new build troubleshoot

[yunger]

i finally finished my first amplifier project. i gutted my pro junior and built a circuit to suite my needs. it sounds unbelievably good, however it hums. does anyone see any obvious mistakes in my wiring that would contribute to the hum?
btw, the filaments have 2x100 ohm resistors to ground acting as artificial ct.

[Enzo]

Hum comes from a million things, it is not one monolithic problem. You need to determine its source.

DO any of the controls affect the hum in any way?

Is it 60Hz or 120Hz hum?

Pull the first tube, does the hum go away?

If not, pull the second tube, I assume the phase inverter. Does the hum go away?

If not, pull one power tube at a time to see if either one affects the hum more than the other, or at all.

[yunger]

enzo, taking out the first preamp tube completely killed the hum. otherwise it is present even at 0 volume and amplified significantly at around the last 20% of the 250k a volume pot. the tone knob doesn't affect the hum until the volume is at that last 20 or so %. it turns the hum into a hiss at full treble....or so it seems. i can't say i'd recognize 120hz. my hum seems to be about the frequency of when you touch the tip of the guitar cable.

[bdcat]

Hi Nice build you've done there!
You may find that using a screened cable going from the jack socket to the first preamp tube helps-remember only to earth it at the jacks socket end and just insulate the screened cable at the valve base end-this should stop it amplifying any stray signals present as that is the point with highest amplification factor.

Also I have sometimes found that if you can put the heater wires so that they run as close to the chassis as possible (ie. poke the wires as close to the chassis as possible). Although doing this would mean re soldering your other wires on the preamp socket so that they come in directly onto the top of the preamp valve/phase splitter-almost the opposite way to which you have your heater wires and all the other wires going to the valve.
You may find that shortening some of the wires and trying to keep them at right angles from the turret board helps.

And finally I think the best way (provided that your experienced with live working-which obviously I don't recommend) switch the amp on and try moving the wires about lightly with a non conductive plastic pen or drum stick and try to see if the noise/hum changes-remember to keep your left hand in your pocket and only poke about using the stick with your right hand!

Goodluck

Brett

[Enzo]

120Hz is power supply ripple. 60Hz is essentially everything else. If pulling V1 kills the hum, then I doubt power supply is an issue. The two are an octave apart, so are the same note, and sound very similar.

Your schematic would help. then we can refer to specifics.

I think his heater wiring is OK. Two schools of though, both of which make quiet amps - the up and over twisted pair like this, and the down on the metal twisted pair like Brett's.

I agree with exploring the input wiring

[yunger]

thanks brett.
i was under the impression that wire were better off next to the chassis as it provided shielding. however, i've seen so many conflicting wiring and grounding methods. i just picked one and ran with it. hopefully i didn't provide an example of how not to do it. i've chopsticked every wire in there to death with no results. i wonder if i have grounding problems. here's how i did it:
input>tone/volume>*ground@terminal below tone/vol pots*<preamp circuit.....power circuit>filters>bridge rectifier>*ground@pt mounting bolt*.....artificial ct>*ground@indicator lamp*.....output is grounded at the jack.
as for shielded wire, i didn't have any. a wild goose chase around town proved fruitless. i need to order some.

[loudthud]

If there is a ground wire connecting all the filter caps under the board, that's the problem. Cut the wire at the last (preamp) filter cap and let it get it's ground at the tone stack.

[yunger]

here's my schematic.

the 22uF that supplies the preamp/pi is grounded with the preamp and tone stack. i should've mentioned that.

[loudthud]

OK. So all the blue grounds from your schematic go to the same point and not back to the grounds of the other filter caps? Is anything connected to the ground lug to the right of the input jack?

[yunger]

yeah, everything in blue goes to ground at the same point along with the preamp/pi filter cap and the input. the ground lug to the right of the input jack is just there to keep the input wire from sliding down near the eyelet board. the hole was already there from the pro junior circuit board so i just used it.

[loudthud]

Yunger,

One of the most difficult things to get right on a guitar amp is the ground. Opinions vary as it seems everyone has their own way of doing it. I for one follow Kevin O'Connors method to a large degree. This is contrary to the practice of many people on this and other boards. The first deviation of the two methods is the grounding of the raw unfiltered HV supply to a transformer bolt. Second is where the main filters get their ground and where the output stage ground is connected. I call this more common method the Fender method because it follows early Fender amps pretty closely. There is a good tutorial on the Hoffman board. In my understanding of how the Fender method works as well as it does, you have followed that method and there is still hum. What has not been described so far is exactly where and how the HV, filter and output stage connections are made to the chassis. A description of these connections should help others here to assist you in correcting the problem. To follow my method you will need to re-do much of you work and your amps looks so nice as it is, I understand why you wouldn't want to do that.

[Diablo]

Maybe I spotted a problem, maybe not.
Above the circuit board main filter caps, you have a terminal strip with a green wire soldered to one side of a terminal, and the other side of the terminal you have a capacitor connected. It looks like the terminal is also the mounting lug for the terminal strip and you are grounding out to the chassis there. Did you intend that to be a ground connection? From the schematic, I think not, but I'm not sure.

[yunger]

here's a map i quickly drew of the grounding points.

[loudthud]

Your grounding looks like I would expect it to following the Fender method.

2x 47uF is quite a bit of filtering for a 2xEL84 amp. If you removed one of those caps, the hum will probably go down. The wire from the transformer bolt to the board should be a nice fat low resistance wire. You can probably measure a DC voltage across it of 10mV. Double or triple it up. Make sure connections are tight.

I'm assuming if you listen to the hum you will hear kind of a low organ note with a buzz on top of it one octave up. To reduce the low note part, move the heater CT from ground to the cathodes of the EL84s.

[yunger]

2x 47uF is quite a bit of filtering for a 2xEL84 amp. If you removed one of those caps, the hum will probably go down. The wire from the transformer bolt to the board should be a nice fat low resistance wire. You can probably measure a DC voltage across it of 10mV. Double or triple it up. Make sure connections are tight.

I'm assuming if you listen to the hum you will hear kind of a low organ note with a buzz on top of it one octave up. To reduce the low note part, move the heater CT from ground to the cathodes of the EL84s.

that's interesting. i didn't know you could over-filter. i have 33uF's...would they be more appropriate? the wire i used was 18 awg...i'll add another one next to it. i'm going to try to narrow the source of noise as per the geofex article also. i'll report back.