Save your skin! Upgrade that scary old amp with an isolation transformer.
Quite a few old amplifiers (and radios) back in the day drew power by directly rectifying the household "mains" wiring. This is an inherently unsafe practice.
Most guitars connect the bridge and strings to the ground (shield) wire on the guitar cord, essentially using the player as a "noise shield." In transformer-less amps, the Neutral wire of the mains is often used as the "ground." With a two-prong cord, Neutral and Hot can be switched (which could place the amp's ground on the Hot wire!) In other words, playing a guitar amp without an isolating transformer could be like sticking a fork in a wall outlet.
Isolation transformers limit the amount of current that can be supplied to the amp (and consequently to the guitar player) if any shock hazards arise, and eliminate possible "hot" ground issues.
In addition, we'll install a three-prong cord, so the amp has a proper earth ground. And a fuse, too. The earth ground and fuse help to maintain a sane ground reference, and protection from shorts.
And we'll incorporate the changes on a small "module," so as to change the original as little as possible. If someone is crazy enough to revert to the original setup...they can do that.
This mod works with radios, too. In fact, many of these amps were called "radio tube" amps, or "AC/DC amps"--like their radio counterparts, a transformer-less amp could be plugged directly into a DC or battery power supply without modification. A decently-sized bank of batteries were required (over 100V), but that was once commonplace.
Quite a few old amplifiers (and radios) back in the day drew power by directly rectifying the household "mains" wiring. This is an inherently unsafe practice.
Most guitars connect the bridge and strings to the ground (shield) wire on the guitar cord, essentially using the player as a "noise shield." In transformer-less amps, the Neutral wire of the mains is often used as the "ground." With a two-prong cord, Neutral and Hot can be switched (which could place the amp's ground on the Hot wire!) In other words, playing a guitar amp without an isolating transformer could be like sticking a fork in a wall outlet.
Isolation transformers limit the amount of current that can be supplied to the amp (and consequently to the guitar player) if any shock hazards arise, and eliminate possible "hot" ground issues.
In addition, we'll install a three-prong cord, so the amp has a proper earth ground. And a fuse, too. The earth ground and fuse help to maintain a sane ground reference, and protection from shorts.
And we'll incorporate the changes on a small "module," so as to change the original as little as possible. If someone is crazy enough to revert to the original setup...they can do that.
This mod works with radios, too. In fact, many of these amps were called "radio tube" amps, or "AC/DC amps"--like their radio counterparts, a transformer-less amp could be plugged directly into a DC or battery power supply without modification. A decently-sized bank of batteries were required (over 100V), but that was once commonplace.
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I'm copying this from my own instructable about tube amp rebuilding :
DISCHARGE THOSE POWER FILTER CAPACITORS!!!!!
Seriously. Do this EVERY TIME you work on the amp. If you don't, DO NOT complain if you loose the use of your hand. DO NOT come back and haunt me if you die....
The power 'filter' caps can store fatal amounts of electrical current, and are sometimes termed "reservoir" caps. The caps are connected near the rectifier and are part of the power supply, and aid in converting AC to DC. In fact, they are a standard component in any power supply.
If you're completely lost, and don't understand this , DON'T MODIFY YOUR AMP . You haven't enough knowledge to work on high voltage/current circuits safely...
There are several ways to discharge caps, but here's the easiest:
FIRST, UNPLUG THE AMP! (But that doesn't make it safe....)
THEN,
-- Jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here... If your jumper has a resistor, leave it connected for at least 30 seconds before you touch anything.
-- OR short the caps with a screwdriver. Lay the shaft on the chassis, then bridge to the positive (+) lead of the cap. Be sure the screwdriver handle is insulated (if it's painted, it might not be.)
This may result in a spark... Obviously, your flesh can act as a jumper also (that is NOT a challenge.)
DISCHARGE THOSE POWER FILTER CAPACITORS!!!!!
Seriously. Do this EVERY TIME you work on the amp. If you don't, DO NOT complain if you loose the use of your hand. DO NOT come back and haunt me if you die....
The power 'filter' caps can store fatal amounts of electrical current, and are sometimes termed "reservoir" caps. The caps are connected near the rectifier and are part of the power supply, and aid in converting AC to DC. In fact, they are a standard component in any power supply.
If you're completely lost, and don't understand this , DON'T MODIFY YOUR AMP . You haven't enough knowledge to work on high voltage/current circuits safely...
There are several ways to discharge caps, but here's the easiest:
FIRST, UNPLUG THE AMP! (But that doesn't make it safe....)
THEN,
-- Jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here... If your jumper has a resistor, leave it connected for at least 30 seconds before you touch anything.
-- OR short the caps with a screwdriver. Lay the shaft on the chassis, then bridge to the positive (+) lead of the cap. Be sure the screwdriver handle is insulated (if it's painted, it might not be.)
This may result in a spark... Obviously, your flesh can act as a jumper also (that is NOT a challenge.)
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Thanks! I upgraded the speaker on this one, and I use it all the time...
I think you'd do fine with a 100 VA transformer. A 0.5 amp fuse should work.
To ballpark: amperage = watts / volts. A 40 watt amp (including 5% added for the iso transformer) is 40/120 = .333 amps. Might have to use a 0.5 amp fuse.
If the setup works witha .35A or a .25A fuse, then that's even better...
Just make sure the new ground comes from the bridge, not from the old wiring setup, which is the "neutral" side of the mains plug...
If you were to run the power through the rectifier first, and feed the filaments DC with a series resistor to drop the extra voltage, do you need the extra bulk and expense of the isolation transformer?
-- Ohms law-- current = watts / voltage.
-- Rectified voltage is about 170 V (120 * 1.4)
-- This isolation transformer limits at 50 watts (50 VA, but close enough).
-- a 1n4007 diode bridge has a 1A limit (but it may take time to fail).
Iso transformer draw: 50 watts / 170V = 0.294 Amp max
1N4007 draw: 1 Amp max
Other rectifying diodes may draw 3 amps or more...
The issue is, that as we then earth (or ground) the chassis anyway, and the circuit ground is generally tied to that, we have just re-referenced our de-referenced ground anyway, which makes me wonder why the heck I'm using an isolation transformer.
According to the math above, if I were to use a 200VA transformer, the amp is suddenly dangerous again? There's got to be something we're missing here?
Don't get me wrong--enough current can pass through a 50VA iso transformer to do considerable damage. It's hardly a harmless amount of current. But there's a LOT more potential current in the mains...with or without a rectifier.
When people are often partly insulated by clothing, shoes, etc., it can make a huge difference.
Earth and neutral are NOT interchangeable. They are usually close in relative potential, but they are not the same. In fact, I bet that's one of the 'ground faults' that ELCBs, GFIs and RCDs trigger on...
Where are you planning on getting the earth reference for the chassis if an earth / neutral connection is by definition a "fault?" (neutral being connected to ground through a full-wave bridge rectifier, or directly with a half-wave rectifier.)
Also those devices also introduce a level of complexity, with their own set of faults-- they can fail themselves. Simply wiring one up incorrectly can render it useless.
Personally, I would never depend on one alone--especially not when I'm holding the signal ground in my hand. I might use a GFI together with a transformer, though.
Not to mention that you're assuming that all wall sockets are wired correctly... And that other people's equipment is safely wired and referenced.
Nothing is set in stone...witness newer switching power supplies which forgo isolation (although all have sophisticated current sensing circuitry). But how many of those power supplies are used in applications where the the user is literally grabbing on to one pole of high voltage?
Transformers limit current by their nature. If you try to pull too much current, the core saturates and that's the limit.
Diodes have a current / voltage limit, but they can exceed that for a time before they fail. Too long a time to protect from electrocution. Diodes can also fail "closed," which would limit NO current. Could still result in a "hot" chassis in the worst case, if only one or two diodes in a rectifier fail closed (and other safety changes weren't done).
Of course, both can fail "closed"...transformers can short internally. And that's why we change the other stuff... Like adding the fuse (and the earth ground). But a transformer short would prevent any current passing beyond that short.
A fuse can't react quick enough to prevent a serious (fatal) shock. It's there to prevent damage to the amp and act as a fault indicator.
Maybe I should drop in an ELCB prior to the fuse?
Most of the amps that are missing power transformers are pretty small, so a small iso transformer isn't very expensive... This 50VA model was only about $12 USD. I think that's pretty reasonable.
The onboard transformer is expecting +/ - 115V (plus and minus 115V). You're only supplying only +115V, that would cut the voltage in half.
Carefully, check the voltages on the secondary of the chassis power transformer. Remember, this stuff can kill you...
I looked up the datasheet.
Yes, you've got the correct wiring for the N-68X @ 230V. For 230V operation, wire the N-68X primary coils in series (they are in parallel for 120V.)
230V mains-- Black and Black/Green.
Connect Yellow/Black and Red/Black together.
(I'll add this to the instructable...)
There's no real difference between the Hot and Neutral regarding transformer primary --the potential of each wire will be the same. The only thing effected is "Phase," and the reference to ground. But to keep things consistent, connect Black on the n-58x to Hot (Black in the USA mains).
Because of the ground reference, there is a difference regarding Hot and the switch and fuse. Place these on the Hot side of the primary. If the amp develops a short, this insures the fuse blows and separates Hot from the circuit...
Current draw can be calculated from wattage and voltage:
I = W / E (current = wattage / voltage)
For a 35 watt amp:
35 / 120 = ~290mA
Add maybe 2-3% for the iso transformer, and the lowest practical value is probably about 500mA (or 0.5A); but use 350mA or 400mA if you can find one...
I can't tell for sure in all the pics, although ALL of the Metal Parts of the Amp,
shall be grounded, and bonded as per NEC code...
Article 250-??? "All metallic parts that have the potential to be energized,
shall be grounded, and properly bonded."
Very good disclaimer and hazard warning you have I must say,
because there are some Not very aware people on here?
The only thing that's not earth-grounded is the iso transformer shield (which is easy enough to do, and should be done.) ... in the event the transformer primary shorts though it's insulation--which could happen, when it gets old. Or if some idiot raps the fuse in foil after it blows...
Unfortunately, it's doubtful that any retrofitted transformer and 3-prong cord would ever make an old amp like this compliant with modern Class I specs. For instance the chassis earth-ground wire is exposed and connected to the chassis with a removable screw. It should be a permanent connection, and inaccessible to the user.
Heck, for that matter you can still stick your hand right into the chassis!
Most amps (even fully isolated and grounded ones) older than 10 years would probably fail modern certification. It's always a balancing act--how much of the original wiring/circuitry to retain, and how much should be replaced...
With this mod, I'm confident at least that the bridge-ground on my guitars are at earth potential, and the fuse will blow if there's an internal short. But it's not "idiot proof," if you catch my meaning...
It also had a tendency to faintly pick up some kind of hispanic radio station(s). Can I presume that this is related?
Older two-prong cord amps (even ones with power transformers) can cause electrocution--they usually had a capacitor between once side of the AC input and the chassis. That cap acted to shunt noise to the "neutral" wire on the outlet, 'cause there was no ground outlet connection. But those caps would go bad; short out, etc. (and were usually underrated to begin with.)
When the caps short out, it's a direct connection of the chassis to the outlet. Bad, bad, bad.
I have an old Ampeg Gemini that had this problem, and I've had mild shocks... Upgrading the cord fixed the problem.
Radio station (RF interference) might be eliminated with a proper grounded cord. There are other possible fixes, too--usually adding a grid-stopper resistor to the first preamp stage eliminates it...
This is an 8 amp, 400V version. Eight amps is overkill, though. Half that (4 amp) would be more than enough.
(Radio Shack has the equivalent, although you can find them cheaper elsewhere...)
I've got several Teisco-ish guitars: Norma, Kingston, Kimberly, Maxitone, Sekova. Fantastically odd guitars! No amps, though. That really looks nice.