Steel String Singer 002 Build - 60Hz Hum - SOLVED

[Krinkle]

Please help me.

Here's my layout. It's not complete but most of the info is there. It's a Bassman head converted into an SSS 002. I only had 4 preamp tubes so I just omitted reverb.

Here's a pic as well. Amp sounds absolutely amazing, thanks to the help of you guys here at TAG, I couldn't be happier with the tone. I need to bug you guys a little more. I've searched the site for all ground/hum posts and tried some things but no luck yet. I don't think this is the rocket science that I'm making it out to be. I really think someone is going to find something really stupid and obvious that I did. Please share your ideas and help.

With no cable or guitar connected I can hear the hum and I can turn up the volume and increase the level of the noise.

So far;

1. Swapped all 4 preamp tubes.
2. Pulled V1, hum pretty much goes away (I would expect a little with the volumes cranked, right?)
3. Shorted Input jack ground lug to chassis with alligator cable.

Fired up the scope and probed V1a plate and saw the 60Hz. Then I scoped V1b plate and was able to turn the first volume pot to see the hum increase when the pot was turned up, just like I can hear it when I have a speaker connected.

I took a pic of my scope screen. You can see the nasty 60Hz that is there. I have the power supply cap grounds for the OT center tap and choke tied together and not to the other filter caps.

I almost forgot, I made a PCB for the FET boost and it is mounted on the side. The noise is there with the FET boost not engaged, and the level increases when I engage the FET boost with a volume increase setting.

The FET boost pcb has a bridge rectifier and a low drop out regulator to convert the 6.3VAC heater voltage to 5VDC. It looks clean to me. It also has a 5V relay to switch the FET circuit in and out.

Sorry if this was long winded, I hope I didn't lose you guys because of it, I just wanted to supply as much info as possible. Anybody got time to help a brother out?

[larsvictor]

The artificial center tap of the heater winding is located near the pilot light?

It could be a problem with the additional FET circuit. I had the same thing when I tried to build a 9 VDC power supply for stomp boxes in an amp using the heater winding. When I engaged the effect, I had this terrible hum. Please disconnect the heater from the FET circuit and hear, if the hum dissapears.

Lars

[Krinkle]

Thanks for the reply Lars. Much appreciated.

I'm suspicious of a few things, one of them being the FET circuit. I'm going to check;

1. Bias rail. I didn't have a 47uF cap as per the schematic so I used a 22uF one. I'm wondering if it has too much ripple. Pulling the 1st tube and the noise going away doesn't give me much hope for this one though.

2. FET circuit, as you suggested. I was going to start desoldering wires from it to run the input straight to V1a grid but your idea of disconnecting the heaters is easier, and better. I'll just desolder the wires from the tube socket. I'll completely remove the pcb and rebuild or split it into 2 pcbs it if I have to. I have the same circuit on a breadboard on the floor in front of the amp so I can switch it back in and do an easy A/B once I remove the heater wires from the circuit inside the amp.

3. Output jack grounding. This is an old chassis and it is dirty, if not oxidized. I'll clip a wire on to it from ground and see if that helps. Not expecting much here either.

4. The lead dress around the 1st volume pot. I really noticed the hum when I changed the pot from 100k to 1M. I don't think there is an issue here though. With the 100k pot, the volume would pretty much go from 0 to VERY loud, so I believe the volume was masking the noise. Once I installed the 1M pot, the volume was properly manageable and I was able to ramp it up as you normally would. Paranoia will still probably get me in the end, and if I don't find the issue I'll probably change that pot, for no good reason.

It's early morning here so it'll be a long 8 hours before I can check some things.

I'm open to any suggestions from anybody. I'm not going to stop until this noise is gone. Then it will be the amp I've always dreamt about. it absolutely nails SRV right now. I've also tried some Eric Johnson tones and they are very authentic. I'm using 2 tube screamers to get the tone from Righteous off of AVM. It sounds very close, and I don't even have my DiMarzio HS-2 in the bridge position of my guitar, I took it out to try regular single coils for a while. Once I get this noise figured out, the first thing I'm going to do is put that pickup in and nail that tone. I never thought in a million years that I would ever have that tone. A HUGE thanks to everybody at TAG!

[pompeiisneaks]

I've found and fixed several noise problems with the old chopstick method, have you done that? If you have the volume up and tap around does any specific area make horrific noise? I've found issues in my ground connections for the input jacks, miswired input jacks, etc. Just tapping the area with a chopstick makes it produce all kinds of funny noise and helps narrow down the exact location. The description you're giving tells me it's either the inputs themselves or the wire going to the first triode. Unless I misread you stated the hum is already there before engaging the FET and the noise is just 'louder' after that, to me that seems to imply the FET is just magnifying it and the noise comes before the FET AND before the first triode, meaning something in the input jack itself. a ground cold soldered, a bad resistor to ground or to the input etc, something along those lines.

~Phil

[Krinkle]

Thanks for the reply Phil.

I have tried with the chop stick, poked just about everywhere last night. No abnormal sounds.

You are correct, the noise is there, it is not too bad until the FET circuit amplifies it.

I am also suspicious of the input jack. I poked and prodded it last night, connected an alligator cable directly from the ground lug to chassis, etc. I reused this jack so you never know, although it looks fine. I don't have another 3 lug jack but I'll just remove it anyways and temporarily put a 2 lug jack on it tonight, just be sure.

I'm the soldering trainer at my company and have been soldering for 30 years, and I just bought a very nice new soldering iron, so it would be pretty embarrassing if it was a bad solder, but never say never, I'll poke my solder joints near the input.

I'm very hopeful regarding Lars' suggestion about the heater wiring powering the FET relay. I'm not 100% sure I had this noise when I was using the breadboarded FET circuit outside of the amp, but my volumes were not working properly.

During my years of troubleshooting, I've learned to consider everything. The reason I usually can't find a problem is because I'm convinced that something is working that is actually not working (sounds like a DUH statement but it's true).

[pompeiisneaks]

Sounds like you've got good ideas about it, I've just dealt with this kind of thing myself a ton, and I'm a neophyte, so I'm sure you've got more experience at it than I do. I did also have jacks recently where the switch wasn't making a good connection and slight tweak to the switch solved it. (again, just thinking of the obvious easy to forget stuff )

~Phil

[Krinkle]

I've been in the electronic testing industry as a tech for 25 years but I learn something new every day. I don't have the patience to delve into tube operation down to the finest levels. I work at electronics all day so when I get home I just want to not think or study and just play

I went home for lunch and I made some progress. I disconnected the heaters and it made a difference. I did this very fast so I have to slow down a little tonight and process what I found. On the scope the level went down quite a bit but I'm measuring a very small level to begin with.

I also added a 2nd 22uF cap in parallel with the 22uF bias cap and that made a difference as well.

I also noticed that my PT HT wires are pretty much on top of the lamp so that may end up being the biggest culprit. I didn`t move them as the amp was on my speaker cab and not on my workbench and I don't stick my hands or anything in the amp unless it`s on my bench. That will be one of the things I do tonight.

I plugged the amp into the speaker cab and I'm not sure if the noise is typical or not, but it certainly seems a lot lower. Tonight I will connect up my floor FET circuit and see how loud it is with it boosted.

At the end of my lunch break I turned both volumes up with no cable or guitar plugged in and it starts to feedback with both volumes at about 8. I'm not sure if this is parasitic oscillation because this amp feeds back beautifully with the guitar plugged, it may just be that lively.

[Krinkle]

Wow, a lot, or all, of this noise is coming from dimmers! I have 4 in the house and I've pretty much decided that they have to go.

While removing the heater connection that powers the FET circuit lowered the noise, it was pretty much because the input wasn't getting boosted. As soon as I used my external FET circuit, I got the noise again.

The more I listened, the more the noise sounded like the noise my dimmer above my bench makes when I turn it down with a halogen bulb installed.

I went over to the dimmer switch and sliding it up and down, well, you could pretty much pay a song with the noise changes.

I've always had noise in my basement, but it's never affected an amp to this degree.

I should probably break down and use some shielded wire 1st to see if that helps.

[pompeiisneaks]

OH man! I had similar problems with my Fluorescent bulbs in the house, swapped out to LED's and it's way quieter. I think upstairs I have a few dimmers. May be time to nuke them with prejudice

~Phil

[Krinkle]

Unfortunately it's not the dimmers, even though they wreak havoc with the frequency of the noise when I slide them up and down.

I just checked the ripple on my bias voltage and it is 1.3Vp-p. On a 400VDC rail that's 0.325%. That seems liek it should be more than okay but.........

The ripple is a pure 60Hz sawtooth wave form.

Here's a pic. For convenience I tied my bias ground to the same lug as the heaters, heading down to move it.

[Krinkle]

Didn't fix a thing. This really looks like it's coming in through the input. I just found a replacement jack but it's late, going to bed, I'll replace it tomorrow. I'll replace the whole front end if I have to. I spent about 20 minutes playing the blues, it sounded so great, it actually cheered me up a little. I'm not giving up, this amp will be perfect once I get rid of that noise.

[davebolden44]

i was going to suggest your lighting in your workshop, i have had that kinda hum before. when i would lay a piece of sheet metal over the open chassis it would shield it and stop the problem. try that if you can before you change everything chasing your tail. all i have in my shop for lights are cheap clip on 60 watt halogens running led lights, no florescent bulbs

[Krinkle]

Yeah, I have a few obvious things to do but I have a dirty habit of turning problems into rocket science.

I put the chassis back in the head case because it has mesh at the top, but I didn't confirm that it was making contact. I had a panel that I used to lay over amps and it got cluttered and I threw it out. I grabbed another one today and I'll give that a quick try, though I'm not expecting much, with my luck.

I found a new input jack last night so I'll install that, plus add some shielded cable, I didn't expect this amp to sound so good right off the bat and I just quickly put it together, without paining over shielded cable, grounding, etc. Now I'm paying the price. The sad part is I know better.

Thanks for the reply.

[Krinkle]

I noticed one weird thing last night. I was turning pots up and down to see how it affected the noise on the scope and the treble pot froze up on me. I turned it down and it only went as far as 3, instead of 0. By turning it up and down a few times it finally went back to normal. I was really careful not to get any debris in the little opening in the pots when I was building them amp, and it didn't feel like I was crushing anything between the wiper and the track of the pot. I think it's just a mechanical defect in the pot.

I really don't like the Alpha pots I bought at Mouser. I much prefer CTS. I have a couple of spare 250k CTS pots, A and L. I'm just going to replace the Bass and Treble pots and along the way I'll order replacements for the rest of them. The 1MA volume pot I used when I replaced the 100k was one I pulled from my buddies' amp when we replaced all of his pots with CTS. He wasn't having problems but now I'm suspicious of it.

[Krinkle]

Here's what I tried tonight;

1. Installed an isolated input jack with shielded cable. One end grounded at V1a cathode resistor/cap ground connection.

2. Installed 0.01/1000VDC snubber caps on all PT diodes.

3. Installed a ground wire from output jacks to the NFB resistor ground connection.

Could some of the big brains here chime in and give me some suggestions. I'm open to anything right about now.

I keep thinking back to the fact that I first noticed this noise after I swapped the 100k volume pot for the 1M. The problem I have with this thought is that with the 100k pot the amp was extremely loud, from low volume to very loud as soon as I got the Master to 2. I'm wondering if the loud volume masked the noise and I just didn't notice it.