50 Hz hum in valve amp

sponger_nz · Posted: Sat Oct 28, 2006 4:13 pm Post subject: 50 Hz hum in valve amp

I have a vintage New Zealand made 50W valve head, and haven't used it in a while. I started using it, it was fine, then a 50Hz hum developed after a few minutes of playing. This hum is present whether a guitar is plugged in or not, and it is independant of volume. It causes the power valves to glow more red than they should under heavy load, so I switched it off before any major damage could occer. I think it's the power supply de-coupling capacitors. I just wanted to confirm that the power supply de-coupling capacitors are the big ones next to the power transformer? As I'm looking at changing these to rectify the fault. Is there any way to test these?

Cheers, Glen.

PlatinumTree · Posted: Sat Oct 28, 2006 5:01 pm Post subject:

Switch them around maybe? It could also be a surge in the power cord.
_________________

You make of it what it is. It makes you what you made it.

sponger_nz · Posted: Sat Oct 28, 2006 5:09 pm Post subject:

I might try swithcing them around, but due to the nature of the fault, I doubt that will help. I don't know what you mean by a surge in the power cord?

PlatinumTree · Posted: Sun Oct 29, 2006 4:39 pm Post subject:

The speakers and head have a resistance called "Ohms". If the Ohms are not matched to specifications, or the cord is faulty and allows tones of amperes to breach through, then even a small little static shock can wreck your tubes, nontheless your amp or speaker.
_________________

You make of it what it is. It makes you what you made it.

greenblob · Posted: Sat Feb 09, 2008 2:18 am Post subject:

physical movement is the most common cause.. if you can make sure the valve (tube) bases havnt lifted from a pcb or something simmilar.. (some heating/cooling of valves screws with the solder pads over time, thus the issue happens once the tubes have heated to "X" temp). Otherwise, the caps are a prime candidate... buy any for anywhere (with the same voltage & uF values.. lol.. duh..) and see if they help.. shouldnt be more than a $5 for both.. if they are.. your being ripped off.. harsh but true..