kmetal

Kyle P. Gushue
Well-Known Member
Joined
Jul 21, 2009
Awsome! Great clarification. lol I had a lot of fun w those Mesa amps.

So in an ideal world would you want to have all three items conditioner, ISO transformer, and battery backup? Is that unecesary resundence?

I'm assuming a case where 'typical' power is supplied from the breaker, i.e. Nothing special as far as circuits go.
 

rmburrow

Well-Known Member
Joined
Dec 26, 2012
My late 60's Fender has a 2-prong, non-polarized plug (which means the 2 blades are the same size, and you can plug it into the receptacle either way), and on the back of the amp is a "polarity" switch. Putting a grounded cable on it is one of the few modifications you can make to a vintage Fender that hardcore collectors will give you a pass on - as long as it's a genuine red plug Fender cable. They don't seem to mind, because it really is dangerous the way they made them back then.

Back in the 70's when I first started playing, some of my friends who were slow-learners, wouldn't think about checking their polarity until AFTER they stepped up to their mic and got shocked in the mouth. I got bit once and learned to check it by grabbing the neck of the guitar with one hand (making sure I had good contact with the strings), and quickly brushing the back of the other hand across the windscreen of the mic. If it was backwards you got a quick,mild tingle of a shock, but it was a whole lot better than getting shocked in the mouth by what felt like the full 110v.

If they want UL to sign off on them, all modern (US) devices have to have a ground to ensure there can only be one orientation of the hot and neutral, assuming the receptacle is wired correctly (which is also a dangerous assumption). If it's a device that can safely operate ungrounded, one blade is wider than the other so you can't get the hot and neutral reversed. If it has neither of those, then it's something they've deemed poses no potential shock hazard regardless of electrical polarity and absence of earth ground.

A power conditioner can absolutely filter out noise, if everything else is kosher - Unless of course you plug your power conditioner into the wall using a 3-prong to 2-prong adapter, at which point all bets are off.



dvdhawk: Is your late '60's Fender amp with the two prong non-polarized plug a tube type unit with a power transformer? If the amplifier uses 6 volt tubes such as EL34, 6L6, etc., a power transformer is probably used to obtain heater and B plus voltages for the tubes. I wrote a comment a while back in response to someone getting shocked off a LA-2 unit.

Some equipment manufacturers connected low value capacitors (typically 0.001 to 0.01 uF, 600 volt paper or mylar) across each side of AC to ground (chassis), after the equipment fuse and at or before the power transformer primary. (These capacitors were for RF or transient suppression.) Sometimes, another capacitor was connected across the primary itself. The assumption at the time (even with the two prong non-polarized AC plug) that the equipment chassis was effectively grounded. With a power transformer, there was no reason to connect one side of AC to the chassis, unlike typical AC/DC gear built at the time. However, the transient suppressor capacitors from AC to chassis did "bleed" an extremely small amount of AC to the chassis, and someone would feel a shock, especially with a mike at the mouth. (The reactance of each .01 uF capacitor at 60 Hz is around 265 k ohms; the current through each capacitor with 117 volts applied is around 0.4 mA.) The obvious repair is to install a grounded power cord (3 conductor) to the amp, and make certain the receptacle ground is good. (If you are a "purist" and don't want to modify the amp, connect the chassis to an effective ground like a grounded METAL cold water pipe; there may be a ground post on the back of the amplifier for that purpose.)

The other scenario is a defective (leaky) power transformer short from a winding to the core. The core is essentially connected through the transformer case to chassis. This scenario could elevate the ungrounded chassis to a dangerous voltage. Again, the obvious need for a grounded power cord. With a grounded power cord, a shorted power transformer to chassis should immediately blow the equipment fuse, or your eyes and/or nose will lead you to the problem.
 

dvdhawk

Well-Known Member
Joined
Dec 18, 2008
Yessir, that's correct. The only amp I still own like that is a '69 Super Reverb, and that's right - 1x GZ34 Rectifier tube, 2x 6L6GC Power tubes, and 6x assorted 7025, 12AX7, & 12AT7 pre-amp & reverb / vibrato driver tubes.

So this is basically what we're looking at, as you've described. I'm not adverse to putting a ground cord on it, but I am adverse to what they're asking for a genuine Fender cable. Ideally, I'd want to change out the receptacle on the back of the amp to a grounded version at the same time.
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The days of me breaking my back lugging around an amp that size are behind me, and the days of traveling with a road-crew are waaaay behind me. So the '69 Super and I will probably both be comfortably planted here for some time, until someone is looking for that signature vintage Fender tone.
 
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