Here's another fine mess I got myself into

Discussion in 'Technical' started by K4TQF, Dec 19, 2014.

  1. K4TQF

    K4TQF Member

    Brian: you must have special access to Google. A search of all of those model numbers turned up only a couple of pictures of the speech amp, nothing on the T8K series transmitters
     
  2. W5HRO

    W5HRO Administrator

    No, just look at page 14 in the attachment. I highlighted the one for CRV-50193.

    As soon as you posted that CRV-50193 number I was able to find it and it goes with the T3K transmitter and not the T8K.

    250W will fully modulate a 500W final too. Be careful of what you are told via the other boards. They are often just full of shit due to their big egos.
    .
     

    Attached Files:

  3. K4TQF

    K4TQF Member

    The consensus was that RCA iron I have was used in a MCW only transmitter and the freq response was crap. Well, this modulator uses the identical RCA iron... go figure/
     
  4. W5HRO

    W5HRO Administrator

    You will also see on page 5 of that sheet...

    CRV-50055 (CRV-50064)
    SPEECH AMPLIFIER
    FREQUENCY RANGE 300 TO 3000 KC (probably a misprint)

    Here's the CRV-50064. It looks like the CRV-50055 was the speech amp inside of it or just the bottom part.

    tblspeech.gif

    TECHNICAL FEATURES

    Design. - Navy.
    Audio power. - 3 watts in 500-ohm load.
    Power.- Supply 110/1/60. Required : 125 watts. Allowable variation in supply line voltage +- 10 %, frequency +- 5 %.
    Input impedance. - 600/200 ohms
    Output impedance. - 500 ohms.
    Overall frequency response.- 300 to 3,000 cycles with +- 2 db of 1,000 cps. value.
    Modulation capability. - Up to 85 percent
    .
     
  5. W5HRO

    W5HRO Administrator

    I think the 300 to 3000 KC on that CRV-50055 sheet is a misprint. The specs for the CRV-50064 say 300 to 3000 cycles. I think they just messed up and put the KC on the end by mistake and it should have been cycles.

    So, that speech amp and modulator is probably only good for a response up to 3kc which is pretty narrow, but for the Navy, Hi-Fi quality wasn't important. Per your graphs of that transformer 3kc would make sense. It will produce plenty of modulator power, but it will just have a narrow Mil spec frequency response.
    .
     
  6. K4TQF

    K4TQF Member

    I think that doc was OCR. Apparently the addition of the CRV-50193 & Speech amp make a TBK transmitter a TBM. There is some confusion with the info posted on the internet because Westinghouse also made a version of this transmitter and accessory modulator.
    That document you found is correct for the RCA version of the CRV-50193. I'll post some info on the mil-surplus list and see if those guys can come up with a manual. BAMA and the other usual sites don't show anything.

    As seen below the Westinghouse is nothing like the RCA 'cept for the speech amp. Prolly painted Navy gray during a depot rebuild in the 1950's.


    IMG_0079.jpg TBM modulator.jpg
     
  7. W5HRO

    W5HRO Administrator

    I went back and looked at you RCA graph again and if by some chance it is right then the attenuation at the high end is not too far below the "0" reference point.

    What I would do is just build a good speech amp with a very high end response to compensate, but for a bit higher than the 3 watts. Make it more like 5 to 10 watts to get a full 100% out of the modulator. The output Z will need to be 500-ohms though unless you modify the input of the modulator to accept a high Z input.
    .
     
  8. W5HRO

    W5HRO Administrator

    P.S. To keep it simple if you don’t have a matching transformer for 500-ohms out is just use a pair of tubes in push-pull connected as cathode followers with the resistors to ground. That would basically work as a buffer to couple to the 500-ohm input of the modulator.

    The CRV-50055 used a pair of 2A3's to drive the 813’s in the CRV-50193 modulator. One thing you could do is use a pair of 6AS7’s which are like dual 2A3’s/6A3’s, but they have separate cathodes and are not directly heated which would be perfect. Just use the second half of each tube as the cathode follower buffers to the modulator :icon_thumbup:

    Untitled.png

    The 6AS7's are also low mu so you can resistance couple to their grids if you want without using any interstage transformer. Just bias everything up to get at least 5 watts out.

    6AS7.jpg
    .
     
  9. W6MQI

    W6MQI Member

    Where did you get those pictures that guy looks familiar I think I've worked him on the west coast military collectors group he has some really nice Navy radios.
     
  10. W5HRO

    W5HRO Administrator

    https://www.qrz.com/db/KD6TKX
    .
     
  11. K4TQF

    K4TQF Member

    Well that's not a big deal. I have the preamp in the "Norton" & all sorts of xfmrs to match impedance.
    unfortunately, I sold all my 2A3, W.E. VT10, 25's, WE 300B ( 1937 engraved base ) etc... to the audophools years ago. The Norton has a "standard lineup of 6SJ7, 6SL7 6J5 & 6N7 . I should have several drawers full of audio xfmrs.
     
  12. W5HRO

    W5HRO Administrator

    I wouldn't use any transformers, they suck and attenuate frequency response. Run the 6SJ7 as the speech amp, then into the 6J5, then that into the 6N7 as the dual inverter amp to the grids of a pair of 6AS7’s. You can find a NOS pair of 6AS7’s fairly cheap.

    If you used the second half of each 6AS7 as cathode follower buffers and did it right you might even be able to bypass right to the grids of the 813's for a bit higher Z than 500-ohms. If not the 500-ohms would be simple to achieve using 250-ohm 6AS7cathode resistors to ground for 500-ohms p-p. The whole point is trying to get the best high-end frequency response possible due to the way the RCA iron is. If you use matching transformers you will lose it.

    P.S. Check to see what voltage they were running the 813's at. If you used a single 4-400A at a low enough plate voltage then it would be around 6K which would probably work fine on the RF side.
    .
     
  13. W5HRO

    W5HRO Administrator

    Mike,

    I took one of my old drawing and quickly modified it. This it what I was referring to. If that modulator has a transformer inside to take the 500-ohms to the 813 grids then just bypass it and do the below. You can use two 1.25K resistors with the negative bias fed thru it's middle where they are connected together which will simulate the secondary winding with it's center-tap . The below circuit uses NO transformers at all which will always work better.

    813_DRIVER.png

    Anyway, just an idea. You already have the 6SJ7, the 6J5 and the 6N7. All you need are two 6AS7's with the second half of each tube connected as cathode followers.
    .
     
  14. K4TQF

    K4TQF Member

    Looks good. I'll need to check my tube caddies and see if I have any 6AS7s. There are two octal sockets on that chassis that were formerly used for the output tubes. There is a Chicago output transformer on there now, but I still have the multi-tapped Kenyon that may have a 500 ohm output on it.


    Wait... I have a new pair of 6080's on the shelf...
     
  15. W5HRO

    W5HRO Administrator

    Just sold that Peter Dahl transformer for $579 :mrgreen:
     
  16. K4TQF

    K4TQF Member

    Hardly seems worth the trouble...
     
  17. W5HRO

    W5HRO Administrator

    Well, I need the money to pay for my vintage Hi-Fi fund. Next on the for sale list is my HRO-50 or HRO-60.

    Fast response huh? I just happened to be sitting here when my iMac sounded the new mail notification :mrgreen:
    .
     
  18. WQ5Q

    WQ5Q Member

    Ka-ching!
     
  19. W6MQI

    W6MQI Member

    More like double Ka-ching!! what's the shipping on some thing that heavy $100.00?
     
  20. W5HRO

    W5HRO Administrator

    I double boxed it, but both boxes were just large enough to hold the transformer. The transformer was wrapped in bubble wrap and the inner box just fit around it then there was a thin layer of bubble around that box wedged into the outer box. Doing that kept the cost down to only $80.

    I still have the original very large box it came in which basically surrounded the transformer in about 2 feet of peanuts all the way around top and bottom, but that would have cost well over $100 to ship. I took a gamble and it got there in one piece. A ham in Tennessee bought it. Anyway, I only wanted about $450 for the transformer so with the $80 shipping and $50 eBay fee I wound up with exactly that.

    Oh by the way, someone told me that Don stated a thread on that other board praising the author of the Debunking the Collins KW-1 Modulation Transformer Myth. I wonder if he knew that was me and it was my transformer? :lol:
    .
     
  21. W5HRO

    W5HRO Administrator

    That guy sent me the pics of the transformer installed. I guess the one in his Desk KW had smoked.

    IMG_1403.jpeg IMG_1404.jpeg
    .