Definitive hum and buzz sticky?

[Luthierwnc]

Operator error on a piece of this. The trace from B+2 to the first series cap broke. That is fixed and the sawtooth 120 waveform now stops after B+2. It still hums like it doesn't know the words so I'll do more digging.

BTW, even though I'm sure I did something wrong, I'm glad to start a rudimentary scope handling discussion. I've never really known how to use this thing for diagnostic. Until now I've run a sine wave into the amp to see how it would distort, stop, get fuzzy, etc. Helpful -- but not the same as using it as a proactive tool. Again, thanks to all for your comments and Tom, if you can sanitize it to keep people from shocking themselves, maybe pieces of it could be used for a permanent sticky. sh

[martin manning]

Ok good. You shouldn't be seeing power supply ripple on the preamp nodes.

Since its making its own signal, use the scope to look at the audio path waveform beginning at the input and after each stage. This might help you determine where the hum is getting in.

[VacuumVoodoo]

BTW, even though I'm sure I did something wrong, I'm glad to start a rudimentary scope handling discussion. I've never really known how to use this thing for diagnostic.

When chasing hum set trigger source switch (upper right corner on scope panel) to "LINE". This synchronizes the sweep with mains frequency which makes the displayed ripple waveform stable on the screen.

[martin manning]

Also, scope the speaker output so you can see the waveform of what it is you are hearing.

[Luthierwnc]

Got the second gremlin. I built this amp with a parallel OD relay tied to two on/off/on mini switches on the panel. One of them is tied to the junction of the two 22m resistors in the LNFB. On the switch it is disabled (grounded) in the up position, on in the middle position. In the lower position it goes to half of the relay and is not grounded (enabled) when the OD is on.

The other half of the relay does the same thing for the bright switch. On the panel, up is on, middle is off and lower is off when the OD relay is on and vise versa. That was the culprit. There was just too much exposed wire not to pick up lots of EMF. I rewired it conventionally and left the LNFB half alone. Works great. There are still a couple little things to chase down but the big noises are history.

Again, thanks to all for the scope help. I hope my dilemma helps others chase down the squeaks and buzzes in their builds.

Enjoy your weekend! sh

[martin manning]

This is sticky-worthy reference material for hum-busting. A general troubleshooting guide would be too much I'm afraid, and besides RG Keen's tube amp debugging pages are already out there.

[Luthierwnc]

Hi All,

There was one more hum to chase down and I found it -- but it did raise some questions. It was the relay supply. I had the transformer wired with an extra 6v winding for a dedicated relay power source. It is set up going into a FWBR with a 1k cap on the + going into a voltage regulator with another 1k cap on the back side. The grounds for the DC conversion circuit are all isolated from the amp ground and it goes to the footswitch.

I disconnected the hot and the amp is dead quiet -- with the volume and master dimed.

Measured across the two leads the voltage is a little over 6 VAC but each lead measured across the amp ground comes in at about 70 VAC. What I imagine happened (and will be shortly told to confirm) is that those two leads are grounded on either the HV center tap or the 100R resistors on the other filament supply so that the real and virtual grounds are duking it out. Of course, it might actually work if I only use the real ground only.

If I don't already have a 6v transformer I'm sure Mouser does and there is plenty of room for it on the chassis. I'm using 5v relays, BTW.

Thanks as always for any comments -- even disparaging ones. Cheers, sh

[martin manning]

By "1k" cap I assume you mean 1000 uF. You need a ground-reference on the DC side of the relay supply. I would ground the bridge to the chassis somewhere on the PT end, and daisy-chain the filter cap grounds to that same location. Don't connect any other part of the relay supply or foot switch to the chassis, and if your relay supply winding has a CT don't ground that either.

[Luthierwnc]

The top one is how it is now. If I understand you correctly, it should look like the lower version. sh

[VacuumVoodoo]

Like this: see pic.

[shoggoth]

If that's a 6v AC winding, under virtually no load you'll only get (6 - (.65 * 2)) * 1.414 = 6.3v DC into the regulator. Under load it'll be even lower, as the bridge rectifier drops more volts. That's ignoring ripple voltage. That's nowhere near enough to regulate, you want a minimum of 8v DC I think going into the regulator, and more to account for ripple. The regulator is basically doing nothing at all right now, all that ripple is coming right through.

You might want to try changing those 1000uf's to the biggest 10v electrolytics that will fit, and replacing the regulator with just a resistor (1 ohm or less, the biggest value you can that doesn't drop you too low, R = Vdrop/Irelay) between the caps, you'll probably get better DC that way.

Take a look at the relay data sheet to see what's the lowest voltage they'll turn on at, and try to stay a reasonable amount above that.

[martin manning]

I think the math goes like this: 6.3 VAC x 1.414 - 1.3 = 7.61V peak. The winding is probably lightly loaded for this duty, and may well be above 6.3VAC. In any case, just measure the regulator input voltage to see where you are.

[Luthierwnc]

I grounded the virtual ground and all is well. BTW, the winding was close to 7 VAC and it regulates right at 6 VDC. The relays are 5 volts so not overpowered but plenty to spare. Now I get to start the tweaking process.

Thanks and cheers, sh

[shoggoth]

Cool - why not use a 5v regulator though?

[Luthierwnc]

6 volts is within the comfort zone of a 5 volt relay and it gives me a little room if the circuit needs to recover. In a single stomp you can activate three relays and two LEDs instantly. If the other relay and LED was already on, the current draw is starting to get up there. Plus the wire run is basically the interior circumference of the chassis.

There is also room to tweak. You can hang a resistor or a diode on the middle leg of the regulator to drop the voltage a little. Much harder to go the other direction. sh