Fisher x100c hum problem revisited

Discussion in 'Fisher' started by mp53, Nov 6, 2017.

  1. mp53

    mp53 Member

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    Hello, This is a follow up on a privious post concerning the buzz I'm getting through the speakers out of mt rebuilt Fisher. I did conclude that the buzz was coming from the 7199 driver tubes. I bought some adaptors and replaced the 7199's with 6gh8 . These 6gh8 tubes came out of a working tuner. I still have the buzzing.
    But I have discovered that I inadvertently left out the heater reference circuit for the amp section . Just dont know the best way to fix it sense I have installed the IBAM and individul carhode resistors. I have so far no discussion here on it so any suggestions or resources is appreciated.
     
    Last edited: Nov 8, 2017
  2. mp53

    mp53 Member

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    77
    Attached is the heater string and how I wired it
     

    Attached Files:

  3. audmod01

    audmod01 AK Subscriber Subscriber

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    MP53;

    I reviewed the schematic for the X-100-C and I only see 12AX7 and 7868 outputs. Recheck your model number as the X-100-C did not use 7199 driver/phase inverter tubes. If we have the correct model number then we may be able to help you.

    I would add that it appears insulated posts have been added near the tube sockets that are not original to Fisher manufacturing. Most of us would not do that. However, heater wiring in general has to be twisted pair wires to prevent radiating 60Hz hum into nearby high gain circuits (if any), so changing heater wiring of tube circuits has to be done with great care and rechecking of performance as changes are made. Keep in mind that changing from 7199 to 6GH8A tubes can be done with amplifier circuits that used them, you are introducing extra parts by using adapter sockets and extra parts add more points of possible failure. I have a Dynaco ST-70 power amplifier that I modified to use 6GH8A tubes by cutting circuit traces and rewiring directly under the tube socket. It works well and no hum was introduced in the process, but the changes were to circuit traces that were close to the socket. Have you tried different 6GH8A tubes? I would suggest that first, providing that it worked well without hum right after the modifications were made.

    Joe
     
  4. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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  5. audmod01

    audmod01 AK Subscriber Subscriber

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    Larry;
    Thanks for pointing me to the previous thread. Now I understand what the approach is. I took time to redraw the partial schematic to make it easier to understand. Here it is:
    7199 mod 2.jpg
    One thing to be careful of is to make sure that the filament of the 7199 has a resistor connecting its filament back to a B+ source somewhat near to the voltage that appears on the cathode of its triode section. It is there to help prevent breakdown between the insulated cathode and the filament. If it does breakdown (or short) it will indeed place a 60Hz hum into the audio. Look closely at how Fisher handled this issue with the original 12AX7 driver/phase inverter. Also, remember to use solid wire for filament strings in order for wires to keep the position you place them in against the chassis. Use about 2 twists per inch or more of the filament wires if you can manage, but don't go overboard. You don't want wires to spring back into the air above the chassis metal. Stranded wire is not the best choice when handling filament circuits for that reason. Dress the filament wires well away from signal grid leads and components. Recheck solder connections of all grounds at the tube socket or any bypass capacitors leading to the 7199 tubes.

    Another unrelated item is to add a 100 ohm resistor in the screen grid circuits of the 7868 B+ feed to the tubes. This will help protect against arc-over inside the 7868 tubes. I believe Dave recommends a 1/4 watt resistor so it can serve as a fuse in case of an internal short of the screen grid.

    Joe
     
  6. audmod01

    audmod01 AK Subscriber Subscriber

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    I went back and re-read your first comments here. You mentioned heater reference circuit - I think you were thinking about the DC voltage at the cathode/heater circuit of the triode (which is where most of the voltage difference would be). According to the RCA tube manual, the 7199 allows a 200V peak difference between the two and a normal steady maximum difference of 100V maximum. All of the tubes in the set have the same filament to cathode breakdown voltage ratings and so does a 6GH8. This heater to cathode voltage may not be a problem. It could still just be due to a poor connection or poor lead dress of heater wires versus components handling signal.

    Joe
     
  7. mp53

    mp53 Member

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    I do understand what you are saying but originally there was no heater to cathode voltage wired in. I was asking the best way to implement one. So this is how I did it. Took two 100 ohm resistors wired each to one side of the ac filament supply then the opposite ends both to the hot side of the cathode resistor. This did help the hum problem. Unfortunately this was just the beginning of more undiscovered malfunctions. Evidentially wherever I soldered any signal wiring the heat of doing so melted the insulation enough to short it to the shield. I've replaced three shielded wires so far and probably replace all of them in the end.
    Right now it still has a hum but better, right channel has a lot of spurious noise and both channels have hiss. Also the volume pot is not consistent .
     
    Last edited: Nov 20, 2017 at 2:33 PM
  8. audmod01

    audmod01 AK Subscriber Subscriber

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    mp53;

    You did it the right way. What you did provides essentially a center tap of the filament AC voltage and attaching the mid-point of the two resistors to the DC present at the cathodes of the output tubes does reduce the DC voltage difference between the phase inverter stage cathode and the overall filament circuit. The cathodes of the output tubes are filtered by the electrolytic present in that circuit and also provides the center-tapped AC ground for your implementation of that design modification.

    Soldering leads near other wiring is always a risk. This is especially true for those of us who are growing older and have hands and fingers that shake. Using a small tip soldering iron with adjustable temperature helps. Also you can use hemostats or seizers to help carry away heat from the wire and help avoid melting insulation. Be careful in your choice of audio coaxial cabling materials as their capacitance from center conductor to shield varies quite a lot depending on brand and type selected. Too large a capacitance per foot can result in degraded high frequency response. Fisher used audio coax cable material that had a carbon film shield with a tracer lead in many of their products. This material had low capacitance properties and yet was small diameter, allowing less space to be consumed under the chassis.

    One way to tell if a particular stage filament circuit is causing hum is to find a way to interrupt the 6.3VAC feed to that particular tube while the rest remain connected. If the hum goes away, your efforts can be concentrated in that location. The audio will remain strong for quite a few seconds until the tube's cathode temperature drops and emission falls. It will give you enough time to judge whether the filament/cathode circuit is contributing to the problem or not.

    Joe
     
  9. mp53

    mp53 Member

    Messages:
    77
    Thanks for everyone's input, I have some more questions and observations to ask.
    1] What I
    have for the coax cable replacement is a 24gauge twisted pair silver plated copper multi stranded wire with teflon coating inside a copper braid. So i dont know what the capacitance would be?
    2] I thought about replacing the two 100 ohm resistors for the heater reference to a 50 ohm pot. That way i could adjust for lowest hum. it would be easy to do because I have a 50 ohm two watt pot and I could put it in the spot where the pot for the cathode adjustment was.
    3] Another concern I see is when I measure with my volt meter set at AC , I get a 70 volt reading at one of the legs of the power transformer AC input and chassis ground, the other leg reads much lower. Could this be a source of hum ?
    MP
     
    Last edited: Nov 20, 2017 at 2:31 PM

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