60Hz hum and 120Hz buzz

[pula58]

Been working on a friends "Sligo" clone of a Deluxe reverb. It has quite a bit of assorted 60Hz and 120Hz (and other harmonics of 120Hz) noise/hum.

One behavior is like this: All preamp tubes removed except the PI tube. I see 60Hz hum at the speaker. If I remove the Pi tube this 60HZ goes almost completely away.

I have checked all the caps in the PI stage (for correct outer-foil orientation), I have tried a shileded cable on the grids of the PI tube (grounding the shield at the ground of the PI stage, only one end of the shield is connected to ground). It had no effect. Yet, when I put my finger near the wire that goes to the input to the PI circuit I see the 60Hz go way-down in amplitude.

I re-wired the heater wiring on one side of the PI tube socket (i.e., the two wires that lead-off from the PI tube to the tube that is in the tremelo circuit). This heater wiring (that was in place when I received the amp) was sloppy, and was near the grid that I ended-up shielding. This had no effect on the 60Hz.

The grounds of the first power supply caps (the ones connected to the rectifier) had a separate ground wire going to a star-ground near the power supply tranny, the other (preamp) power supply caps went to the same star ground, but used a separate wire. I re-routed the ground wires of the preamp pwr supply caps to near the vibrato channel input grounds. Surprisingly, this had no effect on either the 60hHz or 120Hz.

any ideas ?

[Phil_S]

It may seem silly, but try putting a shield over the PI tube. Also, are the any florescent lights, including those new fangled spiral bulbs? Turn them off. I wondering if the PI is somehow picking up external noise.

Do you have the problem both on the bench and when the chassis is in the cab?

[diagrammatiks]

woo finger near the pi input.

you my friend live on the wild side.

have you checked to make sure that the heater supply is ground referenced properly.

[pula58]

yes, the heater voltages are balanced (the heater output of the power tranny has a center tap).

If I turn off all the room lights, soldering iron, o-scope, etc...no change in the 60 cycle noise

[Jana]

It sounds like a grounding problem with the PI and NFB loop. The speaker jacks, presence pot (if you have one) and the tail of the PI should all be grounded to the same place.

[pula58]

It sounds like a grounding problem with the PI and NFB loop. The speaker jacks, presence pot (if you have one) and the tail of the PI should all be grounded to the same place.

This sounded like a great thing to test and I was all excited that this might be "it." Here's what I did: I removed the two speaker output jacks from the chassis and dangled them out the back (all wires still connected to them). The ground that the output jacks were using was the chassis itself. So, for tis experiment,, I use a wire to connect the output jack ground(s) to the ground that the PI uses (the ground side of the 50 ohm resistor that sets-up the feedback along with the 820 ohm resistor).

result: No change in the 60Hz output hum. I was sure that this was going to be it!

It does make me think that the next build I do I am going to use isolated (from ground) speaker output jacks. Even though it seems like this is not the problem with this particular amp, I want to avoid chasing-around 60Hz (and 120Hz) as much as possible!

[Lynxtrap]

One behavior is like this: All preamp tubes removed except the PI tube. I see 60Hz hum at the speaker. If I remove the Pi tube this 60HZ goes almost completely away.

I guess this is my first post here, so Hello everyone.
Had to jump in here since I have the exact same problem with a Silverface Bassman.
I discovered that the first filter cap (one of those in series before the standby) was turned the wrong way. I put in some spare caps (the right way), but no change. I have desoldered and resoldered almost the whole PI circuit, heaters and output jacks, double and triple checked everything, tried different tubes... Obviously, theres is something I'm missing, because the hum started suddenly, and it's loud.
Very frustrating.

[megawhat]

i had that sound in an amp but 50hz + 100hz hum.
It turned out to be a bad can cap. the amp had 1x 100uf+100uf, 2x 50uf+50uf. All F&T's.
Changing one of the 50+50's fixed it.

maybe you have something different going on, I just thought I'd mention that.

[pula58]

this amp has 450V electrolytics in the first capacitor position, but, when in standby mode the voltage on those two caps is in excess of 500V. So, I was going to replace those two 16uF 450V caps anyhow (with two 450V 40uF caps (in series). Maybe that'll do the trick.

[Colossal]

Is this amp solid state or tube rectified? You mentioned it being a Deluxe Reverb type amp.

I would start your troubleshooting at the source. Start with the power transformer, work your way through the rectifier, into the B+ filtering supply and see where that 60Hz has its origin. I am going to take a guess that the current loop formed between the rectifier and the first filter cap may be large enough that it is getting mains frequency injected into it. I would look at the lead dress here, noting where the power transformer leads are running etc. Also, where is your speaker ground connecting to? Ideally, it should be run back to the negative side of the first filter cap.

I think without a very systematic approach (checking one variable at a time and eliminating it) you will not have an easy time of this. I agree with Megawhat's idea that the filter caps are worth checking for DC leakage.

[pula58]

Is this amp solid state or tube rectified? You mentioned it being a Deluxe Reverb type amp.

I would start your troubleshooting at the source. Start with the power transformer, work your way through the rectifier, into the B+ filtering supply and see where that 60Hz has its origin. I am going to take a guess that the current loop formed between the rectifier and the first filter cap may be large enough that it is getting mains frequency injected into it. I would look at the lead dress here, noting where the power transformer leads are running etc. Also, where is your speaker ground connecting to? Ideally, it should be run back to the negative side of the first filter cap.

I think without a very systematic approach (checking one variable at a time and eliminating it) you will not have an easy time of this. I agree with Megawhat's idea that the filter caps are worth checking for DC leakage.

It is a Deluxe Reverb clone. It has a Weber copper cap rectifier in it. I don't know the exact details of what is inside the Weber "copper cap" but I presume that it is some diodes and a series resistance to approximate the voltage drop you'd see with a GZ34 rectifier tube. It can be removed and replaced with a real GZ34 since it is plugged-into the rectifier tube socket.

http://www.webervst.com/ccap.html

I am expecting some new power supply caps in the mail either today or tomorrow.

Some things that I have done to improve things:

1) the mids and bass tone stck caps were installed with the outer foil connected to thw wrong place. I took out those caps, found which end was the outer foil, and connected that end to the 100k slope resistor. That helped noticeably.

2) the bass pot had one terminal not connected. I connected it to the middle temrinal and that seemed to help a little.

I am hoping the new power supply caps will really help. In my mind, it cannot be good that the caps are 450V, but the voltages (especially in-standby) exceed that ating by at least 50V. Perhaps this caused some leakge?

[cbass]

Does the heater winding have a center tap or 2 100ohm resistors going to ground?

[pula58]

Does the heater winding have a center tap or 2 100ohm resistors going to ground?

The heater winding does have a center tap. YEs. The heater voltages look pretty-well balanced. I have though of lifting the heater center tap and using a 200ohm trim pot and see if I can help alleviate some of the 60Hz.

I am thinking (hoping) that the new caps will help with the 120Hz (and its harmonics).

The heater center tap, and B+ center tap, and B+ capacitor ground, are connected at a common ground lug beside the power tranny. The power supply filter caps are in the Fender-style "Doghouse." The other caps in the doghouse are grounded near the input jack.

I routed the B+ wire and the ground of the B+ cap ground wire (from the doghouse back to the ground lug) next to each other (thinking that the currents, flowing in opposite directions would cancel their respective magnetic fileds).

I did notice that the heater voltages (as seen with O'scope) have slight flattening at their maximum pos and neg excursions (of what should be a perfect sine wave, right?).

[M Fowler]

I have too much experience with dirty AC in my home and I can tell you one starts pulling out hairs thinking the amp has ground issues.

I would look for out side source as well as continue with what your doing.

Mark

[pula58]

I have too much experience with dirty AC in my home and I can tell you one starts pulling out hairs thinking the amp has ground issues.

I would look for out side source as well as continue with what your doing.

Mark

I agree, a good thing to keep in mind.

But, the owner of the amp brought it over here because he was hearing excess noise in the vibrato channel. So, I think it is real.