New Mojotone Champ build

Discussion in 'Amp Tech Center' started by Heyhey, Jun 9, 2021.

  1. Heyhey

    Heyhey TDPRI Member

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    Hi guys and gals,

    I lurk here a lot, but don’t post much. I thought I’d share some pics on my progress as I build one of these things this week.

    My dad is a retired EE, and has been bugging me to come by and visit for a week and work on a project with him. He’s a HAM guy, and thought we could build a radio together, but since I’ve always wanted to build a Champ, I brought down a kit.

    We’re building it pretty stock, except I’m using a Hammond power transformer since mojotone was out of transformers, and I could source a Hammond in time from Tube Depot. We’re mounting the fuse in the chassis and using a separate power switch. I’m also using a 1/4” speaker plug.

    We’re halfway through wiring it up, filaments are next, then the ground side of the board. I should complete that in the morning. My soldering isn’t too bad I think, but there are some gloopy spots that need some attention.

    I’ll post more photos as the build moves on over the next couple of days.

    Chris,
    Chris

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  2. Jon Snell

    Jon Snell Tele-Holic

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    I don't mean this as a criticism but capacitors dry out when heated.
    Screenshot 2021-06-09 at 08.13.43.png Cathode bias resistors can get warm.

    Best practise is to separate them slightly I find with my builds, that way they don't fail.

    Looking good.
     
  3. jimgchord

    jimgchord Tele-Afflicted

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    +1 on this
     
  4. Roger66

    Roger66 Tele-Meister

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    You can save a capacitor (or anything) from a bit of the heat from soldering it in point to point, by putting an alligator clip or two on each side of the cap for a heat sink. Maybe a permanent heat sink is indicated on cathode bias resistors, or use 10 watt ones. They are not that big. Maybe 3/8" square and around 7'8" long. (From memory, so I'm probably off a little bit) They are usually white. Actually, I have seen heat sinks for resistors. They are made of aluminium and shaped like a question mark with a long tail, which is the fin, and they snap on to the resistor. I don't know if it's necessary, but they DO exist and might make them last longer. I never tried it though. They usually fry in a few years anyway and should ideally be replaced with the tubes and re-biased. For what it's worth.
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    It looks like it is coming along nicely.

    I noticed you are installing two 100 Ohm resistors on the pilot lamp socket. That is an artificial center tap (CT) for the 6.3V heater circuit. The power transformer already has the green wire with yellow stripe center tap for that function. You should use one or the other but not both methods for a CT.

    Sometimes it is quieter (less hum) to *elevate* the center tap. To do this the CT is connected to the 6V6 pin8. Cathode biased amps have roughly 20 volts at that location so it is a convenient way to elevate the CT above chassis ground.
     
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  6. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    The preamp cathode resistors do not dissipate enough heat for this to be an issue.
     
  7. Jon Snell

    Jon Snell Tele-Holic

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    That is correct but if you separate the one, why not do both. That makes its aesthetics balanced.
     
  8. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I guess if you have enough room and lead length. I personally don't like tack soldering my cathode resistors and caps together if it can be avoided.

    [​IMG]
     
  9. Heyhey

    Heyhey TDPRI Member

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    Thanks. They are air gapped, it’s just not apparent from that angle.

    Cheers!
     
  10. Heyhey

    Heyhey TDPRI Member

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    So basically, I’d run the 6.3v filament center tap, which is capped off right now, to pin eight of the 6v6, and remove the resisters from the lamp completely?

    Cheers!
     
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Correct.

    If it is more convenient, the CT can be connected to the eyelet where pin8 connects to the board. (Electrically it is the same place.)
     
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  12. Heyhey

    Heyhey TDPRI Member

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    thanks, we’ll look into that. My EE dad will want to study the schematic, but this seems like a much cleaner solution.

    Cheers!
     
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  13. Heyhey

    Heyhey TDPRI Member

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    My dad is wondering why exactly connecting the ct to the 6V6 cathode improves the circuit, rather than straight to ground? Is it for noise reasons?

    What are the mechanics of this?

    Cheers!
     
  14. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Here is a Quote from Merlin Blencowe's Valve Wizard site.

    "Elevation means referencing the heater supply to a DC voltage other than ground or zero volts. The heaters still operate at 6.3V or whatever, but this floats on top of the elevation voltage. Some valve stages such as cathode followers require the heater supply to be elevated to avoid exceeding the valve's Vhk(max) rating. But even when not explicitly needed, elevation can reduce hum in AC-heated circuits by reducing or saturating the leakage current between heater and cathode.*"

    *See: Cooper, C. E. (1944). Valve Hum. Electronic Engineering, (July), pp72-5.

    The site can be found here:
    http://www.valvewizard.co.uk/heater.html

    Since your father may have an informed opinion of this topic, I would like to learn more. As I wrote sometimes elevating the CT seems to help and other times there seems to make no difference to the hum level.
    On cathode biased amps it is so easy to just elevate the CT, I always just hook it up.
     
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  15. Heyhey

    Heyhey TDPRI Member

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    Thanks. We’re going to test it first going to ground, then to pin 8 if we have noise issues. We may test it regardless.

    he’s curious what the affects are of the voltage changing at that point in the circuit. Interesting.

    We’re all wired up and ready to test it, so we’ll report back in a bit.

    Cheers!
     
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  16. Jon Snell

    Jon Snell Tele-Holic

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    Elevating the heaters on preamp valves helps to stop emision issues between the heaters and the cathodes, that may inject mains hum from the heater supply, on valves that are suspect or have poor isolation.
    It can also increase the cathode voltage on cathode follower circuits that would otherwise exceed the heater/cathode breakdown voltage. ie, a 12AX7 has a maximum 100volt heater cathode safe working potential.

    Edit; more information added.
     
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  17. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    The heater secondary needs to be referenced to ground, not floating, because of leakage current between the transformer primary and secondary. Also, heater hum is coupled to the cathode because of leakage current between the cathode and heaters inside the tube. Centering the reference will help to cancel out hum because equal but opposite EM fields will be generated by the heater wiring.

    Usually, heater elevation is done to reduce the cathode to heater voltage difference, so that the max (Vhk) is not exceed. According to Blenclowe, heater elevation can also reduce heater to cathode leakage, which should further reduce hum.
     
    Last edited: Jun 9, 2021
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  18. Heyhey

    Heyhey TDPRI Member

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    We’re about to test it. We have a scope, signal generator, variac, and all the safety equipment we need, so I’m pretty confident. I’ll let y’all know how it goes.

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  19. Heyhey

    Heyhey TDPRI Member

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    Thanks! That’s super helpful. He’s insisting we try it with the CT to ground first. He’s a stubborn retired guy. LOL
     
  20. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Hah! My father was a Ham as well. Worked aerospace electrical QC on all of the Apollo missions etc. I should have payed more attention when I was young.
     
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