acoustic 370 humming output

[Enzo]

Maybe no help, but I wonder if reversing the primary leads makes a difference. Might change how it couples.


Hmmm, you think the EQ board is picking it up? Can you mount a steel shield between the tranny and the EQ board? Bolted to the floor in there.

[Phostenix]

I'm going to find a piece of steel to set in there to see if it makes a difference. I'll be curious to see if the other 370 I have will do the same thing.

I'll try reversing the ac leads, too.

[Enzo]

You ought to be able to do the hunk of steel thing pretty simple. Even a sheet of something that wouldn;t be permanent, but just to see if it made the difference.

I remember a SIlvertone amp of some model, and there were two 6L6s right behind the one set of channel controls. There was a stttel plate mounted between the panel and the tubes. I had removed the plate to access the panel controls, then fired up the amp for test without it. WHOA, the controls in the preamp there then picked up what the 6L6s were doing an inch away - instant feedback. Plate restored to position, stable as can be.

[Phostenix]

I did some more experimenting with this today. Checking with the scope, the hum is present on the speaker outs (and equivalent places in the power amp) but not on the input to the power amp or the line outs on the back of the amp.

Here are the things I tried that made no difference:
Swapping the PT primary wires.
Pulling the EQ board & moving it away.
Moving the bridge rectifier around.
Moving the output cap (C404) around.
Moving the wiring bundle that goes to the front panel power switch, pilot lamp, ground switch around.
Lifting the ground on the amp.

Lifting the PT out of the chassis & covering the chassis with a grounded steel plate didn't even make much of a difference.

I even turned off the lights in the room to make sure there was no noise coming from them (1 on dimmer, the other a fan remote).

Rotating the PT to 1 of 2 positions is the only thing that DOES make a difference. Neither of those 2 positions could really be permanently mounted.

The level of hum on this amp is not really noticeable unless the 200 Hz & 300 Hz sliders are all the way up and it may be that this is normal for this amp, but I still find it odd.

I guess I'll get the other 370 one up and running and see how it compares.

[Phostenix]

Here's a pic of the hum. Scope set to 20mV/Div & 5mS/Div.

[Phostenix]

With all the EQ sliders flat, the waveform drops to about 20mV p-p & is not really noticeable. If I push the 100Hz, 200Hz and 300Hz sliders all the way up, the hum goes up to 120mV p-p on the scope.

[Enzo]

Well, the position thing tells us the transformer field is coupling into the amp.

Do you have an old Weller soldering gun? The thing with the two chrome tubes sticking out the front, each with a small chuck the tip is mounted in. Those chrome tubes and the tip are actually a winding around the transformer that is inside the gun. Low voltage and high current. That tip throws off a field. If you trigger on the gun and wave it right over parts on the amp board, you MIGHT find something sensitive to magnetic fields.

[Phostenix]

I actually do have one of those around here somewhere. I'll try that.

After thinking about what I posted earlier, I realized that if the EQ sliders are affecting the hum, it HAS to be there at the power amp input. It is, it's just down in the mud at 5mV/Div on the scope. Grounding the input to the PA kills the hum (buzzkill). The bundle of wires from the preamp board to the EQ board do provide some nice antennas (they are twisted, but still).

[Phostenix]

Using the old Weller as a noise source, it seems like the coils in the preamp are the antennas. With the preamp & the EQ boards free from the chassis, moving them as far as I can within the limits of the wires & rotating them around doesn't provide much relief from the hum. At this point, I'd say it's just part of the design of this amp. I'll see how the other one is after I get it working. I doubt that I'll get to that this weekend, though. Thanks for the help!

[Phostenix]

Putting the old Weller back in the toolbox, I noticed a wooden box that I didn't remember. Turned out to be a 15 piece router bit set that I bought off ebay a few years back. I forgot all about that set & was about to go buy a bunch of router bits for some other projects. You just saved me a bunch of money tonight, Enzo

[Enzo]

Well, when you get to my experience level, you know, these extra abilities kick in, and... uh... my zen approach finds your router bits....

[Phostenix]

For the sake of experimentation, I decided to try a star ground topolgy on this amp. I pulled all the grounds & removed the jacks and the preamp board (which is grounded to the chassis when installed) from the chassis. I soldered a short piece of 14 guage solid copper wire to the lug where the power cord ground attaches to the chassis. I then soldered or jumpered all the grounds back to that wire (power cord ground, 2 filter cap grounds, power amp ground, preamp ground, input jack ground, and output jacks ground). I left the bridge rectifier's DC ground attached only to the big filter cap.

What I got was a very unhappy, oscillating amp. After experimenting with tying the various grounds to either the chassis ground or the negative terminal of the main filter cap or the power amp (where most of them were originally), this amp is quietest wired just as it was from Acoustic. Imagine that. It seems to like its "star" grounding point to be on the power amp board with 1 wire to the main filter cap.

[Enzo]

Your hum was coming from the transformer, I thought we had established. That would be 60Hz hum. Star grounding is usually a cure for ground return ripple current hum, which would be 120Hz.

If your EQ inductors were picking up the field, I am surprised a shield didn;t work.

[Enzo]

Why can't you mount the transformer on its side? Looking at your photo of the good orientation, I see mounting tabs at the bottom of the photo, facing front. A couple sturdy L-brackets mounted to the floor would provide mounting points for those.

[Phostenix]

Oriented with the mounting flange like it was in that photo, but turned 45 degrees (mounting flange angled towards the corner of the chassis) was one of the 2 quiet positions. I thought about mounting brackets in there for that, but they'd have to be pretty beefy. That PT is heavy. The other quiet position was with the mounting flange down and 1 end tilted up in the air 45 degrees. Not very mountable.

I found a U shaped piece of steel that's just about the size of the PT. It made a difference when around 1 particular side of the PT (the bell on the left in the pic above), but not a huge difference. Rotating the 45 degrees makes the most difference.