Conference napalm::guitar

    Grounded power cords really *are* better, of course.  But if it were
    near mint, I would live with the non-grounded cord for the sake of
    collectability.  However, I'd also check my polarity vs. the mike's
    before I kissed the latter.  %^)  Does it have a polarity switch?  If
    not you can just reverse the plug.  Does it hum much?  Maybe you don't
    really need a ground.  Or you could rig up a separate ground wire that
    wouldn't detract much from the pre-CBS-ness of it all.
    
    Dave

    YOu might want to work out the difference in value between a near-mint
    unit and an almost-near-mint unit ... and then compare that difference
    to the cost of a week in the hospital, or a trip to the morgue.

    It's common practice to replace the cords on these amps (even mint
    ones) with grounded three-prong system.  Just keep the old cord for
    if/when you want to sell it (like to me :-).  Replacing the cord is
    totally reversible; the only way you could possibly get into trouble
    was if someone demanded original solder joints!  And I haven't seen any
    vintage amp enthusiasts who were that bad (yet).
    
    If you are really playing this amp (as opposed to just looking at it)
    then there's no question in my mind; you should do the mod.  I'm pretty
    sure any vintage amp dealer/enthusiast would tell you the same.
    
    /rick

    With a Fender amp (or any amp with a polarity switch) the three-prong
    cord doesn't prevent shocks, because if the polarity is wrong, you'll
    still get bitten.  Get a voltage tester ($1 at a hardware store) and
    use it religiously.
    
    
    						Gyro Gearloose

    I think this kind of mod makes a difference only to a highly neurotic
    collector. I consider this an improvement and would do it in a minute.
    Unless you're planning to sell it to a museum, I wouldn't worry.
    Besides, it can always be undone.

    Ground the thing and/or get a wireless.

>    Ground the thing and/or get a wireless.\

    If you go with the wireless be sure to run a ground wire from your belt
    pack to to amp chassis.

    

    just kidding

     I own 4 Fender amps none of which had 3-wire AC cords. I replaced
     them all with 18/3. I don't like shocks.
     	Just recently, I picked up an old bandmaster with the old 2-wire
     AC cord. I planned to change the cord specifically because of that
     reason, but when I got it home, I wanted to play it first. So I
     plugged it, touched another amp and got zapped but good. You just don't
     know which side is hot with a 2-prong plug. That's where a meter
     comes in handy. Even checking it with a meter, you may find the
     voltage will never get low enough so that you won't get a shock.
     That's because the amps "ground" is floating. That means the strings 
     on your guitar could be at somewhere between 0V and 125V, and no
     matter what position the "polarity" switch is in, you still get
     buzzed when you touch your lips to the mic.
     The best way to check this is to put one probe of the meter on the mic,
     and the other on the strings of the guitar and measure the voltage.
     Change the polarity switch for lowest V. If it won't go below 8VAC,
     check the PA to see if he/she is using "cheater adapters" on the
     AC line. Ideally, both the PA and the stage boxes should use the
     safety gnd in the building, and the amp chassis should be tied to
     safety ground to eliminate the possibility of shock.
     	BUT, Brian is right. A 3-wire outlet could be wired wrong and
     a meter will tell you whether the outlet is OK. The small slot is
     hot, the large slot is service gnd (the gnd at the substation) and,
     of course, the U-shaped hole is the safety gnd in the building. The
     voltage from the large slot to safety gnd shouldn't exceed 2-3V.
     If it's 120VAC, the outlet is wired in reverse, which isn't that
     uncommon. I always carry a Grounded Outlet Analyzer along with my
     meter. Get 'em at Rat Shack for $6.

     Neal

re: .all - thanks for the replies!

re: .4 - voltage tester >> how do I use it in this application?
	 polarity switch >> wouldn't this be disconnected as part
                            of the modification?

    Also make sure you buy some cheaters for your gig bag 
    because not every hall has 3-prong outlets handy.
    \Gary

 And don't forget that Alcatraz's electric chair
 was using a 2-wires AC cord !

 J. Manson

    Well, everyone seems to disagree with me here, but I've been using an
    amp with a two-prong cord off and on (so to speak) for over twenty
    years and have only gotten minor shocks when touching mikes and amps 
    that were oppositely polarized.  I consider it an annoyance more than 
    anything, though I'd worry a little more if I were playing in the rain.
    
    Am I wrong in thinking that the shocks are caused by polarization
    reversal and not by one amp being improperly grounded?
    
    In any event, two-prong cords have never been a problem for me
              problem for me
                              problem for me
                                                 problem for me
    Don King

    Haw haw - I just laughed so hard I "aged" my Strat!
    

Actually, the reason you get's a Buzz from your amp is that the P.A. and the AMP
have the Hot, or Live side of the A/C line reversed.  Household wiring has a
Hot and Nuetral side to it.  As someone mentioned already, having a grounded
plug on the A/C cord will not eliminate this.  Only a "Polarity" switch, or
unpluging, turning the plug over, and pluging back in the amp will eliminate it.

If you are getting a "Really Good Buzz" (read, curled hair and crossed eyes)
from the system, it's got a more serious problem in that there is a ground loop
somewhere.  This problem is usually accompanied with a pretty good humm in
the effected system, and at this point should be looked at by someone qualified
to work on it.

The grounded A/C cord is for the purposes of keeping this "Really good Buzz"
from seriously hurting you.  If the ground loop is bad enough, it will pop the
fuse or circuit breaker.

GONZO

    re: .14
    
    Thanks, Gonzo, that's what I thought.
    
    re: I just laughed so hard I "aged" my Strat!
    
    I don't even want to think about *this*.  %^)
    
    Dave

Again, thanks for the great info!

Am I correct in my theory that adding the grounded cord will require
the removal of the polarity switch?  Given the comment about a lot
of house wiring (not MY house, of course....) being done wrong,
would leaving the polarity switch be a plus?

Geoff

BTW - Gonzo, looked at any good cable management software lately???

    I think if you add the grounded cord you must have a polarity switch,
    because now you can't flip the plug over as you can with a two-prong,
    non-polarized plug.  In other words, grounding and polarity are two
    separate issues.
    
    Of course, a friend of mine has a brand new (Carvin) amp that has a 
    three-prong, grounded power cord and does not have a polarity switch,
    so either I'm missing something or Carvin is.
    
    Dave

Nope, leave the polarity switch there.  Just change the cord, and pay attention
to the wire colors.  NWACES::HICKERNELL is correct in that now you really need
the polarity switch cause ya can't turn the plug over.  The major reason 
polarity switches came out in the first place was the introduction of "Keyed"
A/C plugs, that is  the A/C plug tongs are two different sizes, and the new 
sockets are sized to accomodate them.  You can only plug them in one way, 
though this still doesn't do anything for a building that's wired ass backwards.

Unfortunatly, ther are some real dumb commercial electircians out there.

When adding the grounded cord, everything will solder up the same with the 
addition of the ground wire going to the amp chassis.  Get a solder log, 
find a screw on the metal chassis, and connect the ground to the screw.  Easy.

GONZO  (When this company figures out how to make MCC easy to use, them I'll
change)

    re.14
    	Crap!!! If you add the grounded cord the chassis will be grounded
    and in many cases the polarity switch will have to be disabled to do
    this. The buzzing sound may be caused by a ground loop...that's a
    different can of worms, though a severe grounding problem will cause
    the 60 hz buzzing. If the PA is also gounded properly there should
    be no safety issue, but as with many things this is often not the case.
    	Adding the three wire cord to your amp may not solve this problem
    either. Many amps isolate signal ground and chassis ground. The third
    prong provides chassis ground only and if your amp does use this
    isolation scheme the polarity switch will still be active. In this case
    the only way to be sure is to test it using a voltmeter every time you
    set up. 

    Testing using a voltmeter

    Step 1
	(Note: When testing only hold the probes by the isulated parts,
    otherwise severe damage may occur to your internal proccesses!!!)
    
    	Set the meter for AC volts on a scale to read up to 250 volts.
    Touch one probe to the signal ground of your amp (the ground of a cord
    plugged into the amp and the amp on stby), touch the other probe to a
    ground from a receptacle. This can be the metal case of a power bar, the
    screw on the faceplate. This test will tell you if your amp's polarity
    is right. If you see a reading (typically around 120volts) your
    polarity is set wrong, change it and check again. Occasionally you will
    see a reading of under 20 volts, be very careful here, this usually
    means the place isn't wired correctly and the ground that you are
    testing to is floating!!! In these cases find a good ground (water
    pipes are a good bet) or ensure that everything else has the same
    polarity as follows in the second step. Understand that this could mean
    that everything is hot!!!  
	
    Step Two

    	Now check between your amp's signal ground and the other things you
    are concerned with such as the mike, other guitar amps etc. If you get
    a reading then one piece or the other, or both are set wrong. If you get
    a reading of 120 volts then only one is set wrong if you read 240 volts
    both are set wrong and are plugged into receptacles on different sides
    of the 240volt mains coming into the building. You should get a reading
    of zero or almost zero. Change polarity switches as needed to get this
    reading. Note: if you were able to do step one you should never see the 240
    volt scenario, if you do there is a serious wiring problem in the
    building.
    
    	THIS SHOULD KEEP YOU SAFE
    
    	Guidelines to help avoid ground loops

    	Ground loops happen when you plug a number of things together and
    create multiple ground paths. Example: A keyboard with a grounded
    receptacle connected to an amp with a grounded receptacle. The
    keyboard 'sees' one path to ground through its power cord and one
    through the cable to the amp through the amp's power cord. If the amp
    is plugged into a different outlet then there is a lot of wire making
    up the grounding paths and this is essence becomes a large antenna. The
    signal most common is 60 hz noise from the power itself.
    	The easiest way to avoid these nasties is use use a single point of
    grounding...easier said than done.
    
    Thats all for now
    
    	FJE

    re; .19, I appologize and defer to your expertise.  I was assuming an
    Isolated signal and chassis ground system.  
    
    GONZO

Clarification: 

.14 I don't a particular noise problem (however, it sure isn't silent)

.17 and .18 say that I won't need to disable the polarity switch.
.19 seems to indicate that I must?

Soooo.....

I buy an 18/3 power cord (what's the best source - Rat Shack?), pull
the chassis, identify the wiring coming in, heat up my iron, stare
at the polarity switch AND _____________ (fill in the blank)

Geoff

BTW - I've built half a dozen Heathkits in my day and I'm pretty good
with a soldering iron - that doesn't mean I know squat about this
stuff!  

>I buy an 18/3 power cord (what's the best source - Rat Shack?), pull
>the chassis, identify the wiring coming in, heat up my iron, stare
>at the polarity switch AND _____________ (fill in the blank)
    
    ...and probably burn yer hand 'cause you're not paying attention.

    You might also consider replacing the existing polarity switch with a
    "Center off" type if the present switch just moves a .047uF capacitor
    from one leg of the AC line to the other rather than reversing the
    physical connections to the amp. This way you have the choice of the
    middle position (no capacitor) which often is quieter (& safer) than
    either of the others. This type of switch presumes you are using a
    3-prong plug & that the house wiring is correct. 
    For house wiring, refer to .19
    Paul
    

The dirty deed is done!  Thanks for all of the advice and emotional
support.   To keep my purist friend happy, I reverently placed the
old cord in a heavy plastic bag and stapled it to the inside of the
cabinet.  I tried to save the original solder, but I couldn't get
it out of my sold-r-vac   :')

Results - amp is a little quieter, and I can go back to drooling
over my mic with impunity.

Rock on!

    
    
    
    
    Oh, god, I hoped you used a vintage (or at least vintage-reissue)
    staple...
    
    
    

    & a gasoline "blow torch" iron, or at least one of those "handle" types
    & not a gun type. Velcro might have been a better choice than a staple.
    Paul
    

Point was, should noter 0 modify a near mint, classic Fender....

Answer would be something like,,,,, 

Ask one who deals with these things to get a better idea what the
reality check is....


When I modified my Ibanez Mc400, Rich Macduff cautioned me on making
any changes to what was then considered a rare guitar, by todays
standards, mint condition MC500 and MC400 are very rare (from what I
can gather.)

my point is...

I acutally devauled my Ibanez to the point where if I DID ever sell
the guitar, I would lose a bundle on it.  Now they were not THAT 
expensive (list 1199 back in 79).  but stock minth MC500/MC400s
have sold for as much as 700 bucks..

I've worked with a purist, not a nuerotic, but a purist, and if your 
were selling your amp after the modification, he wouldn't buy it.
If it were me, and I was looking for that amp, I'd probably buy it,
and not care about the mod.

The REAL point/counter point is do you plan on keeping the amp
and not selling it?  If so, make your change, if not, think
about asking someone who commonly deals with the buying and
selling of things like this.

In my situation, since I plan on never selling the MC400, I made
a concious desion to modify it (Installed a  Kahler)

    Well, a grounded power cord ain't like putting in an extra hole in the
    front panel and adding master vol circuitry to the innerds. If you plan
    on selling it to a museum, fine, keep the 2 wire cord and put the amp in
    a closet and don't use it. If you want to use it (safely), add the
    grounded cable. In my opinion it doesn't devalue to amp for 99% of the
    potential buyers. (No physical modifications need to be made to
    accomidate a 3 wire cable. It'll fit into a strain relief that fits in
    the same chassis hole.) The other 1% are made of the Smithsoneon Museum
    in Wash DC and a couple of anal retentives in Nashville.

I agree,,, Infact, I would have done it without a second thought..

I thought the question asked was different than what the answers
turned out....


/r

Well, its been a month or so since I made this (quite reversible) mod.
In retrospect, it was exactly the right thing to do for me, what I want
from my equipment, and my plans for keeping the Super.  The points 
raised by -.2 are right on and consideration of same went into my 
decision.

Right now I'm at the point of saying that this amp may be worth X ($1000
to ??? as a collectible), that totally reversible modifications still
might make it worth X-Y (where Y is a hundred plus), and non-
reversible mods make it worth X-Z (where Z may be $300 - $500...).

That takes me back to the issue mentioned in the last reply - do I want
to lock this thing up in a closet.  This was what I call the "purist
dilemma" - what fun is it to have a great old tube amp that I never 
play (hey - come over and LOOK AT my cool amp....).  

Then there was the "investment dilemma" - if I'm holding onto it because 
it is valuable, I must expect some level of appreciation, otherwise I 
should sell it and put the money in mutual funds!

Then there is the "replacement dilemma" - if I lock it away or sell it, 
what do I replace it with?  You folks KNOW that I (or you in the same
situation) would NEED to take this opportunity to get a KILLER rig.
So there goes another $2-3K  :')  :')  :')

So, I modified the amp.  I've got new Groove Tubes and a grounded power
cord.  It still has it's vintage characteristics (including occasional
hums and buzzes - God luv' em!

So now you're all thinking "here's a guy who's got his head on straight".
Well..... no.  You'd think from all of this that I've arrived, but I'm
not over the hump when it comes to more intrusive modifications.  For
example - I've toyed with the idea of replacing the linear volume pot
with an audio taper pot.  I seem to make a lot of fine volume adjustments
between 2 and 3 (when practicing alone).  On these old amps, once you
get above 3.5 the volume slope becomes pretty flat.

Well, I'll keep thinking about it.  I have decided one thing though - if
I'm going to do it I won't tell my purist friend until the work is DONE!!

Geoff

     Re - .30

>not over the hump when it comes to more intrusive modifications.  For
>example - I've toyed with the idea of replacing the linear volume pot
>with an audio taper pot.  I seem to make a lot of fine volume adjustments
>between 2 and 3 (when practicing alone).  On these old amps, once you
>get above 3.5 the volume slope becomes pretty flat.

     Geoff,

     The volume pots *are* audio taper in Fender amps. You could try
     a linear taper pot to see if that works better for you.

     Neal

The volume pots on my amp vary the most between 1 and 3 - at 3 I'm probably
at 60% of max volume.  I thought (but consider the source!) that this was
due to them being linear (on voltage, not on volume) as opposed to audio
taper.  

My (mis?)understanding was that audio taper pots were actually non-linear
from a voltage perspective, but the effect (in this case volume level)
was perceived to be linear.

Am I TOTALLY screwed up, or just slightly mis-guided?  :')

Geoff

>My (mis?)understanding was that audio taper pots were actually non-linear
>from a voltage perspective, but the effect (in this case volume level)
>was perceived to be linear.

     That's right, but pots aren't perfect. They're like �20% tolerance.
     Or maybe they were outta tolerance and slipped by the Fender QC
     guy. Sha..'zif. :^)

     Neal