The dreaded hum

[lowell]

Got an Ampeg V4/V22 on the bench. Came in with bad power tube so replaced tubes and a bad Screen supply resistor (R55). Filter caps for CT and screens are new. Bias looks good, voltages look good in PA and PI. Hum balance pot DOES work and is set at minimum hum setting. There is a decent amount of hum... more than I believe to be normal. When I pull the PI it goes ALMOST completely away. I then put the PI (V4) back in and removed V3 but hum remained. So I've narrowed it down to the PI stage. Ok, so I then disconnect the PI-PA coupling caps C11/13 one by one. Hum remained. Tried a new 12au7 for PI - HUM. I am stuck. I also subbed in a new filter cap for the PI - hum remained.

http://www.freeinfosociety.com/elect...es/ampegv4.gif

[soundguruman]

For one thing, real Ampeg has a lot of hum, as is. It was built that way.
So does real SVT.
This is because the filament tracks are too close to the grid tracks.
Run the PI on DC filaments, just to see.
I think on DC filament, the hum will be gone. But try it.

[Enzo]

Measure the voltage on both PI plates. Are they more or less the same? Or is one way off?

Is it 60Hz or 120Hz hum?

There are a bunch of resistor associated with the PI, check them all, including the ones plate to plate.

[jamesmafyew]

And also check the bias supply. It can help to look at the hum on a scope -- if it is 60Hz sinusoidal, then it's coming from the heaters, if it's 120Hz sawtooth, it's coming from the power supply, if it's 60Hz asymmetric sawtooth, it's coming from the bias supply. It's hard to say if the bias supply could be at fault from your description, because it would probably still cause problems with the PI removed. But if you disconnected the .33u coupling caps and it still hummed, that's the first place I would be looking. C23 and C15 could be bad.

I'd also leave V3 in but put a shorting plug in the external amp jack on the back. See if the hum changes.

[Jazz P Bass]

There a couple of things that you can try.

First off, quantify the hum.
At the speaker terminals, connect a volt meter, set it to read Volts ac.
Write that hum figure down.

One at a time, install a power tube & note that hum figure.
Do the same for all four tubes.
This will help identify a hummy power tube.

Next, install all four tubes a measure the power supply voltages.( A through E).
The Volts dc readings should be close to the schematic (probably a tad high)
Then measure the same points with your meter set to read Volts ac.
This will help identify any bad power supply capacitors.

[g1]

Lowell, your schematic link doesn't work. The one JPB posted is for the later model with distortion, is that what you have?
You mentioned V3 so I think this is the schematic you mean:
http://www.ampegv4.com/images/schematics/V4.jpg
Be sure it is V3 you are pulling, I think they are tough to figure out which is what the way that chassis is laid out.

[jamesmafyew]

Lowell, your schematic link doesn't work.

It does, but the site prohibits direct-linking. You can copy-paste the URL manually and it will come right up.

[Alex R]

Maybe also check the circuit grounds are all good and haven't been altered. I spent many hours chasing hum from the PI area of an AC100 before I decided to put the grounds back to stock. Someone had tried star grounding - I thought, must have been an earlier attempt to stop the hum but no, it was the cause.

[lowell]

AGGGHHHHH.... just had an entire post deleted. So thanks guys for the suggestions. Enzo I measured all the PI components and all were good. I did however find some notable things and replaced the coupling cap that feeds the 2nd half of the PI. That seemed to help and the hum is now copacetic... not great, but ok.

JPB I've tried many times to measure AC on filter caps and my meter goes crazy. Why is that?

[R.G.]

JPB I've tried many times to measure AC on filter caps and my meter goes crazy. Why is that?

Your meter can't cope with ~10V of AC ripple riding on top of several hundred volts of common mode DC.

You can work around it by putting a BFC (Big Freaking Capacitor) of a suitable DC voltage rating between the B+ and a 1M to ground, then measuring the AC across the 1M. Be really careful - that blocking cap will be charged up to B+. Fortunately, with only a 1M to drive, the cap doesn't have to be quite so Freaking Big. It has to be big enough to pass the mains frequency with little loss.

So if you allow 1M for your meter and 1M of resistor to ground, the cap has to pass better than the line frequency with a half meg load.

C = 1/(2*pi*R*F) = 1/(6.28*5E5*50) = 6nF. That's for a 6db drop at F=50, and you want less than that. So a 47 to 100nF cap rated at 500-600V as needed by the B+ voltage would do.

It's a huge help in hum issues to be able to tell mains frequency from twice mains frequency. Mains is an octave lower, and the human ear has a hard time with that, but it can be done. Mains hum means grounding or radiated hum issues - or one half of the rectifier isn't working and the rectifier is making half-wave only. Twice mains means ripple is getting through.

And remember the onion model of troubleshooting. Removing one layer of the onion just lets you see the next layer in better.

[Jazz P Bass]

JPB I've tried many times to measure AC on filter caps and my meter goes crazy. Why is that?

I use a Fluke meter.

What meter are you using?

Maybe it's time to invest in a 'good' one.

[lowell]

Yah cheapo rat shack meter. Thanks for the enlightening post RG. I'm guessing 5E5 = 500k?

[R.G.]

Yah cheapo rat shack meter. Thanks for the enlightening post RG. I'm guessing 5E5 = 500k?

Sorry - yes, it's 500K. I lapse back into the exponent notation when I'm typing equations on character-only keyboards.

The "E" notation is a bit like the included-multiplier version of resistors. 2K2 in some conventions is 2.2K, the "K" replacing the decimal point and telling you the multiplier. Only the "E" notation is usually three significant figures, including a decimal point, with the exponent of ten after the "E".

So 1,230,000 might be expressed as 1.23E6. Your guess was exactly correct.

[lowell]

Ah gotcha. TI82 calculator notation! Thx.

[Debug]

I recently ran into this where my g20 hummed with a buzzy sort of hum even with new filter caps etc. by buzzy hum I mean it had both 60hz and quite a bit of higher harmonics and noisy hash. Tube selection didn't yield anything.
Turning the volumes to zero or pulling the preamp tubes (one per channel) would make it quiet so I ruled out the PI and power tubes and focused on the heaters. I agree with soundguruman that the heater traces are too close to important stuff.


I tried elevating the heaters with a 9v battery and it went dead quiet. I made a quick divider off of the lowest dc rail that gave me about 100v of elevation and it did NOT help the hum or the hash at all that I could tell. I didn't really get this.
Empirically I found that a .001uf cap from each heater line to ground made it quiet again. I suspect the value of the cap isn't that important. However, the cap had to be installed down at the far end of the heater traces and not on the balance pot end.

I'm still pondering why just straight elevation had zero effect when the battery made it perfect. The dc rail is very clean on the scope. I also haven't got my head around why the caps solved it as the impedance is megs at 60hz. I would have expected the hash to go away but not the hum. Also, it's still not quite as clean as the battery but it's more than acceptable as the volume has to be cranked to hear anything at all. I will continue to ponder this.