How much ripple in B+ and other supplies?

Discussion in 'Tube Audio' started by 71R/T, Jan 12, 2018.

  1. 71R/T

    71R/T AK Subscriber Subscriber

    Messages:
    62
    I am trying to isolate a 60hz hum issue in a Eico ST-70. I have replaced all of the electrolytics, coupling caps, I have even tried a new rectifier tube. I have verified all of the wiring is correct. I am seeing about 23VRMS ripple on the DC supplies. I know the electrolytics are low values as far as filter caps go. I used the JJ cans and I even pulled them back out of circuit to make sure nothing had gone bad. What I am seeing is DC supplies W and Y have higher voltages than they should have where Y should be 230 and W should be 165 but I am seeing 275 and 274 respectively. Y supplies the pre-amp stage and W supplies the line amp stage. If I remove W from the line amp stage I don't hear near as much hum. I not using any input signal. I have checked and checked and checked and to be honest I am starting to get frustrated because I cannot figure it out. Bob provided good advice over in Eico Place and I hope I don't wear out my welcome on here trying o get to the bottom of this. There are no open plate resistors on any of the tubes so why the higher voltages? I know these supplies are being loaded. What am I missing? I have worked on this late every night this week. Thanks for any help I can get.
     
  2. kward

    kward AK Subscriber Subscriber

    Messages:
    3,112
    Location:
    Utah
    Looks like you've got two problems. Hum and higher than expected voltages. Try chasing one at a time. They may be related but until that is known I'd treat them as separate problems.

    It's good to verify if the hum really is 60 hertz or 120 hertz. There are different debugging techniques depending on which frequency.

    60 hertz hum usually does not come from the power supply. It usually comes from a ground loop or from coupling of the power transformer or input mains wiring into one of the grid circuits in the amp.

    First thing I'd do is try to isolate which stage the hum is being introduced. Then focus on that stage in more detail once you find it.

    How do you "see" the hum? Do you hear it a the speakers or are you measuring it somehow?

    The way I usually hunt down 60 hertz hum to a particular stage is to start with the output stage and work backwards. Pull all the tubes from the amp and turn it on. It should obviously be quiet but if it isn't you know the hum is coming from coupling of the power transformer to output transformer or wiring of output transformer.

    Next put in the rectifier tube and output stage tubes and try again, moving backward one age at a time. Eventually the hum will show up and it's in that last stage with tubes in it that the hum is being introduced.
     
  3. 71R/T

    71R/T AK Subscriber Subscriber

    Messages:
    62
    I can hear the hum in the speakers and can feel it in the woofers. As far as the 2 higher voltages they feed the the pre-amp stage and line amp stages. I unhooked the supplies from those stages to see what the voltages did and they went higher as expected but I don't understand why they would be higher to begin with when loaded other than a poor ground maybe? That PS is not complicated at all. The 440VDC is fed through a voltage divider network consisting of 3 10K resistors. Withe the supplies loaded I checked the voltage at each stage. I also replaced those resistors so I know they are good. I will pull the tubes and isolate the stage. Thanks.
     
  4. Jim Carr

    Jim Carr New Member

    Messages:
    19
    Location:
    Loleta, CA
    Does the volume control change the hum? How about shorting the inputs? It does look like the voltages are too high especially "W"
     
  5. 71R/T

    71R/T AK Subscriber Subscriber

    Messages:
    62
    What would cause those 2 voltages to be high? I checked the caps and resistors in that divider network and they all check good.
     
  6. Jim Carr

    Jim Carr New Member

    Messages:
    19
    Location:
    Loleta, CA
    I do not have the schematics for this unit but when I see higher than expected voltage then there is increased resistance between ground and my test point. It could be an open component, bad ground, a resistor way out of spec. etc.
     
  7. Jim Carr

    Jim Carr New Member

    Messages:
    19
    Location:
    Loleta, CA
    I think I found your schematics on the net. If so have you checked R90 for short? Should be a 10K 2 watt resistor.
     
  8. 71R/T

    71R/T AK Subscriber Subscriber

    Messages:
    62
    What would cause those 2 voltages to be high? I checked the caps and resistors in that divider network and they all check good.
    Yes I replaced all of those resistors in that network.
     
  9. whoaru99

    whoaru99 Epic Member

    Messages:
    39,769
    Location:
    LoTL
    There are only a couple things that would cause voltages either side of a 10k resistor to be essentially the same when they should be 45V different.

    • There is no load on the downstream/(what should be) lower voltage leg.
    • The 10k isn't actually 10k, whether by component value or connection.
     
    Last edited: Jan 13, 2018
  10. kward

    kward AK Subscriber Subscriber

    Messages:
    3,112
    Location:
    Utah
    Mapping the Eico Schematic labels onto the SAMS labels:
    • Y (Eico) = 230V Source (SAMS)
    • W (Eico) = 165V Source (SAMS)
    • Z (Eico) = 130V Source (SAMS)

    For the 230V source and 165V source to both read at approximately 275V, means there is no current being pulled through the 165V source. This tells me that the stages where the 165V source is connected are not conducting. Either the 165V source is not properly connected to the plate resistors of those stages, or the grounds of those stages are not connected properly to circuit ground.

    Because the 165V source is further downstream from the 230V source, if the stages to which the 165V source are connected are not conducting, it will cause the 230V source to read high also. So check the stages to which the 165V source is connected. I suspect one or more triodes in those stages are not conducting.

    You might also check the stages to which the 130V source connects. It may also not be conducting.

    You can tell if a stage is conducting or not by measuring the voltage drop across the plate resistor of that stage. I suspect when you measure the voltage drop across the 33K plate resistors of both triodes in V2 and V8 (from SAMS), i.e., the stages to which the 165V source connects, you will see there is nearly zero volts drop across one or more of those four resistors.
     
    Last edited: Jan 13, 2018
    thorpej likes this.
  11. jaymanaa

    jaymanaa AK Subscriber Subscriber

    Messages:
    54,480
    Location:
    OZ
    To the original topic, I think 1 or 2 vac is ok on the first stage of the PS. Ripple should of decrease in latter stages, and be in mv at the phono stage (or last stage).
     
  12. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,541
    Location:
    San Francisco, CA
    Indeed. I'd say the incorrect voltages problem is the one that needs to be tackled first.
     
    Dandy likes this.
  13. whoaru99

    whoaru99 Epic Member

    Messages:
    39,769
    Location:
    LoTL
  14. 71R/T

    71R/T AK Subscriber Subscriber

    Messages:
    62
    I went back again and verified the ripple was okay on the DC supplies. grounding issue with scope probe. The hum is 60 Hz, not 120. It is not loud but I can hear it from about 3-4 ft away from speakers. I am beginning to wonder if it is coming from the filament heaters?
     
  15. whoaru99

    whoaru99 Epic Member

    Messages:
    39,769
    Location:
    LoTL
    Sorry, I was actually asking about the two(?) voltages being too high. I see that wasn't clear.
     
  16. 71R/T

    71R/T AK Subscriber Subscriber

    Messages:
    62
    They still seem higher than normal, but the voltage drop across the plate resistors looks good. The tubes are conducting in those stages. I have replaced all of the resistors with metal film ones too. Honestly, the amp biases good and if I can get rid of the hiss and the hum it sounds great otherwise. The hiss is louder than the hum.
     
  17. Bob

    Bob AK Subscriber Subscriber

    Messages:
    1,260
    Location:
    West coast
    I thought this was to be consolidated into one thread. please do so, the hiss is on another thread
    and the hum is also on yet a different thread.

    yikes. put a large cap on the heater supply output if there's a DC PS on it. take a closeup pix of all the
    resistors in X W Y Z. pix of the taps to the circuits would be helpful. pull resistors and measure and
    post pix of measurement. there's two reasons the voltages are over. pix will confirm or rule out one
    and/or the other. take pix of the replacement caps. and the board. do the math for current through
    the succeeding resistors in X W Y Z. to confirm a third possibility.
     
  18. gkargreen

    gkargreen Active Member

    Messages:
    352
    Location:
    Fairfax, VA
    check your filament supply, does it have a return to ground on the center tap? If not, you will need to make one by attaching a 220 ohm resistor to each side of the filament string, ground the other ends of the resistors together. In a number of amps the filament string is grounded through the outboard preamp for these units so that if you don't have the outboard preamp connected to the amp the filament string has no ground return. Maybe this helps?
     
  19. PakProtector

    PakProtector AK member

    Messages:
    3,604
    Location:
    Ann Arbor, MI
    Here is where PSUDii is of great use. Model the PS components in that program, and see what you get, and what shape the ripple is. First time I tried that I was, let me say, unpleasantly surprised.
    cheers,
    Douglas
     

Share This Page