4-400 turning blue.

Discussion in 'Amateur Radio Amplifiers' started by KB4MNG, May 15, 2015.

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  1. KB4MNG

    KB4MNG Ham Member QRZ Page

    I checked the bias with my simpson 260

    op bias is a llittle over 150 dcv
    blk bias is a little over 200 dcv

    Tubes still glowing red. So I guess the problem is from pin 4 to the grids.
  2. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    If I were you I wouldn't keep letting those plates glow so much. I'd pull the tubes until you get the bias sorted out.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    4-400As glowing slightly red won't hurt them, but if they're 'bright' red, over time it could.

    Sounds like you have a disconnected path between the bias supply and the tube sockets...you can use an Ohmmeter to figure out what happened. Shouldn't be difficult. The MODE switch is in that path, so maybe it's a bad switch contact or something. Could be several things, but shouldn't be hard to find out.
  4. KB4MNG

    KB4MNG Ham Member QRZ Page

    Looks like I now have bias. The tubes are no longer glowing red:) I found a cold solder joint. That did not fix the problem. I started checking around the mode switch. I had cleaned it earlier. Sprayed more cleaner and worked it back and forth. That seemed to fix it. VR 75 OA3 is now working.

    I can put in about 10-20 watts and it looks like I am getting out about 400w. One tube is glowing more red than the other during exciter tx.

    The only problem I see now is no grid current reading. I get the two voltage readings, so I know the meter is working correctly.

  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Great that you got it working. Nice work.

    It sounds like one of the tubes may not be up to par. And 1 is doing all of the work.

    The Metering should be a easy fix, If that is the problem.

    What is the SWR going into the amp ?

    Good Luck.
  6. KB4MNG

    KB4MNG Ham Member QRZ Page

    Not sure about the swr, I'll check it. From the reading I have done, probably terrible with this amp.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Actually it shouldn't be terrible; the amp has tuned input on the bands it covers and unless somebody monkeyed with that, the SWR looking into the amp should be fine.

    400W out with 20W drive isn't bad but if the tubes are showing different anode colors, it does sound like they're not balanced and one may be doing most of the work.

    No grid current indicated? Is there screen current indicated?? That's a good tuning indicator. When you tune the plate tank to resonance, as the output power peaks so should the screen current -- also peak, in the same spot.
  8. KB4MNG

    KB4MNG Ham Member QRZ Page

    I was going to try the amp on 20 meters. I switched from 40 to 20 and the bias problem returned. Went back to 40 and it is still there/ Back to the drawing board:(
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    The bias should not be affected if you change bands.

    Are your Tube Sockets Good ?

    Good Luck.
  10. KB4MNG

    KB4MNG Ham Member QRZ Page

    I found another clue. I went back to 40 meters. The bias problem was there. The plate reading was over 500. This is with the unit on, nothing attached but a meter and dummy load, in standby. I adjust the plate tuning, the plate reading drops back to 240 the bias problem goes away. Adjust the plate, the plate current jump to about 280 or more and the tubes start turning red.

    So basically, if the plate control is turned where there is 240 ma standby current, no bias problem. This seems to only work on 40. I tried 80 and 20 and the plate current could never be lowered to the point of no bias with the plate control.

    So in stand by its like the unit is transmitting but there is no exciter and no rf going into the dummy load.


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