Laney LC15R - strange noise

[Cannibal]

Hi everybody,

I have a friend's laney lc15r that has real loud funny noise going on. I was told that he accidentally reversed the order of the tubes, putting two el84 in the preamp tube slot and two 12ax7 in the PA. Now the amp works but it makes so much noise that is unusable. I pulled the preamp tubes and the noise was still there, but when i took out the phase inverter tube the noise disappeared. Then i proceeded to use an audio probe and found that the noise wasn't only on the phase inverter but on every anode of every gain stage (phase inverter included). Therefore i changed every component in the power supply except for the transformer. Could it be a bad transformer or should i look somewhere else? I can only describe the noise as a buzzing similar to a 120hz hum, like the one in this video:

Attached schematics for reference:



Thank you everybody

[lowell]

You replaced every component in the power supply before finding the problem. Generally not a wise approach because you don't know what the problem IS. Always best to locate the problem first. Can end up being a waste of time and parts to do that. Have you checked voltages? Have you ensured that all power supply caps are indeed grounded? A bad ground could cause this. Checking voltages will help narrow down the culprit.

[Mick Bailey]

The EL84 socket has 290v on pin 9. This means a 12AX7 plugged into that socket will have that voltage on the common heater connection and could have damaged the tubes by exceeding the heater-cathode voltage. This can result in heater hum leaking onto the cathode and your audio probe will pick this up at the anode as it modulates the current through the tube. Some tubes are fine with this kind of accidental reversal, others not.

Do you have a spare set of 12AX7s to try? If removing the phase inverter eliminates the hum and the amp otherwise works, then the EL84s are probably OK. Tube swaps are the easiest thing to do - you have to know the tubes are good before moving on to check anything else. I would replace all three to be certain.

Did either of the heater fuses FS1 and FS2 blow?

[Cannibal]

Thank you everybody, I have already substituted all the 12ax7 and the noise is still there. I'll report the voltages back in a few hours, if i'm not mistaken they all read right except for the grid of el84 which were a tad high.

[Mick Bailey]

Make sure the centre-tap for the heater transformer is good. Anywhere there's been a high voltage where it shouldn't be needs to be checked out. Have you inspected the board to ensure there are no breaks or signs of arcing?

[lowell]

Yah what Mick said. Also ensure that you have 3.1vac ish on both heater connections.

[Cannibal]

and SOLVED!

thank you everybody, stupid beginner mistake from me. The humdinger trimmer was dead. Changed it and everything is fine now.

Thank you very much again!

[Mick Bailey]

Ah - I don't see that on the schematic. Good result.