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4-1000A amp restoration

Discussion in 'Amateur Radio Amplifiers' started by KX4DV, Jun 8, 2019.

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  1. KX4DV

    KX4DV Ham Member QRZ Page

    Sorry misspoke. 2&4 on the tube are grounded. The 3 is metered.

    I am missing the 10ohm resistor. I need to incorporate that into the power supply setup I made.
  2. KU3X

    KU3X Ham Member QRZ Page

    Unless I am not reading something right or missing something, if there is no VOX relay, just an antenna relay, once you apply high voltage to the amp and there is no "VOX" or cutoff relay, the tube is on. That means it is going to draw idle current and the tube will get red. No grid current is a good sign but the tube must be turned off. You should have a 50K ohm @10 watt resistor in series with the B-. This cuts the tube off. When you go into the transmit mode, a relay is shunted across the resistor to take it out of line and complete the B- connection to turn the tube on.

  3. KX4DV

    KX4DV Ham Member QRZ Page

    There's not any provision for it from what I've found in the amp. There was another relay, but I think that was for the primary side of the HV transformer. (It had some very ratty wiring and ooooold wire wrap resistors) I ditched all of that and used a modern relay for the HV power supply for a soft start.

    I know my henrys run with HV across the tubes with no drive. There's no shunting relay. Just the T/R relay gets switched in and out to pass drive through the tubes. And yes, idle current is to be expected in that case. But what I saw during intial testing showed about 140-150ma at "idle". Which is a little high for nothing. And the plate grew red almost immediately after HV apply. After cutting nearly a 2" length of 12ga solid core wire length to the tubes and switching to the copper straps, it behaves like I'd expect with no plate redness. At most maybe it turns a slight straw color. And no more plate draw at idle like before. There also was a 2" section of the wire spanning across the tube pins. Basically all sorts of length for this thing to start oscillating.

    I'm going to pull the tube back out next time I get to play with it and clamp in a 90 ohm over the filament pins and tune the input with my analyzer.
  4. AI3V

    AI3V Ham Member QRZ Page

    A minor nit,

    You are describing "self bias" , and a tube cannot be truly "cut off" (zero current) with self bias, as you depend on current flow thru the resistor (which is of course in series with the tube) to generate the bias voltage across the resistor.

  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I prefer using a little supply to ensure cutoff.
    WA7PRC likes this.
  6. KX4DV

    KX4DV Ham Member QRZ Page

    Swung by the shop. There is no vox relay at all. No resistor provision for the tap, heck the tap wire broke when I touched it. Guess I'll be adding that vox relay circuit!

    Attached Files:

  7. N4HZ

    N4HZ Ham Member QRZ Page

    I built that same amplifier. My center tap of the secondary of the filament transformer has a 25K 10 Watt resistor going to ground. I use Dow Keys on the input and output of the amp which have auxiliary contacts. When the amp is keyed the auxiliary contacts short the resistor to ground. My amp runs about 4800V. When keyed with no input signal idle current runs about 90ma if I remember correctly. Note that I modified the K9KLA circuit because I couldn't get his input match circuit to work. I just used some old MFJ tuner parts and rigged up something that works on 75 thru 20 Meters. Other slight changes were made based on what parts I had. May not be perfect but works fine.

    Attached Files:

  8. KX4DV

    KX4DV Ham Member QRZ Page

    I was rummaging through my spare parts bin. I don't have any 25k resistors. But I did find a few 50k 5w's. I'm going to just gang two up in parallel. I know I have a handful of relays and sockets around as well. I'll toss one in there for the vox. Also overkill, but I found a 10 ohm 100w resistor. I'll shunt B- with that.
  9. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    If there is no VOX relay, does the PS B- go through a 10 ohm 50 watt resistor to ground?

  10. KX4DV

    KX4DV Ham Member QRZ Page

    Not yet. It will tomorrow though...

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