Isolation transformer upgrade for old guitar amps

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by gmoon
 

introIsolation transformer upgrade for old guitar amps

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.

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.
Isolation transformer upgrade for old guitar amps
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step 1ZZZAAAPPPP! It's the Safety Disclaimer!

I'm copying this from my own instructable about tube amp rebuilding :

DISCHARGE THOSE 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 'filter' caps can store fatal amounts of electrical current. 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.)

ZZZAAAPPPP! It\
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35 comments
Aug 25, 2010. 3:51 AMmjsdiy says:
I have an amp with the same 3 tube configuration (the only I.D. anywhere on it is "Model 5000" on the front panel). I'm a tube noobie, but I want to be safe, so I installed the iso transformer and 3-prong cord with true earth ground. Now I have a 60 cycle hum that was not there before. Any ideas? Oh, and any help identifying the amp would be appreciated as well. Thanks for a great walk-through with loads of great info. mjs
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Aug 28, 2010. 5:05 PMmjsdiy says:
Rerouted the AC wires with no change. Powered up the amp and poked around with a pencil looking for anything that changes the character of the buzz--to no avail. I bought this from a repair shop, and they had gone through and completely recapped it. Being a noobie, is there anything ham-handed I could have done to have fried a cap? I was also wondering if I may have fried one of the diodes in the bridge rectifier when I soldered it in. (I started with an iron that was of too low wattage--maybe too much time on the lead). Would you expect a voltage buzz from that? I haven't had it powered up for any lengthy period, but the iso doesn't seem too be getting hot (one good thing). Thanks again for the info. mjsdiy
Jun 24, 2010. 10:00 AMsalad.k says:
I got confused connecting iso trans. it's not working,no light on tubes...I connected, for 220v ac brown wire(ac cord)-->switch-->fuse-->black wire( iso trans)_ blue wire(ac cord)--->green/black wire(iso trans )_ green yellow(ac cord)---> grounded _red wire1,red wire 2--->ss bridge--> +side---> heater -side--->grounded _I tied YEL/BLK-RED/BLK didn't connect to any wire.white wire, didn't connect. I have no idea what is wrong.
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Jun 18, 2010. 9:27 AMsalad.k says:
I got n-68x today finally(take to Korea long time..), but my kay 703c has an original transformer as picture. so should i just put on or remove it? and one more question, in n-68x manual, 230vac, Tie BLK/YEL-BLK-GRN Apply to BLK-GRN/BLK. for 3 prong cord mod, what color wire need to connect to hot wire in power cord?
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Jun 19, 2010. 10:35 PMsalad.k says:
Thanks so much!! helpful instructions!!
Jun 15, 2010. 8:33 AMdiblet says:
great instructable - how do you determine what type of fuse to use?
Jun 13, 2010. 11:03 AMsalad.k says:
I try to safety mod on my kay 703c. even if I install N68-X transformer, still I have to use another transfomer to change 220v to 120v. cos we use 220v ac in south korea. right?
Jun 15, 2010. 2:27 AMsalad.k says:
so you mean n68-x drop electric pressure 220v ->120v?? I don't know I explain well. (wall socket )----220v-----> (n68-x)----120v----> (Amp for 120v) ??
Apr 11, 2010. 7:26 PMampRiser says:
This is Great! 5-stars for sure with the amount of technical info you have here!

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?
Mar 27, 2010. 2:03 PMurbanprimate says:
Well done!  I got one of my first cheap little practice amps (I think it was a 15w Dean Markley) around 20 years ago from a pawn shop and it had a tendency to send a little charge through me, particularly when perspiration was involved.  At the time I was a teenager with no understanding of electricity whatsoever.  Oftentimes it was just a harmless little "buzz," but it was enough to scare me into finally getting rid of it.  I never knew why and had forgotten about it until I saw this.  Now (thanks to you!) I know what to do if I run into that again.  It had a decent sound when it worked the way it should and I hated trashing it, but I didn't feel right pawning it off and maybe being responsible for someone else getting hurt.  Hindsight is 20/20, right?

It also had a tendency to faintly pick up some kind of hispanic radio station(s).  Can I presume that this is related? 
Jan 26, 2010. 10:10 AMjaphy says:
Great instructable!  Can you describe how to select a SS bridge rectifier.  Too many choices!  Thanks.
Jan 26, 2010. 4:20 PMjaphy says:
Thanks very much.  This instructable may just save my life!  I recently bought an old AC/DC tube amp and have been somewhat reluctant to play it.  It's an old Melody, probably made by Teisco.


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Jan 13, 2010. 7:30 PMledscree says:
fantastic
Jan 11, 2010. 4:50 PMjohnbeasley6 says:
Is it possible to build this in a box and plug the original power chord into it? Im not sure if safety mods decrease the value of the amp or not. And this would be less invasive.
Jan 2, 2010. 1:27 PMRe-design says:
Nice article.  Can't find a single thing to argue with.
Dec 15, 2009. 7:15 PMcorbin569 says:
I turned a tube radio into an amp a while back it didnt have an isolation transformer in it, so one night i plug it in and start to plug it in to my efects unit and wham!!! i get a face full of sparks and it kicked my breaker. scared the crap out of me and it melted part of the input jack!!!! luckly i was holding the rubber part of the cable and i missed the input slightly or it would have prob fried my effects unit.....but anyway lesson learned never use amp or radio w/out iso. transformer
Nov 20, 2009. 1:31 AMkotpet says:
I would advice people to be VERY careful when it comes to high Voltage (as in Valve amps), if you don't know what you're doing PLEASE don't even start! Article is good. Thanks man! Be safe!
Nov 20, 2009. 1:35 AMkotpet says:
another comment- read Dave Funk's Tube Amp Book, or something, get yourself familiar with the subject- it's well worth it.
Oct 30, 2009. 10:55 AMPhil B says:
I remember being at a county fair for the evening stage show.  It had been raining and the stage was quite wet.  The guitar players were having a hard time with electrical shocks.  Now I understand better why that was happening.
Oct 29, 2009. 5:07 PMlivesteamfan says:
I have an amp that I just took from an old record player. It has one very large and heavy transformer and two smaller ones. None of them look all to great; the big one is a little rusted and the others look like their coils are seperating. It does work though. Do you think I would need isolater? Also, how much power do you think a 6 tube amp would use? There are two small ones are the control box and 3 small and 1 big on the main body and some metal cylinder that I don't know what it is.