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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

    I'm trying to get a 4-1000a amp back in working order that I picked up from a SK. It appears that the SK built it based on plans by K9LKA. (Grounded Grid) It has a pole pig transformer for the HV side of things. I threw in a full wave diode bridge I had laying around for it and a new henry 7.5kv 16uF cap. I added a soft start circuit for it and a variac to allow control of the power above 120v. I'm going to take a deeper look at the grid meter tomorrow, but I haven't driven it yet. So that might not be an issue. What is a issue is the draw I'm seeing on the HV side of things. Once I apply HV, I notice the plate shows some red at idle, not keyed, and not even a radio connected. Looks to have about 140-150ma of idle current at 4500v. (No grid current shown, but that could be a meter, wiring, or other issue) And once I switch off HV, it drops like a rock on the meter. (With the tube out of circuit, my bleeders take about a minute to bleed the cap) From what I've gathered, I probably should be looking at the grid grounding? I've seen mention of adding parasitic suppression there as well? (And I had one person mention to add a resistor there instead?)

    I'll have a chance to play with it more tomorrow, just didn't have time today to get any further. But wanted to see what everyone suggests.
  2. W1QJ

    W1QJ Ham Member QRZ Page

    Unless you can produce an "as built" schematic of this amp it would be hard to help you out. Something built back in 1963 may have some design techniques that are no longer used. I had to go through and make repairs on a homebrew pair of 4-1000 amp and had to figure out what the designer had in mind. First thing that I noticed is that the grids were hard grounded and the B- was at chassis ground. With that design it is impossible to meter grid current. The list goes on. What you are seeing could be the way the amp is designed or maybe it is oscillating. Hard to tell without seeing the design schematic.
    N2EY likes this.
  3. KU3X

    KU3X Ham Member QRZ Page

    It sounds to me like your T/R relay is in the closed position and the amp is always in the transmit mode. If two relays are used, one for the antenna and one for the cutoff, the cutoff relay is what I am focused on.

    I can email you a schematic of my HB 4-1000 GG amp if needed. I run 4500 volts no load on my 4-1000. It also includes tuned inputs.

    Barry, KU3X
  4. KX4DV

    KX4DV Ham Member QRZ Page

    I'll get a pic of the schematic in the article shortly. I'm about to head over to the shop now. Chassis is also B- as well. I recall that the amp had one grid's neutral pulled out and passed to a meter circuit. (Had a coax leading from it) The other two were to chassis.

    T/R is not in the closed position. It's a open frame style relay. I can engage it, but currently I don't have anything keyed for it. Haven't even hooked up any radio to it yet.
  5. N8FVJ

    N8FVJ Ham Member QRZ Page

    BTW- wicked bottle.
    W1NK likes this.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If the grid is grounded you can’t meter that if the B- is also directly grounded. A full schematic would surely be helpful.
  7. KX4DV

    KX4DV Ham Member QRZ Page

    Here's the schematic for the RF deck. There's some differences from things I've done and things the SK did. There's a single plate choke and only one cap at the base. The plate cap also is 1000pf now (It's what I had laying around, the original was broken). There might be some other differences here and there, but pretty much the same. (There is a time delay circuit for the T/R keying circuit. Course the T/R isn't shown in this diagram)

    I tinkered with it a bit, it's no longer pulling down plate voltage when switched on. And no more plate glowing while idle. The ground leads to the tube were a bit long and made from 12ga solid wire. I removed them completely and replaced them with copper straps as short as possible and up against the socket to the chassis. (Save for the one grid pulled to the meter)

    Attached Files:

  8. KU3X

    KU3X Ham Member QRZ Page

    Does the T/R relay also include the VOX relay, as they refer to it in the print? That is what must be open.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The schematic doesn’t show the grid as being grounded, it’s above ground for metering; the screen is grounded.

    Also B+ is not at chassis ground, it’s 10 Ohms above ground per the schematic. Make sure the “build” actually follows the schematic.
  10. KX4DV

    KX4DV Ham Member QRZ Page

    No. There is no VOX relay to it. The T/R relay is working fine. Bypass when open, through the deck when closed. Closes and releases fine. Confirmed flow as well with voltmeter.

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