My amp still works for a while after I switch it off...

[TheBurbz]

If I switch my amp off I'm still able to play through it for about 20 seconds before the sounds dies off. I took it apart today because it was making some buzzing noises, I did find one loose wire which I soldered back on properly but the buzzing remains (primarily when reverb is used). The amp is a 20 year old Peavey Bandit 112, maybe it just needs a good old check up and service?

[Mike]

Totally normal. Big caps take time to discharge

[chisa]

if it is a valve driven reverb you could try a new valve. or you may need a new tray if it is spring

[TheBurbz]

Thanks Mike, before I soldered the broken wire back on the amp switched off immediately with a loud 'POP' and did not carry on the sound, you think this loose wire was causing that 'POP' all along?

The reverb tank is new as the springs in the old one were broken, the reverb does work very well apart from the buzzing...I tried swapping the wires around on the tank but they both gave the same result. Maybe it's just a characteristic of the amp, I don't use the reverb very often TBH but just wanted to check that I wasn't going to get electrocuted

[Doog]

I've had problems with humming and buzzing using built-in reverb tanks, I think it's a fairly common complaint.

[TheBurbz]

I see, thanks for the help.

[filtercap]

Reverbs can make good noise antennas, especially as their output goes directly to a gain stage to get amplifed. They can pick up a hum from your power transformer, which is one reason why they're often mounted as far away as possible, down at the bottom of a combo amp. As I recall, Accutronics recommends that you mount their tanks against the amp's baffle instead of its floor, as this reduces the spring area exposed to interference emanating from the power trans. Of course, most amp baffles don't provide 4-6 inches of blank space below the speakers to do this.

You might check to see that the metal shell of the tank is grounded one of two ways:

either

(A) to the grounded shield of one of the cables (not both -- that would cause a ground loop & likely pick up noise).

or

(B) To neither cable shield, but instead to the amp chassis via a separate wire.

Regardless, both cables to & from the reverb should have their shields grounded to the amp chassis, to protect the to/from signal from additional cosmic ugliness that might strike the cables.

[TheBurbz]

You might check to see that the metal shell of the tank is grounded one of two ways:

either

(A) to the grounded shield of one of the cables (not both -- that would cause a ground loop & likely pick up noise).

or

(B) To neither cable shield, but instead to the amp chassis via a separate wire.

Regardless, both cables to & from the reverb should have their shields grounded to the amp chassis, to protect the to/from signal from additional cosmic ugliness that might strike the cables.

What do you mean by cable shields? The cables that I have are very thin wire, 3mm in diameter or so. I can post pics if it helps.

Both of the cables going into the reverb tank have one core of the wire going into the back of the connector (which plugs into the tank) and one core actually soldered on to the outside of the connector. both of the cables then go into some kind of molex connector and plug into the circuit board, there is nothing fastened to the chassis of the cab and no signs that there ever was.

[filtercap]

I was assuming that your amp used a shielded cable with quarter-inch or RCA-style connectors to send signal from amp to reverb, and another such cable to return reverb to the amp. Internally, these cables are like guitar cables, with a stranded wire center for the "hot" signal, and a braided wire layer around it that's generally grounded. The braided layer is also called the shield, as it's intended to shield the "hot" wire from outside interference that'll cause noise. The center wire is connected to the tip of the plug, and the shield is connected to the outside of the plug, where it contacts the part of the jack that's grounded to the amp, and thus grounds the whole braided shield layer down the length of the cable.

Do take pics if you want. It sounds as if you've got some specialized connectors that use two unshielded wires side-by-side. That could contribute some noise, but I don't know if it would be easy to improve.