Hombrew 5E3 Hum Problem

[markammon1979]

I've converted an old HH Scott Audio Amp into a 5E3 Deluxe Clone. Sounds great apart from an annoying hum. I've checked the Tubes, wiring etc. and can't find the problem. It hums when there is no guitar plugged in and when there is one plugged in and it gets louder as the volume increases. Any ideas on why? It's driving me crazy!

[Bruce / Mission Amps]

Pictures of your layout would be helpful.

[markammon1979]

Pics of amp

Pics are at: http://cid-966258abf718898d.skydrive...20Build%20Pics

[markammon1979]

Attached pics

Attached files

[tubeswell]

Can't tell too clealry from your pics but here are some ideas to try.


Heaters

Your heater wires look like they are all bunched up with your signal wires going to your tube sockets. That will induce hum in the signal path. Better if the amount of length of heater wire alongside signal wire is minimised (try and keep them at least 1 inch apart), and ideally where heater wires and signal wires cross next to each other, they should be at right angles (to each other).

The heaters in your photos look like they are wired in parallel on each side of the winding. They seem to be twisted together part of the way, but its hard to see under the bundle of wires they disappear into in the pictures. If there is no center tap on your heater winding, you can make a defacto centre tap by connecting a 100 ohm resistor between each heater wire and the chassis (ground) at the PT end of the heater wires. This should reduce hum. If your amp is cathode biased and you still have some hum, you can run the heater CT-ground/100ohm-resistor-ground-node to the cathode pin of the output tubes - this biases the heater circuit to the cathode voltage.


Signal wires

Can't see clearly from your pics, but its a good idea to shield the signal path from input sockets to grid pins, especially if these wires are any longer than about an inch, as this is the most sensitive signal path in this amp. Ground the cable shield(s) to the respective input socket ground(s). Similarly, shield the signal path from each vol pot to the grid of the next stage, with the cable shield grounded to the vol pot ground.


Grounding

Minimise ground points on the amp chassis to two points. Have all the pre-amp grounds (including the pre-amp B+ supply filter cap ground) running to one input socket ground. Have the power amp grounds - including first filter cap ground and screen supply node filter cap ground and cathode ground going to the other ground on the chassis, along with the PT ground and AC Mains ground and the Heater CT ground.

[redmax61]

Yeah, what he said about the split ground. I had a ground buss running across the front of my High Octane build, and it had a fairly nasty hum. I was advised to split the buss between the preamp and power amp, and that cleared it right up.