Debugging Noise on a Home-Made Amp

[Theashe]

Hey guys,

I've got a home-made push-pull tube amp with some hum and strange noises - maybe you can help me sort them out. I've attached a picture of what the component layout in the amp looks like.

There's a small amount of 120Hz hum that stays constant no matter what the volume controls are set at - it might be that the cathode resistor is too close to HV.

There's also a large amount of 60Hz hum that seems to be coming from V1 and the feedback loop. As you turn up the first volume pot, the hum gets louder, but if you turn the first volume pot all the way down, none of the other controls affect the hum. If I disconnect the feedback loop, some of the 60Hz noise goes away. If I reconnect the feedback loop, the 60Hz coming from that path stays at the same volume no matter what the position of the feedback control is. If I turn the amp off, the 60Hz noise dies instantly, while the 120Hz noise dies down slowly (there's no standby switch in the amp).

The amp occasionally makes small "popping" and "hissing" sounds. I haven't been able to figure that one out yet. They come and go.

If the feedback control is turned down (or disconnected altogether), and the master volume is turned to a specific position, the amp starts making a high-pitch whine. Adding feedback reduces the problem, and the problem never happens when feedback is turned all the way up.

Some more info:
- The amp uses a star grounding (despite that the diagram makes it look like there's a bus bar), and it's grounded at the input jack.
- The preamp grounds and the power stage grounds are connected together at the star.
- Checked all the ground connections, and they're all solid - no impedance to the chassis.
- The power supply is built on the back of the turret board.
- The tubes are all brand new, and so are the power supply capacitors.

I'm also faced with higher-than-expected voltages in this amp. I built and tested it in my hometown where the mains voltage about 120V, but I'm temporarily living in a factory city where the mains voltage is 138V. The result is that my B+ is about 50 volts too high here, and my heaters are at about 7.5V.



Any thoughts would be more than welcome. I realize debugging this problem won't be easy, since it's an unorthodox amp. When I get home from class, I'll try out suggestions and let you know what the results were.

[tictac]

A schematic accurately drawn would be helpful....

Your grounding scheme isnt clear from this layout and I don't see any ground for the heater circuit either.

If everything is grounded to a buss that's grounded at the input you should separate the preamp & preamp filters from the power amp & power amp filters. The PA filters, PA cathode resistor, and heater ground should be tied to the same point at the PA side of the chassis. Your chassis ground should have it's own connection near the AC input jack.

Your speaker ground should be grounded at the same location on your ground buss as the phase Inverter.....

You've got alot going on in your layout, if you've got a schematic it may reveal other things...

TT

[Theashe]

Hi tictac,

I'm going to be honest with you here - I hadn't grounded the sleeve of the speaker jack. Rookie move!

I grounded it and the whole amp quieted right down! And the negative feedback control, resonance control, and presence control, all of which weren't working properly, now work perfectly. You just saved me a ton of time going in circles.

Now there's just a bit of hum coming from V1 when it's turned up, and that's it. Easy enough to find and fix.

[Stevem]

138!
I gotta tell ya your tubes are not going to last long with the filiments 16% up from where they should be and the amps gain levels are going to be way off as well, the hum and buzzes you have are secondary!

Also you really need to let the power company know that they have step down transformer on line in your area with a bad secondary leg and as such any electronics that you have go south and you will see them in civil court!
I had a relative go thru 3 flat screens do to a situation like yours!

[Leo_Gnardo]

138!
I gotta tell ya your tubes are not going to last long with the filiments 16% up from where they should be and the amps gain levels are going to be way off as well, the hum and buzzes you have are secondary!

Also you really need to let the power company know that they have step down transformer on line in your area with a bad secondary leg and as such any electronics that you have go south and you will see them in civil court!
I had a relative go thru 3 flat screens do to a situation like yours!

What Stevem said ^ ^ ^ !

I had to do a big rock show in Sao Paolo 1982, same 138V on stage. We didn't plug in any gear until the promoter came up with a huge variac just before show time. Got the stage dialled down to 120V, I kept one hand on variac's "steering wheel" knob and the other on a voltmeter while mixing monitors & we got thru the show without unexpected pyrotechnics. If you don't get cooperation from your power company, get a variac or build yourself a bucking autoformer rig - cheaper than it sounds - for $50-60 including transformer, box, cable for your amp.

[Theashe]

There's a store nearby me that sells variacs. I've been meaning to pick one up, maybe I'll finally shell out for one.



I was about to start working on smoothing out the overdrive of the amp today, but your posts got me curious - so I connected my multimeter to mains. 154V! I'm gonna send an email to the utility or pick up a variac before I continue.

[rp]

woah!

Grandpa hushed Herman and turned on the power. The ray machine vibrated to a start. It hummed . . . it whirred. The eggbeaters pulsated; and, around town, the street lights exploded one by one! Finally, the main building at the power station went up in a terrific blast!

Grandpa’s machine, too, blew up . . . a total wreck. Two soot-covered Munsters survived the explosion in the basement. Herman emerged from the litter with an eggbeater stuck in his mouth!

[JazzGuitarGimp]

It looks like your meter's battery is in need of replacement. You should replace it and measure again before you spend any money - as your meter may be lying to you.

[Theashe]

It looks like your meter's battery is in need of replacement. You should replace it and measure again before you spend any money - as your meter may be lying to you.

Good call, it was my battery. Measuring pretty close to normal now! Although I should mention that I read it at 138 with another meter about a month ago. Measuring 126 today.

Thanks gents, I'll get back to my tinkering.

[Structo]

Strange that the wall voltage is that high.

My power is usually right around 120-122vac.

Maybe hydro power is more consistent?

But yeah, if the heater voltage is higher than 7 volts, you are going to
lose some tubes.


Here is a great little book about the life of tubes.

[SilverFox]

Tell me more about living in "Factory Town". Makes me wonder if this is in the US as most "Factory Towns" are KaPutt. Maybe the power company needs to check the voltage regulator at the substation...

Is the remaining Hum at a normal level? Perhaps your done.

silverfox.

[Theashe]

Hey Structo,

To be safe I called the control room at my local utility, and they were reading 250 volts on the feeder in my area, 125 per phase. That agreed with my voltmeter's reading within 1V at the time. I re-measured the heaters, and I'm getting about 6.5V now. Amazing how far off a low battery will make your meter. Thanks for the book!

Silverfox:

I'm in Hamilton, Ontario at the moment - until April. Home to Stelco, Dofasco, Bitumar, a Siemens plant, and more. I figure being close to those factories might result in a poor quality mains signal, especially since some of them have such high power motors and arc furnaces that extra generation has to be added to the province when they're turned on.

There's only one place I'm still getting a little bit of hum in the amp - at the reverb recovery when I crank that all the way to full. I just got some more shielded wire from Digikey, so I'm replacing the long runs between the tube socket and control pot with that.

I'm driving down to Alabama for a week though, so I won't be able to do too much more until next week. Maybe I'll keep an eye out for part suppliers. I've got a 16" reverb tank right now, which can get kind of washy when it's cranked. Might look for a 9" one, since shipping to Ontario is murder.

[Stevem]

Set the amp open side chassis down on a big sheet of aluminum foil, does that help the issue?

[Theashe]

Set the amp open side chassis down on a big sheet of aluminum foil, does that help the issue?

No, the hum is the same whether the amp is open or sealed. I tried wrapping the reverb tank cables in an extra layer of grounded foil to see if the hum was being picked up off the PT, but that does nothing either. The recovery circuit is a 12AX7 in series with the triode side of a 6GH8 with a control pot between them. I haven't been successful using shielded wire runs yet, either, but I haven't tried shielding every wire that crosses my power supply.

[Structo]

Adding too much shielded cable can affect the tone of the amp.

Only use it where it needs to be.

Study your lead dress to see if there are any wires that shouldn't be near preamp stages.