My Laney LC15R goes DDUUURRRR intermittently. It seems to be OK with softly picked single notes, but a decent brraaannngg becomes a brraaan-DDURRR, and sometimes it just does it without provocation. Sometimes it stops, sometimes it doesn't. Both inputs are equally affected.

Any ideas?

Is "DUUURRRR" a low frequency humming noise or a higher-pitched feedback sort of sound?

Rodabod wrote:Is "DUUURRRR" a low frequency humming noise or a higher-pitched feedback sort of sound?

It's a loud abrasive low-frequency, earth hum sort of sound

How long does the hum continue for, and did you take a look at the output valves to see if either looked odd - eg. bright orange? If your output valves are a few years old then its possible they could be shorting under the vibration as I've seen this before.

Otherwise, check the input jack is good and clean before taking a look over the PCB (not sure how easily the LC15 comes apart though).

i've got an amp that does exactly this, Dudley. can you control it with the amp's volume control? once mine starts it just keeps going:

"durrrrrrrrrm"

no matter what i do, except turn it off.

the helpful people at the music electronics forum gave me this advice but i haven't tried it yet

Rodabod wrote:How long does the hum continue for, and did you take a look at the output valves to see if either looked odd - eg. bright orange? If your output valves are a few years old then its possible they could be shorting under the vibration as I've seen this before.

The tubes look normal - certainly not glowing particularly fiercely, and also this amp was new a year ago, and it's purely a bedroom amp - pretty low volume, never been gigged or even moved, so the wear on the tubes has been minimal.

HCT wrote:i've got an amp that does exactly this, Dudley. can you control it with the amp's volume control? once mine starts it just keeps going:

"durrrrrrrrrm"

no matter what i do, except turn it off.

the helpful people at the music electronics forum gave me this advice but i haven't tried it yet

I can turn the "durr" down with the vol knob. I can also sometimes induce the "durr" by tapping the jack.

Once the "durr" (ahem) starts, can you make it quieter by reducing the gain control as opposed to the master volume?

My 1973 Marshall Artiste went through a bout of this a few years back. It was a sort of loud-as-fuck reality-being-cleft-apart DURRRRRR that started cutting in as the amp reached fighting temperature in rehearsal rooms. It was apparently dry joints causing intermittent contact to be created somewhere when things started heating up.
Go over all the solders, see if that helps.

Rodabod wrote:Once the "durr" (ahem) starts, can you make it quieter by reducing the gain control as opposed to the master volume?

Ta for the help, Messrs. Abod and sidca. I have been trying to test this since you posted it, but the DURRR has eluded me, no matter what I've done to try and induce it.

Dudley wrote:

Rodabod wrote:Once the "durr" (ahem) starts, can you make it quieter by reducing the gain control as opposed to the master volume?

Ta for the help, Messrs. Abod and sidca. I have been trying to test this since you posted it, but the DURRR has eluded me, no matter what I've done to try and induce it.

After three weeks of sweetness, it's back with a vengeance.

Turning down the gain reduces it a bit. Turning it up produces more of a sweep/WAH effect than just increase in the volume of the DURRR.