHICAGO         IL
840    WHAS    231     50000   50000   LOUISVILLE      KY
890    WLS     284     50000   50000   CHICAGO         IL
990    WNML    483     10000   10000   KNOXVILLE       TN
1000   WMVP    313     50000   50000   CHICAGO         IL
1070   WFNI    170     50000   10000   INDIANAPOLIS    IN
1120   KMOX    188     50000   50000   ST. LOUIS       MO
1130   WISN    418     50000   10000   MILWAUKEE       WI
1160   WYLL    286     50000   50000   CHICAGO         IL
1190   WOWO    311     50000   9800    FORT WAYNE      IN
1300   WOOD    459     20000   20000   GRAND RAPIDS    MI
1510   WLAC    320     50000   50000   NASHVILLE       TN
1530   WCKY    285     50000   50000   CINCINNATI      OH


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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Feb Wed 28, 2018 10:33 pm 
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Had a cleaning party last night :D
Attachment:
DSC05392.JPG
DSC05392.JPG [ 126.49 KiB | Viewed 1883 times ]


Getting ready to order coils, caps and such.
Got a couple questions.

I found some RF chokes on Ebay. https://www.ebay.com/itm/50PCS-0410-Col ... 1438.l2649

Found this crystal: https://www.ebay.com/itm/1-MHz-Full-Can ... 1438.l2649
I would like to verify if these are the correct parts before I order anything. Had no success locating these parts on Mouser. I have trouble with the search system they use.

I assume on the 680pf and 470pf caps that silver mica is best? Or disc? Not important?

I just want to be sure that a tuning cap from a AA5 will work?

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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Mar Thu 01, 2018 12:40 am 
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That oscillator will work for $16 but for $3 more you can get 10.
I found the oscillators from Jameco wouldn't take even a 10th of a volt above 5V.
Jameco sent me 5 for testing for free. They are in the trash.
No problem with the 10 Saronix oscillators.

https://www.ebay.com/itm/QTY-10-1-MHz-O ... 2749.l2649


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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Mar Thu 01, 2018 7:27 pm 
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Thanks Mike for that heads up! Now I know which crystal to order. I had a little party with an old AA5 that came from a barn. It provided a good output transformer, tuner cap, osc coil and set of tubes (yet to be tested). I tested the cap and it looks like the larger section has a range from 20 PF to 350 or so PF. and the small section read 30PF to 120 or so PF.

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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Mar Thu 01, 2018 8:07 pm 
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Here is a schematic of Norm's circuit that I plan to use. Also is a schematic of the 2 tube circuit. Been kind of going back and forth about which to do, but I do like the simplicity of the 1st. So, this is the diagram I will start with. If I need the extra tube, I can add it.
Attachment:
2q33w9i (2).jpg
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Attachment:
6c46888080910.jpg
6c46888080910.jpg [ 57.56 KiB | Viewed 1832 times ]

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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Mar Thu 01, 2018 8:17 pm 
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If you decide to start with the basic, feed it a lot of audio and be prepared to be amazed!


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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Mar Fri 09, 2018 5:53 pm 
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I found a thread that may have a variation of Norm's circuit that I like. One question that has been brought to my attention, will I be able to feed line level input to the one tube circuit or speaker level? If it has to be speaker level, I may use the additional tube to up the gain. Another thing I like about the 2nd version is that the tuner cap is not at B+ potential and can be mounted directly to the chassis. That really simplifies things. I am working on a layout design that I will post and get your opinion. I want to keep hum, feedback and noise down.

Here is a diagram I am considering:
Attachment:
6888-TX-3%20small Colins.png
6888-TX-3%20small Colins.png [ 61.19 KiB | Viewed 1734 times ]

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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Mar Sun 11, 2018 4:38 am 
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Mike Toon wrote:
You notice that tight fit too. :oops:
I moved the socket under the chassis.

Image


Any chance I could get a parts list? I'd love trying to build one.


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 Post subject: Re: Tim's AM tube transmitter build
PostPosted: Mar Mon 12, 2018 3:22 am 
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battery-maker wrote:

Any chance I could get a parts list? I'd love trying to build one.


I've responded twice to your yahoo email.
Good luck.


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