Why does the reverb hum?

clive swindell · Post by **clive swindell** » 3 Aug 2000 3:39 am

I have two Nashville 400s which I have adapted to UK spec. My technical man has now got them running silently until I turn on the reverb and the I get a hum.

My question is: how does the reverb work? - there seems to be some tube or other in the bottom of the amp, and does it need earthing?

There is a letter in the latest PSGA newsletter which makes reference to a 'reveb tank' and it being wrapped in aluminium foil for shielding so it looks like I am on the right track.

johnopsg · Post by **johnopsg** » 3 Aug 2000 5:13 am

Clive,
I too experience the same noise & hum problem
on my Nash-400. I don't know what the problem
is either, but I work around it several ways:
(1)I sometimes use the "Pre-Amp"-In input and
use a Hilton super Sustain box as the pre-amp...on my amp it runs real quiet this
way...(2) turn off the reverb on the 400 and use an external one...I use a couple different reverb boxes and each is far less
noisy than the 400-internal reverb. But maybe
somebody has the "real" answer...
Regards, John O'Neill .

Jack Stoner · Post by **Jack Stoner** » 3 Aug 2000 5:46 am

There is some inherent hum in the reverb circuit and you will never get it out. However, being close to high AC voltage (mains) will cause additional hum. Sometimes, moving the amp at 90 degrees from the AC power source will help.

The reverb tank (spring unit mounted in the bottom) can be picking up the AC or it could be one of the connectors in the external reverb interface connector (Molex connector) is not making proper contact and causing the hum.

Some have tried shielding the open bottom on the reverb tank but I've never heard anyone mention whether it helped or not.

Bob Metzger · Post by **Bob Metzger** » 3 Aug 2000 11:04 am

The internal reverb in the Peavey Nashville 400 will add a small amount of noise when turned up. If it adds alot of noise or hum when turned up, probably something is wrong with it and it needs looking at by an experienced tech. What you can do: There is a molex connector extending out the bottom of the amp chassis and running to the reverb tank (which contains connectors, transducers and reverb springs) in the bottom of the amp cabinet. Gently and slowly remove this connector, retension the female sleeves, use a good contact cleaner on both sides and reconnect. Take the reverb tank out of its bag in the bottom of the cabinet and visually inspect it, looking for broken springs or wires. If everything looks ok, repeat above cleaning/retensioning instructions to reverb tank connectors. Now get a 3 ft. length of wire and strip both ends of the wire. Turn the amp on, advance the reverb control until you hear it hum and press one end of the exposed wire against the metal exterior of the reverb tank. Now press the other end against any EXTERNAL point of the chassis (like the heat sink, in the rear). If the hum disappears, you've got a grounding (earthing) problem. If it doesn't, you can experiment with reverb tank orientation (put it at various angles to see if the hum diminishes) or experiment with further shielding techniques. You can also do this test with an ohmeter or a continuity tester. After that, it's tech stuff.

There are no vacuum tubes (valves) in a Nash 400. The reverb is driven by an opamp and two discreet transistors and recovered by another opamp. The springs in the tank at the bottom of the amp are how the reverberation effect is created. This is an old technology that some people hate but others love. It has its inherent flaws but when spring reverbs are good, they can be very good. Stay tuned for some updates to the Nash 400 reverb.

Bob[This message was edited by Bob Metzger on 03 August 2000 at 10:38 PM.]

Bob Metzger · Post by **Bob Metzger** » 3 Aug 2000 9:39 pm

The other reason that reverbs hum is that they don't know the lyrics!

Bill Llewellyn · Post by **Bill Llewellyn** » 4 Aug 2000 6:40 am

The reverb, if it's conventional guitar amp reverb, is made up of a mechanical spring (or 2) stretched out ins a box with a transducer on either end. The tranducers are just ferrous (magnetic) metal with electrical wire wound around them; one is use to induce vibration (sound) into the spring, and the other picks the sound up from the other end, functioning just like a guitar pickup. And, just like a guitar pickup, the pickup transducer is subject to magnetic fields from things other than the reverb spring(s), such as AC adaptors, transformers hidden inside equipment placed nearby the amplifier, florescent lamp ballasts, AC wires with significant current in them, and so on.

There are a couple things you can do:

1. Keep all wall-warts (AC adaptors), other amplifiers, or any equipment with built-in power transformers away from your amp.

2. Re-orient (rotate) either your amp or the source of the offending magnetic field (other equipment), since there is a directionality to magnetic fields which you can use to your advantage.

Now it's also possible you have a problem with poor shielding of the pickup transducer's wiring, but unless that's somehow been modified, the stock arrangement Peavey put into the amplifier should be fine.

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Bill (steel player impersonator) * MSA Classic U12 * email * my online music

Lefty · Post by **Lefty** » 5 Aug 2000 5:24 pm

I solved a hum problem in my Mesa amp by simply rerouting the reverb send and return cables. The position of these in respect to other components is important in some amps, as is conection and grounding. Just another thing to explore and eliminate as cause.

Curt Olsen · Post by **Curt Olsen** » 8 Aug 2000 5:21 pm

I too have the same problem to some extent on my two 400's.I have never tried this, but a method called hum-bucking has and still is used in the electronic field to cancel 60/120 Hz noise.Has anyone tried wraping a copper or aluminium shield completely around the reverb tank (it must be isolated from the metal tank)and then connecting this shield to the chassis?

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