How hard is it to move a Broadcast transmitter to 3.880khz from 1570khz?

Discussion in 'Amplitude Modulation' started by K8CCA, Apr 27, 2021.

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  1. W7TFO

    W7TFO Premium Subscriber QRZ Page

    Some of us come to hamming from broadcast. In my case, over 5 decades of it.

    When one gets used to equipment that was designed and built to run 24/7/365, you just never warm up to 'ham' gear.

    It's all about mV/M, however it is achieved.

    73DG
     
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  2. K5UJ

    K5UJ Ham Member QRZ Page

    The words of someone who obviously doesn't operate AM every day year in and year out on 75 and 160 m. A great antenna certainly helps and is key to being a strapper, but few can achieve this. I can probably count the number of tall ships on the fingers of one hand. For most, 20 w. is the path to being continually piss weak, frustrated and dissatisfied. A Valiant is certainly an improvement, but to reliably provide an enjoyable QSO experience, one needs power from a transmitter that can handle heavy processing, namely a broadcast rig, or something homebrewed to similar standards, or a ham rig with the RF and audio sections built up. This is particularly true on 160. It's a sliding scale--on the low bands, a Valiant with a D104 and average antenna can provide some fun, but expect to be shut out when condx are not great, or there's a lot of QRN or a QRMtest is going on. A two operators running mauls can carry on in the middle of the Sweepstakes. It's a whole different ballgame on 10 m. with a beam on a tower. There, if the band is open, a 20 w. plastic radio will work well. Not my thing, but some operators love it.
     
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  3. W3SLK

    W3SLK Ham Member QRZ Page

    Rob, it all depends on how you derive your pleasure regarding AM operations. Some folks like running the BC rigs. Quite a few years ago I was pining after a BTA-500MX, but the deal fell when the owner, (my elmer and good friend) went SK. I usually run 250~500W with the MK-214D and the HN-500. But there are times I like to work the people close to me and don't need the power 'high'. I remember Joe, N3IBX(SK) and I having endless QSO's on 160M in the middle of summer days running only DX-100's, (one time he used a Globe Chief complete with 15 watts out!). So if you want to chat with people further away, there is nothing wrong with 'putting another tube on the fire', (or MOSFET if you are into Class E), but some are ok with not running QRO all the time.
     
  4. WA3VJB

    WA3VJB Ham Member QRZ Page

    AMEN.

    I am coming up on 30 years owning & operating a 1946 Collins 300-G that was reconfigured for VFO use on 160-80-40. The power is inconsequential, at just 250 watts. The appeal is the substantial design, true broadcast-quality componetry, and the allure of the art-deco style it offers.

    Every time I power up, I appreciate the craftsmanship of Jim, W8MAQ, a retired broadcast engineer, who was loyal to the standards of his career as he designed and configured the modifications that let me enjoy this transmitter on our shortwave bands. He performed the same project on a second 300-G that he has continued to use from his home in Ohio.

    And, there's yet a third 300-G on the air from the home of KB3WFV, Brian. Here's a video of Moving Day, to illustrate that part of the adventure of bringing one of these home.

    http://qsl.net/wa3vjb/MovingNo.147.3gp
     
    Last edited: May 1, 2021
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  5. KA4KOE

    KA4KOE Ham Member QRZ Page

    The neophyte that I am, I researched lots of conversions, most from ER Mag. The designs invariably showed a TGTP driver. Now I know that another name for a TGTP is an oscillator. Very unstable and has to be tuned up with a spectrum scope or the sucker will spit out garbage. When I get back on it, going to find a way to neutralize the driver or direct drive with an exciter. The Tx is a Bauer 707. And thanks to Dennis for parts help in this effort.
     
  6. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    The driver stages shouldn't be that unstable. Maybe the problem is that the driver stages were designed solely with the standard AM broadcast band in mind, and are simply not compatible with HF service, and must be modified to work properly on 75m and 40m. One example of that was my experience with the RCA BTA-1R; I was given the carcass of one (minus all tubes and modulation transformer) just for hauling it away. I parted it out without even thinking of resurrecting it for amateur service. The PA stage had leads between the various components far too long, IMO even for the BC band. The thing was too damn tall to move into the shack anyway.

    My converted BC1-T originally used a single unshielded wire (incorporated into the wiring harness just like the B+ and filament leads to various stages), as the lead from the plate of the 12BY7 in the oscillator unit to the grids of the 807 RF drivers. I replaced it with a piece of shielded wire (a short piece of RG-59). That alone cured much the squirrelliness in the driver stage, and I use it only on 160m, which is supposed to lie within the manufacturer's specified tuning range for the stock transmitter.

    If a Valiant or a KW-1 can be built to not have self-oscillations in the driver stages, so can a BC transmitter, duly converted for operation in the ham bands.
     
  7. K5UJ

    K5UJ Ham Member QRZ Page

    The driver(s) I've seen or heard of have been 807s, 12BY7s, 4-65As, 813s, IOW not triodes and no neutralizing. If the oscillating is VHF, I've had that happen with an 807 once and the problem turned out to be output network caps not well grounded.

    Bauer 707: 1. I think that rig has some solid state stages? Can't remember where or which. Probably in the audio. 2. I am pretty sure that rig uses 12BY7s for oscillator/driver and a pair of glass tetrodes in the final. As such, it probably doesn't need much drive. I'd try an input pi network to the PA grids and try using a source that can provide ~20 w. continuous duty. You can get that with an 807 novice CW rig. You may need to put in a bigger transformer because some of the old rigs for CW had iron barely able to make it. For 160 the stock lineup should work with a signal generator in place of a crystal.

    That could be because the ER articles used vintage harmonic VFOs like the 122, VF1, WRL 755 etc. Virtually every ham VFO in the '50s was rich with harmonics, so the stages that followed had to have wave traps to progressively tune out the harmonics until you had one intended signal coming out of the rig. You need a driver or oscillator source that has unwanted signals attenuated by at least 30 or 40 dB in my opinion. Broadcash rigs used crystals.
     
  8. KA4KOE

    KA4KOE Ham Member QRZ Page

    The article I use modified the driver stage with a tuned inpoot and modified the output to be resonant in the 75M band.
     
  9. W7TFO

    W7TFO Premium Subscriber QRZ Page

    One really needs to use some type of bandwidth monitor, best a spectrum analyzer when redesigning oscillator/driver stages to eliminate spurs. I've seen them in the damned places up & down the dial. Some had to be parasitics, as they had no numerical relationship to the fundamental frequency.
     
  10. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    When I first put the HF-300 rig on the air, I used an 807 to drive the 211/VT4C RF driver. The rig had an annoying backwave on CW. I traced it down to RF passing through the 807, which was the only keyed stage, during key-up. So I replaced the 807 with an 802, which is a pure pentode, not a beam tetrode, and better shielded. The 802 tuned up more smoothly and was less squirrelly than was the 807, but I still had the damned backwave. Finally, I tried neutralising the stage. Sure enough, that did the trick. I was able to null out the backwave completely, and it held across the full frequency range of the transmitter, 160 through 20m. Sometime, even the minuscule feed-through of a screen-grid tube can be problematic enough to require neutralisation.

    I don't think harmonics from the VFO make any difference. Even a crystal oscillator has harmonics, since most of them operate in class-C, and the following driver stages are usually class-C. So, even if you feed a pure sine wave into the first RF stage, there are plenty of stages with high harmonic content between the VFO and the PA output network. A proper plate tank circuit at each stage should sufficiently attenuate harmonics.

    OTOH, maybe if any of the driver stages are designed to operate class-A or class-B, then a harmonic-free signal source would be more a necessity.

    It's always good practice to make sure each stage really is operating at the fundamental frequency, or that you have selected the correct harmonic frequency, before proceeding to the next stage during the initial smoke tests.
     

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