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What could be causing that annoying 60hz (or 120/50/100hz) hum?

All ToneWheel Hammonds hum.

The tones your organ produces start in an electro-magnetic pick-up, after all. So some hum is normal and unavoidable. If you think you suffer from excessive hum, here is a list of some possible causes.

Where's your Leslie?

All ToneWheel Hammonds have a MatchingTransformer, which is used to match the low impedance output of the ToneGenerator, to the high impedance input of the preamp. The MatchingTransformer is particularly sensitive to stray magnetic fields, such as those caused by electric motors (and to a lesser extent power transformers).

In most (all?) Hammonds the MatchingTransformer is on the left side (viewed from the front) of the organ. Placing your Leslie on the left side of your organ is guaranteed to cause hum. Best keep that Leslie at least several feet from the organ. (Don't put that HammondClock on top of your organ either!)

In any case, Hammonds are sensitive to magnetic fields caused by electrical appliances around them. If you have hum try moving things (including the organ) around.

Note: Leslies which use a single-ended input scheme, like the 147 inherently pick up more hum than the differential-input Leslies, like the 122.

Loose or Bad Cables

Bad connections in your Leslie cable can cause a big whopping hum.

Bad grounds

The impedances of the ToneGenerator output vary from a few ohms to a few tens of ohms. This is not much --- any extra resistance in the ground paths within the organ can cause hum or other funny noises.

In particular, check and tighten the ground connections at the preset panel, and the grounds at the ToneGenerator and the manuals. It is sometimes hard to tell by visual inspection whether a soldered ground connection is okay or not --- some people recommend resoldering all the grounds as a matter of course.

TG RC Networks

Around 1965, Hammond started added RC filter networks to the outputs of certain tones of the ToneGenerator. This additional filtering was added primarily to address ?CrossTalk on those tones, but they also help attenuate hum.

(This additional filtering was only added to tones 37 through 48 --- middle C though B on the third (8') drawbar.)

RobertHayton has a scan of a Hammond technical service bulletin describing how to retrofit RC filters onto older organs on his web site at:

Old Capacitors

Old, tired electrolytic capacitors in the power supply filter networks of your preamp or amp are another common cause of hum. It's likely that if bad filter caps are your problem, then you'll be experiencing 120hz hum.

See CapacitorReplacement.

Bad Tubes

Bad, or weak tubes in the finals of your amplifier are another common cause of hum. If your hum gets loud when you initially turn on your organ, then gradually disappears, this is likely to be your problem. (Replace the finals in pairs.)

Heater to cathode shorts are another cause of hum. (Likely to result in 60hz hum.)

See also CausesOfCrossTalk.


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