Ameritron Amplifier Problem

Discussion in 'Amateur Radio Amplifiers' started by K9JBX, May 9, 2020.

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  1. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Well, if someone put 2 little puffer caps on the filament lines under the tube to 'fix' a non-existent problem, for example, that could cause what you are seeing. This amp isn't tough, something duhhmmm is going on. It's time to search around for other extra parts.
     
    Last edited: May 23, 2020
  2. K9JBX

    K9JBX Ham Member QRZ Page

    I pulled the tube socket and looked it over.
    There is one small ceramic capacitor on one of the filament lines to ground and it's not on the schematic but it doesn't look like it's been added.
    It was soldered the same time as the wire was as near as I can tell.
    Should I try disconnecting the capacitor?
     
  3. N8VIL

    N8VIL Ham Member QRZ Page

    I would try it. I could understand a capacitor across the filament pins but not from filament to ground on the tube side of the choke.
     
    K9JBX likes this.
  4. W1QJ

    W1QJ Ham Member QRZ Page

    Can you post a picture of that? There is always two .01uf bypass caps at the connection of the filament choke to the transformer leads. And often times a .01 between the filament leads at the tube.
     
    K9JBX likes this.
  5. K9JBX

    K9JBX Ham Member QRZ Page

    The two bypass capacitors are there at the connection of the choke and the transformer leads. I'm assuming they are the right values and haven't been changed but I can't find them anywhere on the schematic to check the values.

    I put the correct capacitor back in according to Ameritron (C212 68pf) on the 10/12 meter coil and clipped the capacitor that was going from one filament lead to ground on the bottom of the tube socket and tested it.
    There was no difference in swr @ 10/12 meters when keying tha amplifier when powered on.

    Next I put a 110 ohm resistor from the one side of the filament to ground and with no power to the amplifier I manually keyed the antenna relay and the SWR on 10/12 meter band dropped to 1:2.8 --- the lowest I have seen.
    When I reconnected the small ceramic capacitor under the tube socket that was connected to one filament line and ground the SWR went down to 1:2.5
    I removed the 110 ohm resistor and powered up the amplifier and the best SWR when keyed was 1:15
    I tried another tube with the same results.
     
  6. W1QJ

    W1QJ Ham Member QRZ Page

    That 68pf is a mica cap. I’ve seen them go bad. It’s a bad spot to put it but they do. It’s not on the schematic.
     
  7. K9JBX

    K9JBX Ham Member QRZ Page

    Ameritron told me they had done away with (C211 - 220pf) and replaced C212 with a 68pf capacitor.
    That's why it doesn't show on the schematic. I have tried a different 68pf capacitor but no change.
    It seems strange that I can get the SWR down with a cold tune (1:2.8) but with the amplifier powered on and keyed the SWR is 1:15
    I'll get some more pictures and post them.
     
  8. N8VIL

    N8VIL Ham Member QRZ Page

    I wonder if the problem is before the input tuning board. Possibly from the input on the amplifier, through the relay and the coax to the input tuning board.
     
  9. K9JBX

    K9JBX Ham Member QRZ Page

    I will check that tomorrow however the input to the tuning board is just a wire (no coax) but the return from the tank circuit is coax.
    Here are a couple of pictures, one showing the capacitor under the tube socket and the front of the input board.
    Once again I checked the swr on the 20 & 40 meter tuning slugs and they were just fine.

    ameritron_tube_socket.jpg ameritron_input_board_front.jpg
     
  10. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    That is really odd. Does the device you are measuring SWR with tell you the values of reactance? 15:1 is a drastic change.

    Let's zoom out from the input board and look around the amp to other places just for grins. How is the suppressor doing? Can you give us a pictorial tour of the amp? A few nice hi-res pictures of the tank, that switch wafer, the suppressor, etc. Are there extra parts on the relays? How's the plate choke doing? Maybe 8 pairs of eyes will find a clue. Someone has done odd things to this unit so there has to be an interesting surprise somewhere.

    I like that 3-500 socket. It's been discussed here but I haven't seen one. If I had an SB-220... but I digress. :p
     
    Last edited: May 23, 2020
    K9JBX likes this.

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