Apmpeg V4 Hum Issue - Please Help!

[Floridabwoy]

Hello,

I just wanted to see if I could get an expert opinion before I take this to a tech. My V4 just started having a hum for no reason. I checked to make sure all the tubes are firmly in there sockets. I also turned the amp on and ran a cable out of the preamp to a mixing board and I could still hear the hum in the mixing board. I adjusted the hum balance but it does not do anything. Something that I noticed if I change the OHMs it stops for a second and then comes back in. I had a tech look at it when I bought it about a year and a half ago and he said everything is fine besides the reverb tank does not work. I am wondering if it is something simple (somebody mentioned in another thread about something having to be grounded to the plate) or if I need a whole cap job. Thank you for your help!

Here is a video of the hum. It is louder than the normal tube hum but not super loud as it sounds in the video.




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[beans-on-toast]

The hum can be due to many things. Your tech will sort it out.

A bad tube can be the cause of the hum. If you have any spares, try swapping out the old tubes one at a time. This is about the only thing that you can try before taking it to a tech.

The pinging sounds like thermal expansion within a tube. There are welded metal bits inside the tube and one of them may have come loose. As they heat they can pop out of place making a pinging sound. The simple solution is to locate where the sound is coming from and if it is a tube, swap it out.

It could be that you need to change your power supply caps. This can also be a source of the hum. If your caps are old and one is gone, it is more economical in the long run to change them all. If one is bad and it needs to be changed, it doesn't cost that much more to do them all. Otherwise if you just change one and it stresses the other older caps, they could go and you might find it necessary to take the amp back in a month or two to have more work done. Better to take care of everything all at once and pay the bench charge only once. On the other hand, if only one cap is bad (it can happen) and the others are in good shape, there is no need to change them all.

When you switch the impedance switch, it should only be done with the amp off. If in the middle of sliding the switch there is a dead spot where no speakers are connected, it can damage the amp. This might be why you loose the hum, you are momentarily disconnecting the speakers.

 

[Bassmec]

The hum can be due to many things. Your tech will sort it out.

A bad tube can be the cause of the hum. If you have any spares, try swapping out the old tubes one at a time. This is about the only thing that you can try before taking it to a tech.

The pinging sounds like thermal expansion within a tube. There are welded metal bits inside the tube and one of them may have come loose. As they heat they can pop out of place making a pinging sound. The simple solution is to locate where the sound is coming from and if it is a tube, swap it out.

It could be that you need to change your power supply caps. This can also be a source of the hum. If your caps are old and one is gone, it is more economical in the long run to change them all. If one is bad and it needs to be changed, it doesn't cost that much more to do them all. Otherwise if you just change one and it stresses the other older caps, they could go and you might find it necessary to take the amp back in a month or two to have more work done. Better to take care of everything all at once and pay the bench charge only once. On the other hand, if only one cap is bad (it can happen) and the others are in good shape, there is no need to change them all.

When you switch the impedance switch, it should only be done with the amp off. If in the middle of sliding the switch there is a dead spot where no speakers are connected, it can damage the amp. This might be why you loose the hum, you are momentarily disconnecting the speakers.

All the above and a big + for checking to see if its been re-capped ever.
you just won't know how quiet or powerful it can be without first changing out the tired old electrolytic caps.
There is zero mojo in old electrolytic caps.

 

[Floridabwoy]

haha... well it was at a tech a year and a half and he asked me what was wrong with it after he benched it. He said everything looks fine. Maybe, the hum just came on all of a sudden. Maybe I need a new amp tech. : )