The AM Forum

My very first homebrew transmitter. I won't call her a work of art, but she's my design, my toil, and she's working great. Parallel 254W's modulated by 811A's. 1800V@225mA, 300W carrier output. Modulation looks great on the scope. It'll be good to be back on the air after 5 years.

Ron W8ACR

Awesome and inspiring!  Well done OM!

Very nice work!!

...and those tubes are really nice!

I like it!!! Great tubes, cloth covered filament leads, fahnstock clips.....very nice    Steve

Nice job, Ron!

Welcome to the exclusive Homebrewer's Club.  Only one way to join - and you have successfully completed the requirements... ;)


T

Beautiful. And a nice selection of rare tubes in the final. I can't wait to hear it on the air.

She's definitely purty. Nice choice for PA tubes. Big old buzzard types with a nice warm glow.
You will have to show us when they start blushing.

Fred

Nice looking rig. Great job.

Dave

that's very nice !

I recall a few threads on it, particularly related to the
circuit...can you post your final version as built ?

Peter

Thanks for the kind comments. I guess I started seriously working on this rig in September in my unheated garage. There were a few cold days along the way - below freezing indoors! She has four decks - RF, speech amp/modulator, and two HV power supplies. Almost all the wiring is done with old, but high quality cloth covered wire. I have about a half mile of that stuff from an old FM BC rig that was junqued. Only problem is that it's all big stuff - 16ga or bigger. So there are a few areas that have newer teflon covered wire that is smaller. Most of the wiring is laced as well.

She uses a BC-610 plate transformer on the 2000V tap. 866A rectifiers. The modulator has a separate power supply @ 1350V under load. I'm gonna run the 811A's zero bias although that is a bit more plate voltage than recommended for zero bias. I'm getting about 60mA static plate current on the 811A's. Diode rectifiers in bridge configuration for the modulator power supply. 811A's driven by 6A3's (thanks to Skip K7YOO) through a UTC S9 driver. Mod transformer is a UTC S22 with 50H audio reactor and no RF B+ on the secondary.

This is a single band 160 meter rig. Kept it that way for simplicity. RF driver is a Viking 1. Grid coil is a B&W JEL 160. Plate tank is pi network. I think I must have underestimated the inductance value of the plate tank coil. The tank resonates at about 240pf on the tuning cap. Tank Q at the values given in the previous post is about 10 or so. Seems to work fine at that value. I could raise the Q by reducing the plate current or by tapping down on the coil. Using the high voltage tap on the plate transformer would also help.

I have to disassemble the rig to transport it to the shack which is about 50 yards from the garage. I'll take some more pics when she's apart. Hope to have it on the air this weekend.

73, Ron W8ACR

One correction. There is RF B+ on the mod transformer secondary, but no plate current through the secondary. I'm using a 1uF 5000V oil cap directly to ground from the bottom end of the mod transformer secondary. I'll post more pics and a partial schematic later.

Ron

Hi Ron,

It is indeed a real kick to breathe life into a homebrew rig!

It looks like a runner, great choice of finals! ;)

73 DG

That is very cool-looking. The satisfaction of building and using your own design and equipment is the best thing.

Beautiful work, Ron.

Steve KL7OF.....

The Fahnstock clips were pure improvisation. I ran out of plate caps and they were the only other thing I could think of to do the job. I ordered ten of them from AES so now I can also use them on my stash of 35TG tubes in case I ever build something with them. They also dredged up some good memories of my model trains from my youth.

By the way, my son Jesse is now a student at Whitworth Univ in Spokane, so next time I visit, I might just take the drive out to TumTum. Looking forward to talking to you soon. Your signal is always BIG here in Lewistown.

73, Ron

Gud Job Ron, When will you have it speaking at us??

I have a hand book from 1941 that uses that tube on some ham band. The 254W was a rather futuristic looking tube in a day when most tubes looked like light bulbs. I think there were at least five companies that manufactured the tube. I think it was Penta, Eimac, Lewis & Kauman, Vacuum Tubes Products Inc. and one other. The aforementioned are in my collection.

They were used here on the east coast in one application with the Tennessee Gas Pipe line for communications with their people in the field. I also believe  they were in many voice two way paging systems.  

Nice looking transmitter, Ron. I, too, look forward to hearing it on the air. Those 254Ws have a plate structure that looks like a 75TH on steroids.

It's always interesting to see something less expected or even unexpected being used in a HB project like this. Adds character to the mix of rigs on the air....

 

Top and underside of modulator deck. Speech amp is at top right. 6SJ7 followed by 6N7 phase inverter. 6A3 drivers at bottom center. UTC S9 to their left. Left side of chassis top to bottom is UTC S22 mod transformer, 811A's, HV oil cap 1uF@5000V.

Ron

top view of modulator plate supply.

Top view of RF HV plate supply. The plate transformer is bolted to the cabinet floor. All other components are mounted on a metal chassis that is not attached to the cabinet. The 866's are top center. The 50H audio reactor is bottom center. Both HV power supplies are old school choke input with swingers and smoothers.

Lastly, one more view of the RF deck out of the chassis.

Here is a schematic of the major RF deck and modulator components.

Wow. Great job. This is a perfect blend of extremely buzzardly components, sound layout and construction techniques, and good old-fashioned home-brew ingenuity. Love those finals!

Very inspiring!

Oh, do tell us you drew this on a cocktail napkin or the back of an envelope!  'How romantic'.    :)

Nothing that romantic sorry to say. Just a piece of plain old copier paper. Took me about ten minutes to draw from memory. That is one more advantage of homebrewing, if something goes wrong, you usually know where to look for a problem. You know your own creation.

I was once in a restaurant in Kalispell, MT that encouraged you to do some artwork on your "table cloth" which was nothing more than a large piece of white paper. They had a large can of crayons on each table for people to use to draw whatever they wanted. The better specimens of artwork were then used as wallpaper and ceiling decorations. Of course, I drew a schematic of a push pull triode amplifier. 8000 tubes I believe. My wife just rolled her eyes, incredulous. I haven't been back, but I doubt that my artwork made the cut.

one benefit of such tubes is that you can build "Manhattan-style" on
an aluminum rack shelf like the 19"x14" one shown below.

  "  you can build "Manhattan-style "

Yeah, but who do you pay off?

klc

Ron,
It was great to hear the new rig Saturday night.  Get some heat in the shack, we'll be there on 1945.
bob woc

Yeah...Ron is dedicated (and tough)...I worked him on 160 during the Heavy Metal Rally..He was in his unheated shack at 10 degrees with only his transmitter to keep him warm....The rig sounds great Ron!