humming issue, and grounding schemes...help..

[swt]

Hi!!. I've been wondering if someone could help me with this issue.
I have a d**ble hybrid style amp clone. It works and sound good to my tastes. but i have a noise issue that's killing me. Here's the detail story...
If i ground input of power amp (driver) the amp's dead quiet, so no problem here. If i turn down drive pot, it's also quiet. The problem lies at output of stage 2. if i ground that, it's quiet. So i have isolated the problem to stages 1 and 2. If i turn down the gain, the HISS goes down, but still have some bad humming. The ground is a bus, it goes: ot, first filter, bias, and screens filter, to same point, then the string is wired as tut books, with distributed capacitance, so it's driver filter and 390 res, od filter to od's cathodes, and clean stages to last filter. At first i had the input jack isolated from ground, and everything connected to the same filter point, cathodes, stack, pot, etc. It didn't work. Now i have tried grounding jack through chassis, and cutting the bus from the chain, so only first filter and clean stages hung from that point at input jack...it's not working also.
Don't know what else to try...and the amp simply smokes...so i need to solve this. Any ideas??. Thanks a lot for your help or advice.

[Corona Blue]

You've checked for cold joints in the filter section I assume? Did you set the amp up with the virtual center tap for the heaters?

[SlidePicker]

Consider positively biasing your heater circuit. Early preamp stages are notorious for humming from emission from the ends of the heaters, which with AC heater power typically swing through a wider voltage range than the cathodes of the preamp tubes. When the heater is more negative than the cathode, it emits a pulse of electrons to the plate that causes hum. If the heater circuit is biased consistently positive relative to ground, emission from the heaters is effectively prevented. The center tap of the heater secondary (whether a real one on the transformer or a synthetic one made from two resistors) can be connected to a resistive voltage divider from a B+ node that gives something in the range of +15 to 40V, or where there's a cathode biased output stage, the cathode(s) of the output tube(s) can be a convenient source of positive bias voltage for the heaters.

[Don Symes]

One other thought, in case you haven't tried it -

You have tried at least one other guitar - one with humbuckers, ideally - and a known-good cable, right?

Turned off all the fans, ceiling fans and light dimmers?

[SlidePicker]

And of course, it may be humming because it doesn't know the words.

[swt]

Thanks a lot for the kind replies!. you know the amp's noise comes even with nothing plugged at the input. So grounding the input is useless. Gain pot at zero, or grounding grid of v1b, is also useless. If i disonnect the wire at the master or v2a (od chanel in case it's connected), noise is gone. That's why i think it might come from other places. The power amp, driver, od, etc has no noise. I've used a dc heater for all preamps tubes, with a bridge rectifier, and big filter ( around 7000uf) and it's referenced to ground through to 100 resistors, as tut book suggested for rec dc heaters. I'll keep trying and let you know...thanks again.

[Enzo]

Are the pot covers grounded? Many pots ground their cover via the bushing, but not all, so if the cover of the pot has no continuity to ground, add some. ANything that affects the hum is after the source, and anything that doesn't is before it. You can ground anything that doesn't have DC on it to see, and even then you can add a big old cap - .1 for example - in series with your ground lead and ground off DC offset points in the signal path. That .1 might not squash the hum, but it ought to put a dent in it, so you should be able to pinpoint the spot where the hum comes in.

Are there any channel switching relays in the curcuit? The ground return currents from those could be involved if so.

Signal leads parallel to AC wires can pick up induced hum. Does moving any wires do anything.

Is the hum 60Hz or 120Hz? 120Hz is from ripple, 60Hz is picked up from radiated noise or is from poor grounding somewhere. They sound very similar, so if in doubt, use a scope.

Does the amp do this in more than one location?

Is there ANYTHING else running off the mains within several feet? Or even a wall wart?

DOn't guess, track it down.

[swt]

ok...so far this is where i am right now. I will change the ps resistors to metal film, to hear what happens. By the way i also have some hissing. if i move wires...nothing happens. I'll let you know any further improvement. thanks again.

[dai h.]

changing the resistors to metal film is not going to fix hum or buzz. Try finding a working grounding scheme and copy that exactly, or do some reading about why noise happens (internal, external sources, grounding, etc.) and figure out and fix the problem. It's more efficient in the long run, and next time you have a hum problem, you will have a better idea what to do. IMO learning the basics of dealing with hum and noise is one of the easier things to learn because it requires close to zero math (only an idea of more or less). I suppose overall, a basic idea of how a tube works, how power supplies work, then what DC and AC paths are present (there was such a chart showing these in an amp at the Hoffman amps site IIRC) are what you need to gain an understanding of what to do (and not do).

[swt]

The rpoblem is that although i read a lot...there's not a consistent method to do this. For example...in tut3 there's a detail story about what to do (galactic ground), and although it works great in another amps, It's not working on this one. I'd really like to know how the real ones came grounded...mm??

[dai h.]

which parts don't make sense?

[LooseChange]

The usual reason why a grounding scheme does not work is because there is some kind of fault in the production. I've discovered in my own builds that a jack that I thought was isolated was not because the faceplate that was plastic was actually conductive. I use the galactic ground and add the ground lift to most of my builds. The nice this is that when you lift the ground, you can test to see if it really IS lifted with a meter.

Test everything...

[dai h.]

Possibly... in a production amp, but I'm not so sure about that would be the case in a DIY amp, where I think the reason would be lack of conciousness of what is happening in the ground lines, therefore wiring done with little regard to the current flows which can lead to problems manifesting as hum and buzz.

[angelo]

swt, About what/which note(s) is/are the humming/buzzing making? did you post the schematic? I could not find it, granted I'm in a rush at the moment. Can you describe the power supply? My experience is that most of the problem comes from the power supply, then you clean up the residual with the grounding stuff. I wired a voltage tripler ass-backwards once and the amp still sounded good, except it was buzzing like crazy. Leo Fender himself never would have gotten the hum out through proper grounding.
You'd mentioned grounding in the "real amps"...
Someone in the old board made the point that production amps are a result of trial and error on a scheme we can't even imagine. They will literally make 20 prototypes--one that hisses, one that buzzes, one that buzzes faster. Thats why they seem to work with less-robust grounding schemes. There is a black magic to noise reduction.
You are probably right to criticize the lack of a coherent voice in amp noise reduction. It seems like alot of noise sometimes. It is an elusive science at times.

disclaimer: I don't know what amp you are talking about. I'm assuming you built it from scratch.

[swt]

thanks everyone!. It's a dumble style amp, two channels, loop, class ab1 with a pair of 6l6s at 470 volts. It works like a charm. but the noise is killing me. After od2, and input to the driver, noise is zero. But i have a lot of noise at the output of clean channel. By the way...this amp is number 15 i have built, and never have this issue. I have a lot of literature, and been hanging on these forums since 1998, that's why i'm lost in here. Fact is, that i've changed tubes, caps, resistors, different grounding techniques, shielding, different position of ground to the chassis...and nothing seems to help...