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Thread: MODS

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    MODS

    Just curious what everyone's favorite mods are to the stock circuit?

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    Supporting Member Jazz P Bass's Avatar
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    I prefer factory correct circuits.

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    Old Timer tedmich's Avatar
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    Noodle of Reality Steve Conner's Avatar
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    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Senior Member hasserl's Avatar
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    My favorite mods would be 4.7uf cathode bypass on the input stages, .022uf coupling caps in the pre, .047uf coupling caps to the power tubes.

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    Supporting Member loudthud's Avatar
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    There are a few updates that everyone takes for granted. The power cord is updated to a 3 wire with safety ground. The "death cap" and ground switch are removed and replaced with a standby switch. The heater wiring is updated to a balanced type with a virtual or actual center tap. The center tap can be connected to the 6V6 cathodes for a little lower hum. The 16uF caps in the power supply are sometimes increased to 22uF for availability reasons.

    There are several mods to the coupling caps that are frequently done because of excessive or flabby bass. See Hasserl's post above.

    Grounding can be an issue. Everyone seems to have their own opinion on how to get it right. Good luck with that!

    Tube swaps can be done to change the gain and/or headroom.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Steve Conner View Post
    Man Steve, it's a good thing you aren't a plumber...

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    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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    Supporting Member deci belle's Avatar
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    The center tap can be connected to the 6V6 cathodes for a little lower hum.
    Please tell me about that, Loudthud!

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    Supporting Member loudthud's Avatar
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    Preamp tubes can inject hum from the heater circuit into the signal for a number of reasons. One is that the heater wants to emit electrons (because it's a hot glowing metal) and those electrons can be attracted to the cathode. Not a problem if the cathode is grounded or fully bypassed to ground with a big capacitor.

    If the cathode is always negative with respect to the heater, the cathode will repell those electrons. This is accomplished by making the heater slightly positive, about 20V. This reduces the 60Hz hum that sounds like a low organ note, not the 120Hz buzz that comes from the power supply. The amount of reduction will vary from tube to tube.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    Supporting Member deci belle's Avatar
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    Okó now I know why, Loudthud!

    So on a 5E_, the center tap shares the ground connection with the cathode/resistor bias circuit?

    Is this the green/yellow 6.3v center tap?

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  11. #11
    Supporting Member loudthud's Avatar
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    Quote Originally Posted by deci belle View Post
    Ok— now I know why, Loudthud!

    So on a 5E_, the center tap shares the ground connection with the cathode/resistor bias circuit?

    Is this the green/yellow 6.3v center tap?
    Your wording makes me a little nervous. In this thread, post #4, I posted a layout of an amp with elevated heater supply: http://music-electronics-forum.com/t12663/

    Different output tubes were used but it's basically a 5E3. Note how the green yellow striped wire goes to the plus side of the cathode bypass cap of the output tubes. The layout also shows a good grounding scheme.

    In this thread, post #10, I posted a schematic of a 5F1. That transformer didn't have a center tap on the heater winding so a pair of 100 ohm resistors were used to create a virtual center tap. http://music-electronics-forum.com/t16311/

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

  12. #12
    Supporting Member deci belle's Avatar
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    Different output tubes were used but it's basically a 5E3. Note how the green yellow striped wire goes to the plus side of the cathode bypass cap of the output tubes. The layout also shows a good grounding scheme.
    Yes, Loudthud. I see the ctr tap in the first example (5E3) and the two 100Ω in the second example (5F1) connecting at the same spot: the positive side of the cathode bypass cap for the power tube(s).

    I did not understand which side of the cap you meant, which IS a good reason to be worried. I've only been at this for two years and am just in the middle of my first scratch build. Thank you so much for including the links to both examples!

    In my case, even though I have a heater ctr tap, I want to ground the 100Ω resistors per example two. My PT came with a warning NOT to use the heater ctr tap unless it was on my schematic (since I drew my schematic it isn't there haha!!).

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    Last edited by deci belle; 02-01-2012 at 05:29 AM. Reason: got my examples mixed up!

  13. #13
    Supporting Member loudthud's Avatar
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    Use one or the other, not both. If you hook one to the cathodes and ground the other one, you will short out the bias for the power tubes! (That's very bad!)

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

  14. #14
    Supporting Member deci belle's Avatar
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    Haha! I'm joining the ends of the 100Ω Rs and that'll go to the positive side of the pwr tubes' bypass cap. I'm lifting the heater CT.

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Adding to this... you can just lift the filament winding's center tap (usually green/yellow here), then solder a 1/2 watt, 47 to 100 ohm resistor on the end of it and solder the other end of the 47 to 100 ohm resistor right to the positive end of that same cathode bypass cap previously mentioned.
    You don't really need the 1/2 watt resistor but I always do it in case there is short from lug 2 to 3 somewhere on one of the power tube sockets.....and my small brain makes feel like the 1/2 W resistor could at least blow open if too much Hi-V goes into it.

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    Bruce

    Mission Amps
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    www.missionamps.com
    303-955-2412

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    Supporting Member deci belle's Avatar
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    Bruce, why is the bright channel's coupling cap .047 and the normal channel's cap .022 on the Tweedy specs? Guess you can tell I haven't wrapped my head around the 5E3's volume/tone circuit yet!

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  17. #17
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Not sure where that info came from unless you might be talking about someone's mod and they just used my altered artwork... which was altered from old Fender art.
    Using smaller coupling caps to the volume pots is an old and true way to rid some of the flabbiness from the 5E3 circuit.
    The normal channel is very flat and dull sounding to me so a smaller cap vs the bright channel is a good thing to try.
    I've even gone from the stock .1uF to as a low as .0075uF and the amp still sounded really good.

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    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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    I personally like these:

    Some Blindingly Obvious Things...

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