Express Hum and Layout (changed from EMI Filter)

[mumford]

I've had what I believe to be 60 hz hum in my express since I built it--it's the same frequency as the hum in my fridge, lights, etc.

I've been in law school since I built it, so I've had NO time for tweaking. I've got an upcoming gig, though, so I figure I'd better figure something out in a hurry. I bought an EMI filter power jack to see if it made a difference. Because it might not fit in the chassis, and would definitely require cutting it, I wired the filter onto a power cable. This also meant I could test to see if it made a difference.

To cut my already long story short, I couldn't tell any difference. (It didn't seem to affect the tone, either.) Granted, it was only a $2 part, so no loss, but I wouldn't bother using one.

[LOUDthud]

Those filters are meant to prevent noise from getting into the power cord where they will be radiated or get into the mains wiring. They are usually necessary on any kind of digital gear or when a switching power supply is used.

There are two types of hum. 60Hz (line frequency) hum is usually from lead dress, or heater supply issues, or possibly coupling between the power transformer and the output transformer. 120Hz (2x line frequency) hum (more of a buzz) is usually a grounding issue.

Does the volume control affect the hum? Listen carefully as you put the amp in standby. Does the hum go away instantly as soon as you hear the switch clink off or does it fade away for a second or two? Did you follow the ground layouts posted here exactly?

[mumford]

It's been a while since I've been able to look inside, but I suspect that I didn't follow the grounds exactly, something I'll have to fix. I know how I think, and seeing multiple ground wires coming off those big caps is the kind of thing I would have been certain to eliminate.

I'm not sure about whether it fades out or not... what should that difference tell me?

[LOUDthud]

Hum can be caused by the magnetic field going directly from the power transformer to the output transformer. You'll hear this coming from the speaker as soon as you turn the amp on before the tubes warm up. You'll have to move the transformers farther apart or turn one of them to fix this.

If the hum is coming from the heater wiring or possibly a bad tube, it will fade out slowly as the amp goes from 'Play' to 'Standby'. This could be an issue with the heater center tap or a wire too close to the heater wiring.

Normally in an Express, the first filter cap stack is on the 'cold' side of the Standby switch. The current pulses that happen at twice the line frequency to charge up the filter cap flow through the standby switch. These current pulses get to ground via the transformer center tap and could be picked up by the preamp if certain grounding rules are not followed. The important thing is that the pulses are cut off as soon as the switch is opened so the hum should stop before the B+ dies out.

[mumford]

Finally got around to poking around in there... my grounds were fine, and then... what's this? One of the wires coming from the filter caps to the power supply board had broken off. Haven't plugged it in yet, but I imagine that's most of my problem.

[mumford]

Got to check it out tonight, and the hum is still there but a LOT lower. (And yes, it fades in/out when standby is flipped.)

I had to check the layouts to reattach that wire, and I noticed a difference between the layout in the build files, the ceriatone one, and my actual build.

In the two layouts, the heater wires leave the transformer really far from the filter caps. In mine, they come into the chassis all but touching them. Also in one layout, the heaters trailing from the PT are not twisted, they're just separated, but the ceriatone layout has them twisted.

So in my amp, my heater wires come into the chassis right next to the filter caps and are twisted. Do I need to rotate my PI to get the wires away and/or untwist them?

[rooster]

No, the wires should be twisted, leave them. This is not your problem. What is the grounding on the filament supply? Does this build use the center tap of the filament supply, or does it use the two 100 ohm resistors to ground? The latter is better so change to this if you feel inclined, your call. Ken used the 100 ohm resistors, BTW.

Pull all three preamp tubes and turn the amp on. Still hum? Bad power tubes? Perhaps. Replace them and try this again. (BTW, eh, typically if an amp has been sitting more than 6 months the power supply caps need to reform. Do this by letting the amp run - unattended - for 4 hours or so. Put all the tubes back in then and turn all controls to '0'. Walk away...)

OK starting over then, pull the three preamp tubes and start this again.

Put the PI(V3) tube back in. Hum?

Put V2 in. Hum?

Put V1 in. Hum?

This will pretty much isolate your hum source. V1 can stand to have the wires leaving pins 2 and 7 sheilded. Is this your amp? If you heard the hum increase as you installed V1, then you can see the need for this. And sometimes the 25uf caps on pin 3 and 8 can go south which will cause a HUGE buzz. .......Which doesn't sound like your problem.

That's all I got. Reading your account of what you are discovering within the amp, the broken wire in particular, makes me wonder how this amp was stored. Is it in a head case? I would check all connections out at this point and it sounds like you are doing this. Carry on then.

Oh, some questions: Did this amp ever have a life without hum? Is this a new twist or are you just now just getting things sorted out? Do lawyers have better powers of recall than criminals?