Tube CB to 10-Meter Conversion Project

Discussion in 'Technical' started by W5HRO, Oct 4, 2014.

  1. W5HRO

    W5HRO Administrator

    This last July I made the trip back to Tulsa to see my folks and I found a bunch of old radio stuff I had and used back in the 1970's. One of which was an old 3 channel Executive model 50A with an all channel receiver via a built in tuner. It was made by International Crystal Mfg in 1962. I think it was still new in the box and unused when I got it. I must have moved the mic jack to the front, but never did finish it because it's still setup for the CB band. I plugged it in and turned it on and it works fine. I searched around and did find the manual for it online.

    Anyway, I just received it last week because before I left there in July I had boxed a few things up and told my folks to ship them to me when they had the chance. What I might do is order some 10-meter xtals from International Crystal in Oklahoma City because they are still in business. I love using old tube CB's on 10-meters because they have squelch control and you can just leave them on 24/7. What I can do is build a linear for it with a built-in receive preamp and I am open to suggestions.

    I did find the attached drawing I drew up for it back then and looks like I was probably going to modify the mic input for higher gain and needed to know what the stock circuit looked like. No Internet back in the 70's. It would need an amplified D-104 unless it's modified. I might have to throw in a transistor to drive the circuit a bit before its over.

    50A.jpg
    Audio_Section_Drawing.jpg
     
  2. W5HRO

    W5HRO Administrator

    I was looking over the manual and schematic today during lunch and there are 2 oscillators and 2 mixers in the receiver. There is an adjustment cap then two fixed crystal positions. Basically it takes the receive frequency input and converts it to 10Mc first. Then that 10Mc signal gets converted again with 10.455Mc via the 2nd osc. That yields 455kc for the IF strip.

    So to receive 29.000Mc the 1st oscillator or crystal needs to be 19.000Mc. What I'm going to do is tune and modify the "TUNE" receive position to make it receive just below 29.000Mc and just above 29.200Mc. Then I can also get one 19.000Mc fixed xtal.

    On the transmit side it just uses crystals that are 1/2 of the operating frequency. The oscillator tunes to the 2nd harmonic of the crystal. So to transmit on 29.000Mc I need a 14.500Mc xtal. What I can also do when I have time is build a little external oscillator to act as a transmit VFO or as a dual VFO with a converter to work on both transmit and receive so it will track. The VFO would be all solid state though.

    Anyway, this old tube CB was really well designed compared to others I played with many years ago. I can also add the 3 diode, 2 resistor negative peak limiter to beef up the positive audio a bit. The thing about the mic input is it was designed for a very high impedance mic, but it will need a bit more audio before its over. The problem is the additional attenuation circuity after the output of the mic amp tube, the 12AX7. The 12AX7 will amplify the mic fine, but it gets cut back before it reaches the next stage. Too many series resistors and series caps in the middle due to the connected squelch circuit and doubling as the receive audio. I may have to change one of the resistors and caps in the middle somewhere. It's either that or drive the mic input a little.

    The manual is attached below...
     

    Attached Files:

  3. W5HRO

    W5HRO Administrator

    The 10-meter amp or linear needs to something that will accept low input drive coming from the transceiver, only 3 to 5 watts of drive. I wonder how a pair of 4D32's would work in grid driven push-pull? I'm looking for something that will produce around 200W carrier output.

    It's just an idea at this point, but it would help split the drive power to each tube and output full power on 10-meters because it's in push-pull.

    10-Meter_4D32_PP_Amp.png
     
  4. W5HRO

    W5HRO Administrator

    I plan to order the xtals for this project soon, but I'm also still looking at the push-pull amp design.

    I guess since no one has suggested other tubes means the 4D32's must be a really good choice. The 4D32's control grid to plate "C" is only 0.4pF which is super low and perfect for 10-meters. I think a grid driven push-pull 4D32 amp biased in class BC mode would be about the most efficient amp I could ever build and would give me the power output level I am looking for. I have a few split stator caps that were pulled out of Johnson 250 matchboxes and those caps are low pf caps and can be reconfigured. They can also be made even lower pF by removing a plate or two. I must have torn down at least 3 or 4 of those matchboxes through the years so I have the parts including the knobs.

    If the transceiver's output was still a little too hot in the end then I think just simply placing a non-inductive series resistor with each grid should work fine and might be a good idea anyway just for the added protection.