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RCA MI-4288-G Troubles

Discussion in 'Tube Audio' started by mbates14, Nov 29, 2010.

  1. mbates14

    mbates14 Well-Known Member

    Messages:
    770
    I have a MI-4288-G theater tube amplifier that I am working on.

    uses 2 5Z3s, 4 6L6s, and 2 6C5s.

    Recapped the entire thing, and had to replace a few open ballast style resistors, probably where someone powered it up with bad lytics.

    Anyway. It sounds fine, but the 6L6 at V-6, nearest to the rectifier starts to go cherry red. Sign of overcurrent.

    So, i went through and checked the voltage drop across the cathode resistor. All 4 tubes share the same cathode network. 94ohm resistor, somewhere between 29.8v to 30v drop across the resistor when fully warmed up.

    Also, when the amp is silent, you can hear some popping/crackling in the speaker when idle.

    So i started checking resistors. most of them read a slightly bit high.

    I checked all 4 grid resistors, R11, R12, R14, R15 are 47K ohms.

    R11 reads 55K
    R12 reads 55.1K
    R14 reads 50.7K
    R15 reads 58K


    47ohm plate resistors R24, 25, 26, 27 read as follows:

    R24 reads 48.1
    R25 reads 41.4
    R26 reads 40.7
    R27 reads 41.7

    Any ideas? some values might seem low, some might seem high. who knows.

    Thanks.
     

     

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  2. battradio

    battradio Electron trainer

    Messages:
    5,184
    Location:
    Near ST. Louis MO
    Have you replaced the coupling caps and filter caps yet , move the red plating 6L6 to another socket and see if the red stays at the same socket or moves with the tube .If it stays with the tube the tube is bad if it stay swith the a cap resistor or tube socket is bad .
    http://www.theused.com/manuals/rca/rca_mi-4273_4297.pdf
    Mark
     
    Last edited: Nov 29, 2010
  3. mbates14

    mbates14 Well-Known Member

    Messages:
    770
    As i mentioned in my first post, i fully restored it. all capacitors have been replaced.
     
  4. battradio

    battradio Electron trainer

    Messages:
    5,184
    Location:
    Near ST. Louis MO
    The first 6C5 would keep the amp from working if its bad, have tested the tubes or switched positions of the 6C5's or 6L6's . R10 if is missajusted would keep the amp from working .What is the B+ on the 6C5's and the 6L6's . On most PPP 6L6 amps R-13 should be about 250 ohm instead of 94 ohms
     
  5. mbates14

    mbates14 Well-Known Member

    Messages:
    770
    The G series schematic that I have, as mine is a G series, it shows 94 ohms. and it shows 94 ohms in the unit. It was bad, so i had to replace it. I need to test all of the tubes, yes. But the amp is working fine. just the plate will start to glow rosey red, and the tone starts to change until it distorts. and mine doesnt have an R-10 pot. I need to find where R-10 is though. its not a POT, its a fixed resistor.

    I bet the tubes are not matched. But the tubes have tape on them that say good, 3500 on them.

    Also the amp has a volume knob and an input interstage tranny. schematic doesnt have that.

    When i get back to the shop tomorrow, ill re-read the B+ readings. if not mistaken i remember them being around the 350 to 400v range. it shoots over 500v until the tubes warm up and draw current.
     
    Last edited: Nov 29, 2010
  6. mbates14

    mbates14 Well-Known Member

    Messages:
    770
    If i calculate the 30V through the 94 ohm resistor, i get roughly 319.1ma of current flow.

    i guess thats a little high.

    Well i have to divide that by 4, because thats across 4 tubes. So 79.7ma of current draw per tube?
     

     

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  7. battradio

    battradio Electron trainer

    Messages:
    5,184
    Location:
    Near ST. Louis MO
    I would like to see about 35 volts bias try increasing the 6L6 cathode resistace to 125 ohms and see if the readplating goes away . My PPP baldwins use 125 its the PP 6L6 that uses 250 . If you used larger filter caps or todays higher line voltage may have the B+ high
     
  8. mbates14

    mbates14 Well-Known Member

    Messages:
    770
    you might be right with the line voltage thing.

    the 6.3v filaments are running at a hotter 7.2v. If i stick the fuse in the 110v side instead of the 120v side, that goes up to about 8.5v its crazy. my line voltage is 131.2vac

    Either way i need to get those 50, 55, 58k reading ohm resistors outta there and replace them with solid 47k as the schematic calls for.

    Also the 6L6 tubes i doubt are matched. I am going to have to at least try and find a matched set as the cathode network applies for all 4 of them.

    Then again, i could modify the circuit and have a R/C setup for each tube cathode.
     
    Last edited: Nov 29, 2010
  9. mbates14

    mbates14 Well-Known Member

    Messages:
    770
    that still doesnt explain the crackling/popping sounds im hearing in deadkey conditions. no sound playing.
     
  10. battradio

    battradio Electron trainer

    Messages:
    5,184
    Location:
    Near ST. Louis MO
    Old carbon comp resistors do that , replace the imput resistors with carbon film .

    Mark
     
  11. mbates14

    mbates14 Well-Known Member

    Messages:
    770
    ok, I have 335V on the screens, and 410 plate voltage. thats directly on the plate, through the output tranny, etc...

    taking the 30v drop across 94ohms, and 410plate voltage, thats rougly 32watts of tube dissipation per tube??? Is that right?
     

     

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  12. mbates14

    mbates14 Well-Known Member

    Messages:
    770
    put a 200ohm resistor in place of the 94 ohm and now i have a drop of 42v and a plate voltage of 468v.

    With the same calculations, im getting 24.57w per tube?

    Also the resistor gets significantly hotter.
     
  13. mbates14

    mbates14 Well-Known Member

    Messages:
    770
    I put a 100ohm back in. the 200ohm was causing the plate voltage to exceed the value of the filter capacitors of 450v. i didnt need a BANG in my face.

    450v caps were originally in the amp, and thats what went back in as new ones.


    The grid voltage is roughly 0.1v and varying. varies with the signal going through
     
  14. battradio

    battradio Electron trainer

    Messages:
    5,184
    Location:
    Near ST. Louis MO
    Sounds like you need to make a bucking transformer for your amp , connect it to a variac to find out how voltage reduction you need to get the 6.3 volt to be spot on .
     
  15. Radiotubes4u

    Radiotubes4u MC75, EICO HT-20, Dynaco ST-70, Lafayette 600A, Subscriber

    Messages:
    78
    Location:
    New Castle, PA
    I bet one of your 6L6's has a bad/loose element and when it gets a little warm, it starts making a popping sound. they can test good, but when they get hot, they change composition a bit inside and any loose elements will definitely do that. I had a bad 350 that did the same exact thing once it warmed up, but it tested excellent.......
     
  16. mbates14

    mbates14 Well-Known Member

    Messages:
    770
    You do realize you dredged up an 8 YEAR OLD thread, right? I cant even recall what I did to fix it (I think it ended up being a transformer or something) or if I even fixed it. Not to mention I dont have that job anymore, or the job after that, or even living in the same location!
     

     

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