60 cycle Hum when plugged in - 18w plexi build

[Garrett335]

Hey guys I had a question about a possible problem in my build.

The amp is dead quiet with nothing plugged in but when I plug in my strat I get a noticeable 60cycle hum in the low channel, and in the high even more so.
With the les paul (humbuckers) it is still there but much quieter.

Another thing I noticed when the cable is plugged in and I touch the end with my thumb (Bass Channel) it makes the standard low frequency buzz, but when plugged into the high channel it makes a squealing noise, high pitched. Also there is no feedback loop in this circuit.

My goal is to reduce the hum while playing. I'm going to try several other amps on the same circuit and see if it's doing the same thing.

Any thoughts?
Thanks :D

[shooter]

amp is dead quiet with nothing plugged in

that's a good indicator you've got a good build.
there could still be an issue, but I would relocate the amp, change guitar and cable, things that "couple" outside noise in, like bad ground in cable/guitar, is the bottom of the chassis covered, like fender foil, metal bottom.

[Garrett335]

amp is dead quiet with nothing plugged in

that's a good indicator you've got a good build.
there could still be an issue, but I would relocate the amp, change guitar and cable, things that "couple" outside noise in, like bad ground in cable/guitar, is the bottom of the chassis covered, like fender foil, metal bottom.

Thank you for your input! It is not covered. I'll order some of the shielding for the bottom of it and try that, as well as the different cables, guitars, circuits in the house. Thanks!

[Colas LeGrippa]

did you hook up the 120VAC correctly? The the gnd lead must be  attached to the chassis alone , not with the center taps or filtering caps grounds. The signal shielded cables in your amp must be groun.
ed only at one side to avoid loops. Did  you use grid stoppers  on preamp as well as on power tubes ? If it is a cathode bias  amp, did you bypass the resistor with a cap ? Start by the first preamp tube. Sometimes, no grid stoppers at this place cause problems like you described.


Colas
 

[Crate50watt]

I have issues with high voltage dc wires inducing noise on signal wires sometimes. Use a wooden skewer or pencil to move wires around and see if anything gets worse or better.
Poorly grounded pots, filter caps and signals will cause hum too.
Center taps on power transformer need to be grounded well too. If there isn't a center tap for the heaters, you can use the 100ohm to ground parallel resistors to cut back on hum.

[Garrett335]

did you hook up the 120VAC correctly? The the gnd lead must be  attached to the chassis alone , not with the center taps or filtering caps grounds. The signal shielded cables in your amp must be groun.
ed only at one side to avoid loops. Did  you use grid stoppers  on preamp as well as on power tubes ? If it is a cathode bias  amp, did you bypass the resistor with a cap ? Start by the first preamp tube. Sometimes, no grid stoppers at this place cause problems like you described.


Colas

The 120 is grounded to the chassis alone.
Then I have a Shared ground for the:
-Heater Centertap
-Filter cap
-Fuse for high voltage
-Output speaker taps (I unhooked this and tried grounding this to another ground and nothing changed)
The other ground I have is at the opposite side of the chassis for the inputs and the bus.

*One thing also to add is that if the volume is low and treble is turned down, it does not make the squealing sound.
When volume is low and I turn up the treble pot (Finger on tip of guitar cable) it makes the high pitch squeal.
If treble is on zero, and I bring up the volume it will make the noise once I get past 12 o'clock.

I have issues with high voltage dc wires inducing noise on signal wires sometimes. Use a wooden skewer or pencil to move wires around and see if anything gets worse or better.
Poorly grounded pots, filter caps and signals will cause hum too.
Center taps on power transformer need to be grounded well too. If there isn't a center tap for the heaters, you can use the 100ohm to ground parallel resistors to cut back on hum.

I'll try looking at the DC wires, I know I have the Filtercap wires zip tied close to the PT HighV wires, also one of the OT primaries, is that a problem?
I'm deff going to start moving wires around and see if that changes everything.

Lastly, I have built this exact model twice, and they both do the same exact thing.
Attached is a photo of the PT section

[sluckey]

Show us a schematic.

[Garrett335]

Show us a schematic.

[shooter]

it makes the high pitch squeal.

I'm not sure I've ever had an amp where I only had a cable plugged in that didn't get all pissy when touched

with tone 5, vol 8, guitar plugged in, vol 0;
what guitar volume makes you forget all about the "issue"

[Garrett335]

it makes the high pitch squeal.

I'm not sure I've ever had an amp where I only had a cable plugged in that didn't get all pissy when touched

with tone 5, vol 8, guitar plugged in, vol 0;
what guitar volume makes you forget all about the "issue"

Maybe I'll make a video of it and put on youtube. I hear what you are saying though, who cares what it sounds like when its not plugged in, just make some rocknroll and be on with it :D

[sluckey]

I think your amp is behaving exactly as it should.

[shooter]

+1

My current build has more "hum" than the last 4 put together, I is somewhat annoying, but I'm 98.7% confident it's normal "as designed".  my amp and yours uses a full wave rectifier instead of the bridge I normally build with, that alone adds some hum.  mine is SE, that doesn't allow for the normal hum cancelation you get in P-P.  But when I plug in, turn the guitar to 2, NOBODY hears the hum.