getting started Debugging a buzz/hum (AB763 deluxe, new build)

[The Dude]

A little bit of noise is to be expected. Have you checked bias yet? There's the possibility that you're idling hot and a small noise is made large by excessive bias.

[mikepukmel]

A little bit of noise is to be expected. Have you checked bias yet? There's the possibility that you're idling hot and a small noise is made large by excessive bias.

Thanks Dude, yeah, checked a couple of times. I have a really crappy meter, but I think its OK for this. The tubes are Electro Harmonix 6V6's, rated 14w max. I have them biased pretty cold, around 6.5W to 7.5W (depending on if measuring across the 1 ohm bias resistor, or using the OT resistance and drop method. When I crank the amp, it doesn't break up, that's how cold it is.

The hum that comes on with the vol on min and when I switch the standby "on" is bothering me most, worried something is wrong with the power supply caps or output tubes.

[The Dude]

Have you checked for ripple on the B+ as suggested yet? That will tell you if you have power supply issues.

[mikepukmel]

Have you checked for ripple on the B+ as suggested yet? That will tell you if you have power supply issues.

Tried to, didn't work. I think its because of my toy meter. I can measure sine wave AC, e.g. wall outlet voltage, and the HV secondaries OK, but when I tried the AC setting on the voltage coming out of the rectifier around the filter caps I get a garbage number (meter is reading something like 800 on a 500 scale) Will order a new meter soon.

[Enzo]

Set the meter to AC, and add a capacitor in series with the probe. A 0.1uf or 0.047uf would work. That cap will block the DC from your meter, and the cap will pass the AC.

[mikepukmel]

Set the meter to AC, and add a capacitor in series with the probe. A 0.1uf or 0.047uf would work. That cap will block the DC from your meter, and the cap will pass the AC.

Thanks Enzo, can't wait to get back from work and try it tonight!

[R.G.]

I've typed up guides to grounding and hum several times. It's worth seeing if you can find one. I'm not in a typing mood right now.

However, I can tell you a few gotchas to look for.
One is nearly always missed by beginners. This is the critical wiring of the transformer lead to the negative side of the first filter cap. In an amp with a typical center tapped HV secondary, the center tap of the power transformer must be wired ONLY to the negative lug of the first filter cap. Not to the chassis, not to a star ground, not to anything else, only the negative lead of the first filter cap. This is because large currents go into and out of the first filter cap synched to the power line and caused by rectification. You simply must not let the return wire to the transformer CT share any other ground wire in the amp, or you will get small pulses on ground at 120Hz, and this will be heard as hum.

Another: magnetic field transmission of hum depends on large conductor loop area for effective transmission. To normal humans, this translates as "keep current loops small" and that devolves into "to the extent possible, make all AC-power wiring be twisted pair". It minimizes the area of the hum loop and keeps mag-field transmission low. Make AC heater wiring as much twisted pair as possible; there are people who think this doesn't matter, but it sure can't hurt.

Another: use tube rectifiers or fast, soft recovery silicon diodes to make rectifier turn-off soft and stop these every-120th-second transients from shock-exciting all the wiring and giving you a 120Hz buzz much like fluorescent lights.

[mikepukmel]

I've typed up guides to grounding and hum several times. It's worth seeing if you can find one. I'm not in a typing mood right now.

However, I can tell you a few gotchas to look for.
One is nearly always missed by beginners. This is the critical wiring of the transformer lead to the negative side of the first filter cap. In an amp with a typical center tapped HV secondary, the center tap of the power transformer must be wired ONLY to the negative lug of the first filter cap. Not to the chassis, not to a star ground, not to anything else, only the negative lead of the first filter cap. This is because large currents go into and out of the first filter cap synched to the power line and caused by rectification. You simply must not let the return wire to the transformer CT share any other ground wire in the amp, or you will get small pulses on ground at 120Hz, and this will be heard as hum.

Another: magnetic field transmission of hum depends on large conductor loop area for effective transmission. To normal humans, this translates as "keep current loops small" and that devolves into "to the extent possible, make all AC-power wiring be twisted pair". It minimizes the area of the hum loop and keeps mag-field transmission low. Make AC heater wiring as much twisted pair as possible; there are people who think this doesn't matter, but it sure can't hurt.

Another: use tube rectifiers or fast, soft recovery silicon diodes to make rectifier turn-off soft and stop these every-120th-second transients from shock-exciting all the wiring and giving you a 120Hz buzz much like fluorescent lights.

Thanks RG! Reading now. Also, I'll search for your other posts.


Re "This is the critical wiring of the transformer lead to the negative side of the first filter cap. In an amp with a typical center tapped HV secondary, the center tap of the power transformer must be wired ONLY to the negative lug of the first filter cap. Not to the chassis, not to a star ground, not to anything else, only the negative lead of the first filter cap."

Aha, ok, I did the exact opposite here, like in the photos of the old period (noisy as hell) AB763 chassis. My center tap wire goes right to ground very close to the power transformer. And the first ground wire from the first 2 filter caps is grounded to the same lugs. So, I should lift the ground wire leading back from the first filter caps, lift the center tap, and wire those together, no ground. Will do.

Re twisted wiring, yes I tried to do that as best as possible, all bundles are twisted.

Not sure i understand the last one.

[mikepukmel]

Tried the capacitor/ac thing, didn't have a big enough cap, could not measure anythig. Will order a couple of 0.1 and 0.047 non polar caps, or try to pick some up on the way home from work tomorrow if I can find an electronics place (very rare these days around here).

Downloaded 60 and 120 hz test tones. Fairly sure that the low hum in my franken-amp is 60hz. (could hear the beats between the amp and my computer speakers. So, this seems to indicate heater wiring inducing into a signal someplace, and probably not hv from the rectifier?

[mikepukmel]

RG, read through a bunch of hum/grouding threads, could not find one of yours, but some good ones. I'm still not completely down on the wiring routing. I got the idea why the CT from the power trans HV needs to first get routed to the neg filter caps, but no ground at all? I found a few threads that had a whole bunch of different configurations and looked like the gurus were saying to route first to the filter cap, then run another wire from that connection to ground. I mean, the filter negative has to get grounded at some point, its not left floating, right?

Thanks,
MP

[stokes]

He wasnt saying they should not be grounded.What he said was to ground the first cap with the PT CT at the same place,usually one of the transformer mounting bolts.I had gone thru this in a different post from you.I said to move the screen cap ground from the point where the preamp cap is grounded to the point with the first or main cap and PT CT.

[R.G.]

What's going on here is that (1) wires are not short circuits, they're low value resistors and (2) single ended tube circuits have zero ground noise rejection.

Taking #2 first, any noise on the "ground" that a tube's grid is referred to will ride right in with any signal and be amplified by the tube. So you have to keep the ground with as little noise pollution as you possibly can.

That's where #1 comes in. Wires are resistors - if you shovel current through them, the only way to do that is with a voltage across them. This is an expression of Ohm's law: V = I*R. R may be small, but if current is big, or the use of the voltage is to be amplified a lot, the resulting voltage can be truly annoying. Well, Ohm didn't say truly annoying, but you know what I mean.

We have to have some place to designate the One True Source of Zero Volts in the amp. That's usually the negative side of the first filter cap, although it may be in other places depending on special needs. So we want all those triode grid resistors to go to the filter cap ground and nowhere else. Problem solved. Well, except that the other part of the tube are busy funneling current through the plate, into the cathode, and draining that off to ... ulp! ... ground. The wire leading from the cathodes and cathode resistors carries current, and that current, times the wire resistance, is a voltage. For a single stage, it's not too bad to connect the grid wire to the cathode ground, it's a bit of positive feedback, and small at that, but where many tubes and higher currents - like the output stage, and speaker ground - get into it, you can come up with screaming oscillations because of the unintended feedback.

Same with rectifier ground noise. The rectifiers have only a tiny part of the AC half-cycle to get all the current stored into the first filter cap to hold up the supply voltage till same time, next half cycle. So there are BIG current pulses when the rectifiers turn on into the caps. Big enough to cause the wires back into the transformer at the CT to wobble up and down with it. If you've connected the CT somewhere off on the chassis and the signal ground reference to the first filter cap, and the input jacks to the chassis, the input jacks' "ground" is being jerked up and down by the rectifier pulse currents, and you hear it in the output.

The way to fix this whole mess is to force currents to flow only in specific wires, so the voltage drops in the wires are at least known. And if you have critical "reference" ground wires, make it so no currents flow in those wires, so the voltage drop in the wires is zero.

We designate the negative side of the first filter cap as the One True Ground. The CT connects here. The current pulses from rectification cross only the wire back into the transformer, so they can't yank the first filter cap's negative lead up and down. A second wire leads from the first filter cap to the output tube cathode circuits. Big currents here, not so big as the rectifiers, but they have to follow their own wire, so they can't muck up the input stages. Speaker ground? Goes right back to the first filter cap negative, not the chassis, but for different reasons.

The second, third, fourth, etc. bypass/decoupling cap negatives connect by wires to the One True Ground. The AC they "bypass" goes through those wires and not into mucking up signal references. Finally, the grid and cathode grounds for tubes - they get one or more wires back the One True Ground. The currents are small, the resistances are small, so the perturbations are small.

Ground may mean:
>> A source of zero volts as a reference
>> A shield for shunting interfering radiation or conduction away from sensitive points
>> A "sewer" that returns used electricity back to the power supply for recycling.

Mixing up the uses of these and forcing them onto a single wire can cause heart - and head-aches.

[Dave H]

Tried the capacitor/ac thing, didn't have a big enough cap, could not measure anythig. Will order a couple of 0.1 and 0.047 non polar caps, or try to pick some up on the way home from work tomorrow

The cheap meter is probably half wave rectified so it won't work with just a cap like a bridge rectifier would. Try a 0.1u cap in series with the +ve meter lead and a 1M resistor across the meter inputs on the meter side of the cap. That might work (or not) depending on the meter's circuit.

[Enzo]

Why won't a cap work in series with a half wave? Isn't that how most taps off HV bias supplies work?

[stokes]

Have to ask,Mike,how are you connecting the leads to measure the ac on the filter caps?Even the cheapest of meters should get a reading.Dont quite understand what you are saying here "(meter is reading something like 800 on a 500 scale)"