Homebrew transmitter project

Discussion in 'Technical' started by NR5P, Nov 3, 2014.

  1. NR5P

    NR5P Member

    I received a home brew transmitter for free off Craigslist. I've finally got the schematic done and powered up the rf deck by itself. It used a 4-250 or 4-400. I was told that it was a transmitter but the guy didn't know anything about it or the guy he got it from, and had no modulator. Well the operating voltages are about 2500vdc on plate and -74 grid and 320v screen. This is all unloaded. Looks to have been a linear to me. It is grid input and screen voltage comes from its own power supply and is fixed. In thinking of just leaving it as is until I can maybe find a mod transformer. Maybe just run QRP into it. Pictures to follow. I like the worksmanship
  2. NR5P

    NR5P Member


    IMG_20141027_194507274.jpg IMG_20141027_204242847.jpg IMG_20141030_200244309.jpg IMG_20141030_200310664.jpg
  3. W5HRO

    W5HRO Administrator

    What's on pin 4 of that RF tube socket?

    I'm asking because it could have been a grid modulated 803. Same socket. It would depend on how much the 2500V dropped with the tubes load, but even 2000V will work fine for a grid modulated 803. It looks like a small panel is missing and maybe it was for a small grid modulator that was removed. 320V is a little low for the screen though on all of those tubes.

    Just a possibility to rule out, but there's a chance is was for grid modulated AM. See if there is a small positive voltage on pin 4 of that socket to rule out an 803 or not. The 803's suppressor required a small voltage for control grid modulation.

    The blower fan does tend to suggest a 4-250 or a 4-400, but you never know...
  4. NR5P

    NR5P Member

    Pin 2 and 4 are connected together with a copper strip. Thanks though. I didn't know 803s also shared the same socket. And yes it looks like there is a panel missing. The only thing that was out of place was the plate voltage was not hooked to the plate current meter, which is the way current was monitored on this. Made me think it was a plate modulated rig and the modulator was missing. But the bias seems wrong for that.
  5. W5HRO

    W5HRO Administrator

    Yep, if pins 2 and 4 are shorted then either a 4-250A or 4-400A.

    I was just curious because I've come across a lot of old HB setups like that before where hams got really creative with whatever parts they had available. Thought maybe the missing rack could have been for a small grid modulator with the RF driver and buffer stages using an external remote VFO. That would also be an idea you could consider. All it would take is a small transformer with the secondary in series with the grid supply to where it modulates the RF drive with a feedback line connected to the RF driver tube to generate the correct and full 100% AM modulation envelope.

    My only suggestion is to run the screen up higher and the control grid down lower. Those bias supplies are off even for a linear. The screen should always be+500V and the grid lower than -100V. I bet the currents always ran way too hot on both grids because the screen voltage was so low. I'd use a 4-400A and not a 4-250A. The guy was probably using a 4-250A in grid driven AB2.

    Good luck with it...
  6. NR5P

    NR5P Member

    It will definitely teach me a lot. Looks like it's the voltage for ab2 looking at the tube data. It would be easy for me to redue that part of the transmitter. To add adjustable screen voltage etc.
  7. W5HRO

    W5HRO Administrator

    If you are going to play around with the screen and grid voltages to where you can experiment and move both of them around then I wouldn't rule out trying something simple like control grid modulation. Then you wouldn't need to find a modulation transformer. 4-250A's and 4-400A's control grids will handle a lot.

    Back in the old days guys use to just build circuits from the handbooks then improvise when necessary, but they often never obtained 100% modulation unless they were smart enough to think outside of the box and try something that wasn't in any of the books. You know how when you whistle into your mic with a plate modulated transmitter and adjust the RF grid drive current level to get the maximum peak modulated output power? Ever tried that? Same thing. If you use a feedback line to control the RF driver tubes output during control grid modulation and you find the correct operating point for both the screen and control grid voltages then 100% modulation would be really easy.

    Anyway, just a final thought...

  8. WD5JKO

    WD5JKO Member


    I might be able to get you parts that you are looking for to finish that rig. or something totally different. I have some big iron, and other odds and ends around that hasn't been on the air since I got married, and that was in 1986...If you can make it over to Round Rock, I'm sure I can put some ballast in your automobile. :mrgreen:

    I distinctly recall John Mohn W5MEU (sk) handing me a NOS 4-1000 tube socket. He reluctantly gave it to me after making me promise I'd use it. Well I broke that promise. Maybe I can fix that by putting the burden on you to do something with it....I also have a tube that fits into it...

  9. NR5P

    NR5P Member

    A 4-1000 may be above what my man card will allow :smile:

    I'd sure like to see you shack and what you have though. My wife and I are actually going to be in round Rock tomorrow (Saturday) to see her friends new house. I think we are supposed to be there at 2:00 for that. If some time tomorrow doesn't work let me know your schedule. I'd love to come by.
  10. NR5P

    NR5P Member

    Thanks Jim(wd5jko)! Great to come by your place and look forward to coming by again when I can stay longer. If the roads had been icy we would have had much better traction on the way back;) I look forward to getting a really cool AM transmitter on the air that I'll cherish for a lifetime. When is the AM transmitter rally going to be. Wasn't it April or may last year?
  11. W5HRO

    W5HRO Administrator

    Hey by the way Nathan, what ever happened to your other wooden framed transmitter project with the charcoal staining? Did you abandon it? If not you might want to start a second HB topic.
  12. NR5P

    NR5P Member

    Yeah we sold that place and made two moves since then. I pretty much went inactive for that entire time. Finally am settled in the place we plan on spending the rest of our lives at and I have a shop now. I was still going to use it but now that I have two metal racks I'm going to use them. Much better looking.