Home-brewing a Plate modulated Push-Pull 810 deck

Discussion in 'Amplitude Modulation' started by W2BTK, Aug 29, 2019.

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  1. K4KYV

    K4KYV Subscriber QRZ Page

    I use a magnet on my hardware and never use anything steel, even if nickel plated. Not only for RF, but anything that carries current. I once had a steel nut that someone had put on one of the terminals of a rectifier filament transformer get hot enough to burn my finger. It caused the filament voltage to drop enough that the 866As flashed over on occasion.
     
    Last edited: Aug 31, 2019
  2. W2BTK

    W2BTK Ham Member QRZ Page

    Holy crap. I didn't know about that stuff. I think most of the RF paths are bolted down with zinc-plated hardware..... You know. Stuff I got for free. I am going to have to keep an eye on it or buy stuff. I'm torn.
     
  3. W2BTK

    W2BTK Ham Member QRZ Page

    Also Don- What do you think about the lack of center taps on the driver link and output coils? Do you use a safety Bias on your rig, and how much grid resistor are you using? I'm trying to compare...
     
  4. K5UJ

    K5UJ Ham Member QRZ Page

    At some point in the past, none of us knew it.
    With RF what happens is that ferrous metal carrying RF current vibrates at frequency on a microscopic level. This generates heat. Heat causes resistance to rise (and steel isn't the best conductor anyway), and nuts can work loose over time. What about steel towers used as antennas? Well, obviously anything that massive with that much surface area will not present the same sort of problem.
    Usually what's suspect are nuts, bolts and washers. I usually check homebrew I've purchased, and kits others have built because a lot of guys don't know this, or don't think it matters. It's kind of hard to find nickel hardware, copper costs a fortune, aluminum isn't very strong, so what's left is brass. A lot of hardware stores have brass fasteners and washers but they've become kind of pricey. Going forward start maintaining a stash of sheet metal. Don't throw anything out. Even small scraps of brass, copper or aluminum can come in handy and whenever you cannabalize anything, save the RF hardware and everything else except old leaky caps. Look for brass and copper hardware at hamfests.
     
    W2BTK, AG5CK and (deleted member) like this.
  5. K4KYV

    K4KYV Subscriber QRZ Page

    No need for a grounded midtap on the input or output links, nor to connect one side of the link to ground. I let mine float, leaving no pathway to ground for common mode current. I use grid leak bias plus enough fixed protective bias to keep the tubes well past cut off when there is no rf drive present. Diodes are connected in the protective bias circuit so that the grid leak bias developed with grid current, overcomes the fixed bias voltage and reverse biases the diodes, effectively disconnecting the fixed bias supply, allowing the tubes to function with 100% grid leak bias as if the fixed bias supply weren't there. In one of my homebrew rigs I use broadcast receiver type rectifier tubes as diodes, and in the other, silicon diodes.

    With plate modulated class-C finals, modulation linearity is better with 100% grid leak bias than it is with fixed bias or with a combination of fixed and grid leak bias. I bias my finals to 3 or 4 times cut-off, not just twice cutoff as the handbooks recommend. This requires more driving power but it reduces the conduction angle (shortens the grid drive pulses) and increases efficiency and modulation linearity. Just be sure not to exceed the rated grid dissipation of the tube.
     
    W2BTK, WZ5Q, N6YW and 1 other person like this.
  6. W2BTK

    W2BTK Ham Member QRZ Page

    I need to look into something like what you have there with the diodes, that is very cool. As of now the fixed safety bias is just always on, which I guess will work for initial tests. Thanks for the response, it's very helpful!
     
  7. W2BTK

    W2BTK Ham Member QRZ Page

    I think the MB-150 grid tank is ready to go and all sorted out. Here's a shot of it.
     

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    W2VW likes this.

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