Finished with my valve junior...for the moment

Discussion in 'Amps & Cabs' started by acdcrocker, Jul 11, 2006.

  1. 1Way

    1Way Active Member

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    Thanks Bonfire, apparently I was (erroniously) redirected to the link via the safety verses testing indications early on that page. Zap it to the chassie hu. I suppose that's cool, but I keep wanting to get an earth ground involved. Seems quite useful.

    And THANKS to ESS!


    Here is the entire section. Maybe some tech can relate the most usuful stuff in a consice fashion.

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    QUOTE
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    Safe Discharging of Capacitors in TVs, Video Monitors, and Microwave Ovens

    Why This Matters

    It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that large or high voltage capacitors be fully discharged before measurements are made, soldering is attempted, or the circuitry is touched in any way. Some of the large filter capacitors commonly found in line operated equipment store a potentially lethal charge.

    This doesn't mean that every one of the 250 capacitors in your TV needs to be discharged every time you power off and want to make a measurement. However, the large main filter capacitors and other capacitors in the power supplies should be checked and discharged if any significant voltage is found before touching anything - some capacitors (like the high voltage of the CRT in a TV or video monitor) will retain a dangerous or at least painful charge for days or longer!

    A working TV or monitor may discharge its caps fairly completely when it is shut off as there is a significant load on both the low and high voltage power supplies. However, a TV or monitor that appears dead may hold a charge on both the LV and HV supplies for quite a while - hours in the case of the LV, days or more in the case of the HV as there may be no load on these supplies.

    The main filter capacitors in the low voltage power supply should have bleeder resistors to drain their charge relatively quickly - but resistors can fail. Don't depend on them. There is no discharge path for the high voltage stored on the capacitance of the CRT other than the CRT beam current and reverse leakage through the high voltage rectifiers - which is quite small. In the case of old TV sets using vacuum tube HV rectifiers, the leakage was essentially zero. They would hold their charge almost indefinitely.

    (From: Edwin Winet (ewinet@softwareresearch.org).)

    Some of us work in areas where capacitors are huge, unusual or sometimes both. Many people believe that only "big" capacitors can kill you, knock you across the room, blow a hole in you, or get your attention. Here are a couple of comments:

    When a capacitor is safely discharged, do not stop there. Some capacitors, due to their ability to leak---are "dead" after being safely discharged with a "bleeder resistor" of the right value for the job. Using a resistor that is under-rated - wattage-wise - can result in the bleeder going open circuit DURING a discharge sequence LEAVING some energy! High voltage capacitors, or worse yet, high energy-high voltage capacitors require correct wattage AND correct resistance to be bled safely. Also, high microfarad low voltage capacitors can vaporize a screwdriver and spray metal in your eyes. (Adequate voltage margin is also essential for resistors used in high voltage circuits. --- Sam.)

    Certain types of capacitors are made of VERY good materials, which can hold a charge for YEARS! Putting away charged capacitors of this type is an invitation to disaster!

    Low inductance capacitors that are used in energy pulse circuitry, many times are of the oil-filled high energy/high voltage type. This type can give a MOST un-pleasant surprise AFTER it has been completely drained by a safe bleeding technique. After the capacitor has been bled, IMMEDIATELY short it, from terminal to terminal AND to the external metal can (if applicable)!!! These capacitors RE-charge from their internal fluid and can STILL deliver a lethal, as they "recover" a certain amount of energy! this type of capacitor, or any capacitor of any high (enough) energy value MUST be LEFT shorted.

    Be particularly leery of any capacitor with a broken off lead that is sitting in a drawer! Sometimes, these units break off during testing and don't get thrown out - but remain charged - to kill or shock years later.

    Lastly, the word "electrocution" is used in many high voltage device writings. That's bad, because it was only intended for the "electric chair", short for electro + execution.

    Capacitor Discharge Technique

    The technique I recommend is to use a high wattage resistor of about 5 to 50 ohms/V of the working voltage of the capacitor. This isn't critical - a bit more or less will be fine but will affect the time it takes to fully discharge the capacitor. The use of a current limiting resistor will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage).

    Then check with a voltmeter to be double sure. Better yet, monitor while discharging (monitoring is not needed for the CRT - discharge is nearly instantaneous even with multi-M ohm resistor).

    Obviously, make sure that you are well insulated!

    • For the main capacitors in a switching power supply, TV, or monitor, which might be 400 uF at 350 V, a 2 K ohm 25 W resistor would be suitable. RC=.8 second. 5RC=4 seconds. A lower wattage resistor (compared to that calculated from V^^2 / R) can be used since the total energy stored in the capacitor is not that great.
    • For the CRT, use a high wattage (not for power but to hold off the high voltage which could jump across a tiny 1/4 watt job) resistor of a 1 to 10 M ohms discharged to the chassis ground connected to the outside of the CRT - NOT SIGNAL GROUND ON THE MAIN BOARD as you may damage sensitive circuitry. The time constant is very short - a ms or so. However, repeat a few times to be sure. (Using a shorting clip lead may not be a bad idea as well while working on the equipment - there have been too many stories of painful experiences from charge developing for whatever reasons ready to bite when the HV lead is reconnected.) Note that if you are touching the little board on the neck of the CRT, you may want to discharge the HV even if you are not disconnecting the fat red wire - the focus and screen (G2) voltages on that board are derived from the CRT HV.
    • For the high voltage capacitor in a microwave oven, use a 100 K ohm 25 W (or larger resistor with a clip lead to the metal chassis. The reason to use a large (high wattage) resistor is again not so much power dissipation as voltage holdoff. You don't want the HV zapping across the terminals of the resistor.

    Clip the ground wire to an unpainted spot on the chassis. Use the discharge probe on each side of the capacitor in turn for a second or two. Since the time constant RC is about 0.1 second, this should drain the charge quickly and safely.

    Then, confirm with a WELL INSULATED screwdriver across the capacitor terminals. If there is a big spark, you will know that somehow, your original attempt was less than entirely successful. At least there will be no danger.

    DO NOT use a DMM for this unless you have a proper high voltage probe. If your discharging did not work, you may blow everything - including yourself.
    The discharge tool and circuit described in the next two sections can be used to provide a visual indication of polarity and charge for TV, monitor, SMPS, power supply filter capacitors and small electronic flash energy storage capacitors, and microwave oven high voltage capacitors.

    Reasons to use a resistor and not a screwdriver to discharge capacitors:
    1. It will not destroy screwdrivers and capacitor terminals.
    2. It will not damage the capacitor (due to the current pulse).
    3. It will reduce your spouse's stress level in not having to hear those scary snaps and crackles.

    Capacitor Discharge Tool

    A suitable discharge tool for each of these applications can be made as quite easily. The capacitor discharge indicator circuit described below can be built into this tool to provide a visual display of polarity and charge (not really needed for CRTs as the discharge time constant is virtually instantaneous even with a muli-M ohm resistor).

    • Solder one end of the appropriate size resistor (for your application) along with the indicator circuit (if desired) to a well insulated clip lead about 2-3 feet long. For safety reasons, these connections must be properly soldered - not just wrapped.
    • Solder the other end of the resistor (and discharge circuit) to a well insulated contact point such as a 2 inch length of bare #14 copper wire mounted on the end of a 2 foot piece of PVC or Plexiglas rod which will act as an extension handle.
    • Secure everything to the insulating rod with some plastic electrical tape.
      This discharge tool will keep you safely clear of the danger area.
      Again, always double check with a reliable voltmeter or by shorting with an insulated screwdriver!

    Capacitor Discharge Indicator Circuit

    Here is a suggested circuit which will discharge the high value main filter capacitors in TVs, video monitors, switchmode power supplies, microwave oven capacitors, and other similar devices quickly and safely. This circuit can be built into the discharge tool described above (Note: different value resistors are needed for LV, HV, and EHV applications.)

    A visual indication of charge and polarity is provided from maximum input down to a few volts.
    The total discharge time is approximately:

    • LV (TV and monitor power supplies, SMPSs, electronic flash units) - up to 1000 uF, 400 V. Discharge time of 1 second per 100 uF of capacitance (5RC with R = 2 K ohms).
    • HV (microwave oven HV capacitors) - up to 5,000 V, 2 uF. Discharge time of 0.5 second per 1 uF of capacitance (5RC with R = 100 K ohms)
    • EHV (CRT second anodes) - up to 50,000 V, 2 nF. Discharge time of 0.01 second per 1 nF of capacitance (5RC with R = 1 M ohm). Note: discharge time is so short that flash of LED may not be noticed.

    Adjust the component values for your particular application.

    [size=10pt][tt]    (Probe)
        <-------+
         In 1   |
                /
                \    2 K 25 W (LV)    Unmarked diodes are 1N400X (where X is 1-7)
                /  100 K 25 W (HV)     or other general purpose silicon rectifiers.
                \    1 M 10 W (EHV)   Resistors must be rated for maximum expected
                |                      voltage.
                +-------+--------+
              __|__   __|__      |
              _\_/_   _/_\_      /
                |       |        \ 100 ohms
              __|__   __|__      /
              _\_/_   _/_\_      |
                |       |        +----------+
              __|__   __|__    __|__      __|__      Any general purpose LED type
              _\_/_   _/_\_    _\_/_ LED  _/_\_ LED   without an internal resistor.
                |       |        |    +     |    -   Use different colors to indicate
              __|__   __|__      +----------+         polarity if desired.
              _\_/_   _/_\_      |                   
         In 2   |       |        |
        >-------+-------+--------+
        (GND Clip)[/tt][/size]

    The two sets of 4 diodes will maintain a nearly constant voltage drop of about 2.8-3 V across the LED+resistor as long as the input is greater than around 20 V. Note: this means that the brightness of the LED is NOT an indication of the value of the voltage on the capacitor until it drops below about 20 volts. The brightness will then decrease until it cuts off totally at around 3 volts.

    WARNING: Always confirm discharge with a voltmeter before touching any high voltage capacitors!

    For the specific case of the main filter caps of switchmode power supplies, TVs, and monitors, the following is quick and effective.

    (From: Paul Grohe (grohe@galaxy.nsc.com).)
    I've found that a 4 watt 'night light' bulb is better than a simple resistor as it gives an immediate visual indication of remaining charge - well down to below 10 V.

    Once it stops glowing, the voltage is down to non-deadly levels. Then leave it connected for a little while longer, and finish it off with the `ole screwdriver.

    They're cheap and readily available. You can make dozen 'test-lamps' out of an old 'C7' string of Christmas lights (`tis the season!).

    Editor's note: where a voltage doubler (or 220 VAC input) is involved, use two such bulbs in series.

    (From: Dave Talcott (75711.1537@compuserve.com).)

    I built the capacitor discharge tool. I had all the parts to hand except for the series resistor, for which I used a 2 watt axial unit, since the power dissipation is not critical. I decided to package it in probe form for convenience. Except for the series resistor, which lives in a counterbore, everything is surface mounted and communicates through a LOT of cross-drilled holes. A piece of heat-shrink tubing holds everything in place. The only tricky part was making two small recesses to locate the LEDs. The probe tip is a short piece of solid copper wire salvaged from some Romex house wiring and ground to a point.

    Voltage Checkers

    Whereas a multimeter is intended to measure voltages (and other things), a checker is used mostly to just produce a quick indication of the presense of voltage, its polarity, and other basic parameters. One use is a quick, but reliable indication of the status of the charge on a BIG capacitor. An, example of a simple version of such a device is the "capacitor discharge indicator circuit" described above.

    (From: Ian Field (ionfieldmonitors@ic24.net).)

    The version of the checker that I have, also contains a miniature 12 V battery for continuity checking - any resistance less than about 22K will produce some glow. It's handy for quick checks of semiconductor junctions - in general if it produces a slight glow it's leaky, but transistor B/E junctions have an inherent zener voltage, so there is usually some glow. Also schottky-barrier diodes give a reverse leakage glow - this does not mean they're faulty, check the Vf with the diode-check on a DMM before binning! Any zener diode above 10-11 V can be given a quick test for S/C, lower Vz will produce some glow - again check Vf before binning.
    These checkers are getting hard to obtain, most of the component stockists here only carry vastly over complicated (and expensive) versions with built-in measurement computer and LCD - these wouldn't last 5 min's around flyback circuitry! Some Automotive accessory shops have a simpler version with no battery - always check that it's stated to be capable of measuring AC or DC at 4 to 380 V before parting with money! The internal circuit should contain the LED's, a 15 ohm resistor to limit the maximum surge current when the PTC is cold and the special PTC film-thermistor. The battery can be added with a button from a VCR front panel - but don't blame me if you kill yourself because you didn't insulate the added components properly!

    There is a more complicated non-battery version with 2 LED's close to the front of the handle to indicate polarity and a row of LED's along the length of the handle to indicate the voltage-range. This version contains 2 special PTC's and a discrete-transistor bargraph circuit - there might be room to add a battery inside the case. As for the special PTC this is the only place I've seen them - one possibility that might be worthy of looking into is the Siemens PTC SMPSU startup thermistor for TDA4600 control chips, this usually has a series resistor of at least 270 ohms and is more likely to turn-up in European TV set's, but I have seen it in early Matsushita IBM displays and a few others (possibly Tandon) the PTC thermistor is always blue and looks like a very-miniature copy of the Philips white-plastic PTC degauss thermistor.

    ========
    END QUOTE
    ========
     
  2. Spunky

    Spunky Active Member

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    Look here dude......

    I ain't never discharged a cap in an amp and don't frigging worry.....
    A flyback in a TV set is a different story...

    just clip the wire with the resistor onto a screwdriver and the other end to something metal that is plugged into the electrical wall outlet with a ground. Touch the + ends of all the caps with the tip of the screwdriver. The resistor just keeps the current from burning the screwdriver tip off....

    No big deal!
     
  3. *ess*

    *ess* Member

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    [quote author=Bonfire link=topic=9380.msg134705#msg134705 date=1158760932]
    :?
    [/quote]



    O0
     
  4. 1Way

    1Way Active Member

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    Spunky, alright, so please help this complete novice with some specifics.

    What resistor do you suggest, and where to get it?

    Where do I get the test lead with alligator clips, and how big?

    Thanks! BTW, this is what happened to acdcrocker, quoted from the first post in this thread!
    I guess you identify the largest looking gadgets in the amp and then drain them? Caps, resistors, trannys??? How big/small until it's not worth draining.
     
  5. Spunky

    Spunky Active Member

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    ha ha,

    I got blown back by my 83' JCM800 about 5' and into a wall. nearly cracked my skull! ;)

    get the big aligator clips. The ones that are about 2" long and insulated. use 18 gauge or bigger wire. Go get a 5 to 10 watt wire wound resistor at the local electronics store. Frys or Signal or if none, Mouser.com will have them. solder the ends of the resistor to the wire and onto the clips. I made mine 2 feet long total.
    I plug the amp into the wall with a grounded cord with the power switch in the off position.
    I clip one end of the wire to the chassis and the other to the shaft of the screwdriver. I like to use a rubber handled screwdriver I have that has the tip burned off from not using the 1 megohm resistor....he he...the flyback holds 26500 volts in a TV he he he...

    Use one hand to hold the screwdriver and touch the tip of the screwdriver to all the barrel shaped parts. touch both ends one at a time and keep the free hand behind your back!

    ;D
     
  6. skidshark

    skidshark Active Member

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  7. 1Way

    1Way Active Member

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    I remember that game! Takes a steady hand. If you mess up and touch the sides, I think it buzzes and his nose lights up. Man, the things we did for fun when we were kids compared to these days. We thought it was cool as could be, because it was electronic!!! Today kids can effectively control cartoon programs and race airplanes and such w/video games.

    Let's see, there was light bright and Etcha Sketch too.
    Oooooh, ahhhhhh.
    ;D ;D ;D

    I think those games were precursors to PC and video games.
    8)
     
  8. Bonfire

    Bonfire Active Member

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    UPDATE!!!

    UPDATE!!

    ok! i got my Tung-Sol 12AX7 and JJ/Telsa EL84 today!!!!

    boy am i happy!!! they sound sooo good!!! the difference is mainly in the fact that the distortion is more defined, as opposed to fizzy. its really good. nails that great 70's and 80's rock sound!!! even though i now have a MV, i still love to crank it becoz it sounds soooo good!!!! ooh!!! aahH!!!

    ;D ;D ;D ;D
     
  9. 1Way

    1Way Active Member

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    Alright Bonfire!

    Preamp tube questions. Which Tung-Sol 12ax7 did you get?

    Anyone
    Isn't there supposed to be a really good Mullard sounding tube of current production that people say sounds really good?

    I've been reading about the Epi Valve Jr and the more I read, it just keeps getting better and better! Some interesting reads at TheGearPage website.
     
  10. Bonfire

    Bonfire Active Member

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    i got the normal 12AX7. no gold pins here! well, i dont know what the mullard sound is, but whatever the tung-sol delivers, i like it!!

    [​IMG]

    [​IMG]

    O0
     
  11. 1Way

    1Way Active Member

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    Ok, how about the JJ EL84 then? What number did you get, i.e. 1-10? I hear that the lower the number the quicker the tube breaks up, conversely the higher the number, the more clean overhead you get. It's sorta like a low number for quieter playing and a higher number for louder playing...

    I've heard really good things about the new tung sol 12ax7 that is supposed to sound like a Mullard which was one of the best preamp tubes out there, people have been giving them great reviews, huge tone and great distortion/gain tubes! That might be it, but I can't tell.
     
  12. 1Way

    1Way Active Member

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    [quote author=Bonfire link=topic=9380.msg135651#msg135651 date=1159171192]
    [size=10pt]i got the normal 12AX7. no gold pins here! well, i dont know what the mullard sound is, but whatever the tung-sol delivers, i like it!![/size]

    [​IMG]

    O0
    [/quote]
    First, here's the graphic comparison.
    [​IMG]
    Looks the same to me!

    Check it out! This is from http://www.tubestore.com/ and there are some rave reviews of this tube here on this forum too.


    Nicely done!
    O0
     
  13. Bonfire

    Bonfire Active Member

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  14. Sludgenutz

    Sludgenutz Member

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    I bought a Tung-Sol 12AX7 for my Boogie, and it is awesome! It is more "natrualistic" than the 12AX7EH in the upper ranges...I am going to order some more tonight! The 12AX7EH is good, but it also has more "presence"/thrust in the treble range. This may actually be desired for some amps.

    I have gone through Tube Depot. com twice this month...and they offer a flat rate Fed EX shipping of $3.95 per order (check first). Both orders were well packed, and delivered on the early side of the promise date of 2-3 days.

    The thing I like about the Tung-Sol 12AX7 is the balance from top to bottom, and the distortion in the treble range is very musical (touch and pick responsive) and "tasty" for my high gain Mesa 50 watter.
     
  15. Spunky

    Spunky Active Member

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    Good Job Sludgenutz ;) ;D
     
  16. Bonfire

    Bonfire Active Member

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    sweeet :coolsmiley:
     
  17. Bonfire

    Bonfire Active Member

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    i caught the bug again....the modding bug that is.....ive been over at 18watt, creating a storm....well....not really....but i have been over at 18 watt!! here is my latest update!

    just to re-iterate how earth shatteringly good this tone is, OMFG! I CANT WAIT TIL I GET THE NEW PUP!!!! :Droolin: :Droolin: :Droolin: :Droolin:

    jealous? ;) :D ;D :p
     
  18. 1Way

    1Way Active Member

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    YES! Dang it. I keep seeing these little gems in music stores and I keep kicking myself for buying that Crate Vintage Club 20 (but for a reasonably low price) instead. I would much rather have one of these Jr's which sound Marshall'esque, I don't like the preamp sound in the Crate as much as I thought I would. It's not bad, but not as good as the Marshall vibe.
    :BangHead:
    It was cheap, I have to keep telling myself that you know...
    :roll:
    But I think instead of the Jr, I'd do better with one of the 15watt Galaxies which has a tone stack and MV I think... But really, especially for the vast majority of home guitarists out there, a 5watt mini Marshall tube amp is more than tempting :Droolin:, it's a very sweet deal! :notworthy:

    I like your what you are doing there Bon. I'm interested in hearing sound samples if you get em. I also like comparing the sound of different pickups on the same guitar rig. If you can do that with a sound sample that you play very consistently, and maybe use 2 or 3 samples, that might aid the sonic comparison... ;) I have just soaked a load into a portable PC recording rig, otherwise I'd be pretty much doing what you are doing! Got any eye or ear candy?
     

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