Kendrick 6550 lowrider bass amp hum

[fredcapo]

Was asked to look at a Kendrick bass amp with tube rectifier that has a low hum not scoped yet could be 60 or 120 hz. Amp sounds fine except very low hum. Hum very low but steady no preamp volume or tone control effect. No effect with ground reverse switch. Same hum with both power tubes removed...could it be something with the output transformer..?

I cant locate a schematic.

Should I go right to the electrolytics?

I would appreciate any suggestions. thanks.

[glebert]

Is is a mechanical hum? I have had transformers that were vibrating and rattling the chassis or cabinet.

[nevetslab]

Is is a mechanical hum? I have had transformers that were vibrating and rattling the chassis or cabinet.

That sounds like it would be mechanical, with the power tubes removed. Remove the rectifier tube....that will also eliminate any aspect of the filter caps....though, with the power tubes out, we don't see how you would be coupling any hum into the speaker. This isn't an ancient amp, having an electromagnet on the speaker, I'd presume. There, you can get hum.

[fredcapo]

Thanks for the comments. No, it is not a mechanical vibration. I tried pulling the PI only and the hum got lower.

Confirming yes i hear a lower level of the same hum when the two 6550s are pulled. May e some coupling into the ot.

I have to speak with the owner to find out if the hum started slowly or all at once. No sign of damage or ibvious hacking of the amp, though some components and soldering are not factory looking.

I cant find schematic but was told it is a classic bassman clone.

[fredcapo]

Update on this...put the scope on the output and there are parasitic oscillations present. Did not determine frequency or amplitude yet. With signal generator on input, the output is distorted as expected with the superimposed parasitic signal. When the amp power is switched off, the output clears up as it fades out for the second that the caps are discharging. Not sure if that provides any clue for the source of the oscillation. Dont know if the parasitics are related to the hum. Ready to pack it up and ship it to Gerald...

[TimmyP1955]

Given that it clears up when the PT is powered down, try swapping the rectifier tube (or if diodes, swap them if convenient).