McMartin BA-1K Transmitter Project

Discussion in 'Technical' started by W5HRO, Jul 19, 2016.

  1. W5HRO

    W5HRO Administrator

    Here is the old 1970's McMartin BA-1K broadcast transmitter.

    Starter thread here: McMartin BA-1K

    Anyway, I started stripping it down tonight.

    1. Pulled off the warped and cracked dark plexiglass that covers the black panel below.


    2. Pulled off the meter rack and black panel.


    3. Cleaned out the tube shelf a bit.


    4. All of the relays and connection blocks below will go.


    4. Cleaned out the dropdown RF exciter and AF driver shelf.


    I think the meter rack with the push buttons will go as well. I will probably replace it with a new one that is a little taller then cut down the black panel below it so it is not as tall. If I make that meter panel taller from top to bottom, I can install a new antenna loading cap, switches and a band switch below the meters. I can just get new dark plexiglas and make it shorter so it creates the tube viewing window only. It doesn't need to be as tall as it is. Not sure why McMartin did that, but it was a bad idea making it so tall. It makes it too easy to warp and crack over time which is exactly what it did.

    I can also replace the solid-state AF and RF exciter crap on the dropdown shelf and make it tube-type. It would be too difficult making the RF exciter do 3 bands for example and I want VFO control anyway.

    The main thing with this transmitter is all of the iron is there and is really heavy. The cabinet frame and everything is solid and thick so I will strip it down completely and repaint the inside and out. I think Rustolem makes about the same color in spray cans so I can just use several cans to seal everything and clean it up. Then the new panel and sides covers can be constructed after. What I’m thinking is using nice wood panels on the sides stained and oiled to where it looks like furniture.

    Anyway, I’ll make a complete list of stuff to do later with some ideas as the project progresses further. I still need to finish stripping it down more first.

    Attached Files:

  2. W5HRO

    W5HRO Administrator

    Another idea I just had was to maybe make that whole top compartment an antenna coupler since it’s enclosed and then drop the plate tuning cap, the meters and a band switch below all of that. Then add an RF ammeter at the top with the coupler along with other antenna/dummy-load switches. There's even enough room there for a vacuum T/R relay as well I think. That might be a better all-around modification. There are a few different possibilities I think.

    The plan is to make this thing work on 40, 75 and 160-meters. Going past 40-meters would be a real stretch because of the long RF network length connections. Making it work up to 40-meters is good enough anyway. This thing probably won’t even resemble a BA-1K on the outside when finished, but who cares? It will be better.
  3. KE7TRP

    KE7TRP Member

    Interesting tube. 10 volt filament, Seems similar to a 4-400 but will take another 100MA of plate current. 830 watts key down in Class C plate modulated per tube is healthy. I had a friend with a transmitter exactly like this one but had 4-400s. I wonder if they made on with the 400 tube or maybe he converted it.
  4. W5HRO

    W5HRO Administrator

    4-500A's are pretty much extinct. You'll find them on eBay on rare occasions, but there is usually only one at a time. It would take years trying to find 4 of them that are N.O.S. Most of the time they are old pull-outs so no telling how good they would even be. It's eaiser finding good 4-1000A's.

    I was thinking about using the same 4-400's like my HB transmitter uses, the PL-4-400CG, but it looks like they will draw around 100mA more on max voice peaks due to the fact they draw a little more when in AB1 mode compared to the 4-500A's. It's probably safer to use 4-400A's that will place a little less current draw on the old transformers anyway. It won’t matter because there is no DC load on the modulation transformer's secondary so having the same dead-smack-on ratio needed with conventional class C transmitters is not critical and won't matter. One thing I may want to do though is use the same capacitive voltage divider setup on the two RF amp tube screens to better modulate them like with my HB transmitter.

    Modulation Transformer Setup

    Like you said though 4-500A’s had 10V @ 10A filaments and the 4-400’s have 5V @ 14.5A filaments. I wonder if they ever hooked the filaments up in series when converting the BA-1K to use 4-400A’s? The filament transformer has four separate secondary windings so paralleling two of them for each set of tubes would be 10V @ 20A so it might work. Getting a couple of 5V @ 30A transformers (one for each side) is really not that big of a problem though so it would be an easy switch/modification.

    Anyway, I need to finish up my HB transmitter first before tackling this one. I just wanted to get a start on it then come up with the best plan on what to do and how to do it when I'm ready. I’ll strip it down some more maybe this weekend and then go from there.
  5. W5HRO

    W5HRO Administrator

    Here are the first pics again from the other thread.

    The RF tuning network really only needs the one big coil on the top. The other one at the bottom right feeds into the other one mounted vertical on the far left side which has a motorized inductive plunger that moves up and down inside of it for antenna loading. That would probably only work for the single fixed frequency or band it was on though down around 800kc. It would be too difficult making all of that work on 75-meters let alone on 40-meters so it has to go. It will just have the one main coil across the top when finished. Plus someone disconnected the factory RF ammeter and looped the output all the way across the top and back down on the right side to where a big 7.5A RF ammeter was. You can see on the top right where the meter was plugged in and they had Heliax running down through the top of the cabinet to it. That will all go as well.

    RF Tuning and Loading Network

    The iron below in the bottom is hudge, especially the plate transformer.
  6. W5HRO

    W5HRO Administrator

    Think I found a close enough color to spay the inside and outside of the transmitter.

    Rust-Oleum Putty

  7. W5HRO

    W5HRO Administrator

    I will be playing around with the phase rotator board soon and just wanted to post what I'm sort of planning for the audio section to drive the 4-400A's. This would be the complete audio section from the D104 to the 4-400A's. It would be simple, super cheap and very easy. I won't get to this actual BA-1K project for a long while, but I think the idea below could be used for other similar transmitters where a ton of audio drive is not required. The drawing below is using a low-mu 6AS7 twin triode for the driver.

    Also, I did find a source for NOS 4-500A's and I can get 4 of them still new in the box. They are double the price of new 4-400A's, but I did find them nevertheless. Just not sure I want to pay double yet.


    Attached Files:

  8. NewWave

    NewWave Member

    Very nice Transmitter !! Some details looking similar to the 10 KW Collins with 4 X 3 CX 2500 Tubes i have seen last month in real.
  9. WQ5Q

    WQ5Q Member

    This is a very interesting project Brian. I talked with a broadcast engineer recently about converting old broadcast transmitters to ham use and he said that a lot of guys rearrange the impedance matching network so that it's closer to the output of the tubes (which I think is what you plan to do.) This makes sense.

    Also, you've probably seen it but in case you haven't, there was a nice article in RCA Ham Tips about using the 6AS7 in a speech amplifier. Seems like a good idea for a low impedance driver.

  10. W5HRO

    W5HRO Administrator

    It's going to be a long while before I get to this project. My plan was to just convert it to a simple pi-network configuration with the top compartment being the antenna coupler/tuner or some basic and more simplified configuration. Maybe even a pi-L-network.
  11. WQ5Q

    WQ5Q Member

    I hear you Brian. I've got a Viking II project going now and a Globe King 500 after that, if something else doesn't come up. Plus I've been collecting parts to homebrew a transmitter. Your mention of the 6AS7 caught my eye because I have some 810's on hand that I was thinking about using for a modulator for the homebrew. I've been considering using a pair of 6AS7's to provide enough grunt to drive the 810's. Didn't mean to get off track on the thread but I'll be interested in what you come up with for the McMartin down the road.
  12. W5HRO

    W5HRO Administrator

    :icon_shh: don't let the audiophools find out.

    Yeah, a pair would be the perfect solution to drive 810's if you have a 30W driver transformer.
  13. WQ5Q

    WQ5Q Member

    I think they already have, unfortunately. I have a couple of 20w driver transformers only. I'm on the lookout of something 30w or better like an LS48 or maybe better one of those small multi-tap mod transformers like a CVM-1 or 2. That way I have some flexibility with the turns ratio.
  14. WQ5Q

    WQ5Q Member

    Of course, the mod transformer would need to have a CT on secondary. Not likely.
  15. NewWave

    NewWave Member

    Driver Transformer.jpg Driver trans....jpg

    May be this one ?? Comes from a BC191 set with big VT triode tubes
    Push - Pull CT out to PA grid and two options for driving the input section.
    It's a heavy one +/- one KG or so.

    Output is 2 X 365 Ohm.
    input for tube drive is 580 Ohm.
    Low input is 0 - 4 - 8 - 16 - 32 Ohm

    I think he can handle the 4-400 grid current easily.
  16. W5HRO

    W5HRO Administrator

    The 6AS7 is like a dual 6A3/6B4G so if you used two tubes it would be best to use a transformer with a quad primary unless you can find one that would handle their DC current. If you paralleled the sections in each tube together then each tube would draw just around 110mA so it's whole primary would need to handle a least 220mA peak to be safe. That’s a lot of current for a driver transformer.

    Anyway, those old quad primary transformers are almost impossible to find today so you would need to come up with something else. Maybe like 2 or 4 transformers in series/parallel with series/paralleled secondaries, etc. The good thing about the 6AS7 though it that is has a separate filament and cathodes so you don't have to worry about the imbalance problem the 6A3/6B4G is prone to having and cathode bias will work fine.
  17. NewWave

    NewWave Member

    Ok I understand. It was an idea only.
    Do you have no options to make this transformer at home ?
    I mean if you know all in and out impedances ?
  18. W5HRO

    W5HRO Administrator

    To drive a pair of 810's with say like a pair of 6B4G's it usually requires about a 1.5:1 transformer at 15W. Then a 30W transformer if using four 6B4G's.

    With two tubes it was around 5000 plate-to-plate for the 6B4G's then about half of that grid-to-grid to the 810's or just slightly less. If I remember it is something like 5000-ohms p-p to 2250-ohms g-g or really close to it.

    So, if you use a single 6AS7 then you have about 5000 or 6000-ohms plate-to-plate at the driver transformer's primary. Just calculate it up if using two 6AS7's for 20 to 30W to the 810's.
  19. NewWave

    NewWave Member

    You like to build it on this way, because there is no other option ?
  20. W5HRO

    W5HRO Administrator

    It's just the requirement if you want to use 6AS7's in push-pull to AF drive a pair of 810's in push-pull (class B). I won't need near that much for this BA-1K setup and one tube should work fine in push-pull. I was just trying to give Kelly the information for what he was thinking about doing. If driving a pair of 810's to modulate a DC kW input then you need about 20 to 30W of AF drive to do it. If you are not modulating a DC kW input and it's less then you can get by with only 10 to 15W of AF drive. It just depends on what you are trying to do.
  21. NewWave

    NewWave Member

    to use the 4-400's for your BA-1K is not an option because they are to expensive?
    4-400 is here in Europe the RS 686 ( Siemens / AEG Germany) with graphite anode.
    The Dutch Philips has the same line tubes QB = Tetrode TB = Triode.
    All versions from 250 / 1000 Watts still are available in QB and TB versions.
    Three or four times a year on radio markets for normal prices.
  22. W5HRO

    W5HRO Administrator

    No, I will use 4-400's or maybe even the 4-500's since I did find a source.
  23. NewWave

    NewWave Member

    I would like an old transmitter like this to rebuild in the original version.. so far as possible..
    I have never seen this 1 KW small ones here in Europe.

    One of the last markets i have seen the 4-1000 (8166) boxed for 40 Euro..
    Next time i will look for some..
  24. NewWave

    NewWave Member

    Today i heard ( Web SDR) some US amateurs around the 75 meters with real heavy Broadcast modulation.
    It sounds really nice.
  25. NewWave

    NewWave Member

    RCA zwart.jpg RCA rood.jpg

    6AS7 is for me a strange type number, but in my collection (after one hour) I have found Original RCA Tubes 6080, thy looking similar to the 6AS7.
    One 60-04 (Black label) and the one 67-52 (Red label). It's a heavy small tube, two times 14 Watt anode dissipation, and 2.5 amp. filament current !!
    After 30 Minutes they are very hot, by using filament only..