• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

3 wire power cord conversion (US)


2019-11-07 12:45 am
Is it worthwhile to convert the 2 wire power cord to 3 wire for safety / performance on a vintage amp restoration? (1962 Sherwood S-5000ii). If so what is the optimal gauge wire to use? Does bigger mean better or will you introduce more noise by going too big? Thanks
A good, safe upgrade is to fit a standard IEC type of socket, and use the usual type of power cord that you see on most consumer equipment- commonly referred to as a computer power cord.

If the chassis isn't large enough to accommodate an IEC jack then a standard 3-wire appliance cord of sufficient gauge would be a good option too. Size it to deal with the current draw of the amp. You will not introduce noise by going larger, although here's no real reason to.
You need to get rid of the C56 "death caps.". If 2 wire power cabling is retained, those parts should be replaced with KV./cross line rated 2200 pF. ceramic parts

FWIW, I prefer "captive" power cables over IEC connectors. There's 1 less mechanical connection to cause trouble.


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Paid Member
2011-10-31 1:00 am
If you're worried about safety, run it on an isolation transformer. Failing that, yes! Remove the "death cap" (or snip one lead and leave there so it looks right). Ground loops can be solved with isolation transformers, if necessary. Otherwise, I've never bothered unless it was a matter of replacing the cord because it was lost (find one of those 2 prong tube amp cords new LOL)

If you convert to 3 pin power, use 18/3 or larger. Like Lingwendil said, an IEC socket is awesome if you can fit it because they cost $1 and you can find the power cord in the garbage.

If you go "captive" like Eli suggests, make sure you use one of those cable locking things or at the least use a grommet for the hole, and tie a knot in the power cable inside the amp so it won't rip out if it's tripped over. Seriously though, when was the last time you have to reseat the power cable on anything to make it work again? My Kettle uses an IEC connection and it's a 15A load (even though the standard is only good to 10A IIRC).
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Excellent and important stuff so far. Code requires the safety Earth have its own bolt and nut to chassis - a good idea anyway.

Potential ground loop issues are easily dealt with by plugging all interconnected components into the same power strip, so ground loops will be local and small. Systems with other wiring to the cold, cruel outside world (cable, antenna, etc) should have their shields connected to safety Earth on the strip (and also before entering a building).

All good fortune,