Wurlitzer 200A-ground hum question

[Nelson 54]

I recently got a late production 200A that looks, sounds, and plays great. The internal speakers work fine and the signal is clean and quiet. But the auxiliary output hooked up to a tube combo amp produces a loud hum. A DI box did not work, but dropping the ground on the Wurlitzer power cord did. When I lift the ground on the power cord (plugging the piano with only two prongs into a wall outlet), the hum disappears.

Can anyone tell me whether it is safe to eliminate the ground loop hum this way and/or any steps I can take to keep myself and the keyboard components safe while using this technique? Alternatively, are there other measures I can take that would be as or more effective in eliminating the hum?

Thank you—Nelson

[Electrickey]

Try plugging both amp and piano into the same outlet source. Sounds like a ground loop.

[Nelson 54]

Thanks.  Actually both the piano and the amp were plugged into the same grounded outlet—both with 3-prong grounded power cords—when I got the hum. When I kept the Wurlitzer in that outlet but lifted the ground (using a 2-prong extension cord) the hum stopped. 

[Jenzz]

Hi :-)

This problem in very common. You need a Line-Iso-Box.

The Wurlitzer's signal ground is tied to the pianos earth connection (3-prong connector).
Mostly all tube amps do the same, signal ground is tied to earth (for saftey reasons / High-Voltage).

If you connect a Wurlitzer to a tube amp, you will close the ground loop via the shield of the audio cable.

So, you need someting to 'isolate' the signal path's ground. This can be done by a line iso-box (1:1 audio transformer)... Palmer PLI-01, PLI-05

Jenzz

[Electrickey]

Ground loops are tricky. Could be something is not wired correctly (hot-neutral-ground) and it could be the outlet as well. There are plug-in testers for this. So you're plugged into the wall outlet with both plugs? Have you tried a single heavy duty power strip plugged into the wall with both amp and piano at the strip? At least with the Wurli, you're not going to be touching metal things to get a shock.



Is your piano power cord  wired correctly? I seem to remember an issue where the terminals got reversed at the factory on these.



https://docwurly.com/a-faq-in-process-pt-1-ac-power-cables-for-wurlitzer-electric-pianos/

"Instruments from the 200 and earlier 200A series, from 1968 through around 1978, used an oval connector with 3-round pins, plugging into the female end of an AC cable that was also used by Hewlett Packard.  The connector is called a PH-163.  It is a less than ideal component to the instrument.  The pins tend to get loose.  Many of the power cables Wurlitzer used resolved to an unpolarized two pin AC plug, and even some of the 3-pin plugs had a fake ground plug.  Polarity wasn't wired consistently in the instrument, and cables used were randomly either #17280 or #17952, as detailed here:"




There were TWO PH-163 power cord models used by Wurlitzer at the time (according to Doc Wurly's blog) and the wires were made reversed by the cable manufacturer. Wurlitzer did not check for polarity when sending pianos out the door. So depending on which cable came with your piano there would be a polarity issue.

Update:

Re-found this info:
For a Wurli, the correct cord is the "reversed polarity" version.  The proper Wurli cord has the hot on the left and the neutral on the right when you look at the connector from the end, with the ground contact above the other two.

According to the photo of the outlet above that would make the Wurli a standard wiring if you consider the plug's right lug (when plugged into the outlet) will connect to the "HOT" side of the wall outlet.


I have two Wurli AC cables. The one that came with when I bought it from the previous owner, a  brown VICTOR , and a Belden gray that I bought later. The VICTOR is wired HOT on the LEFT and NEUTRAL on the RIGHT (looking at the piano socket with the ground pin hole on top of the other two) but the ground does not appear to be connected to the ground pin from the piano's socket. The Belden's is the same HOT/LEFT NEUTRAL/RIGHT but the ground works.

Keep in mind with the above description of the socket end of the Wurli AC cable, AT THE PLUG END the right lug will be standard HOT and the left lug will be standard NEUTRAL. Whew...

Now how is the socket on  the back of the piano mounted? With the ground pin on the bottom?