Power section hum?

[dehughes]

In my two channel AC30-inspired amp I find that the 12AX7 channel has a more pronounced hum present when turned up than the EF86 channel. The hum is similar to what you'd hear if you brought your pickups a bit too close to the transformers, but it is a pretty muffled, warm "hummmmmm", not the typical buzzy/hissy preamp noise. Definitely sounds like 60hz hum, and it doesn't change in volume or meter when I poke around the amp. The one thing that DOES change it is when I turn the master volume up on the 12AX7 side (pre-PI master....), as when it is 3/4 or all the way up you'll start to hear the "hummmmmmmm". When you turn the master down past 3/4 of the way the hum is barely audible, and with the master down low you'll hear almost none of it. Turning up the preamp gain on this channel doesn't bring in any hum or hiss, so that's good, but I'd like to figure out where this is coming from. The hum is not noticeable on the EF86 side unless you turn that up ALL THE WAY, and even then it is so very slight...this channel is just volume/rotary tone switch, straight into the PI.

Odd note: I notice that the power transformer (Mercury Magnetics Woden clone) itself makes a noise very similar to the hummm I'm hearing on the 12AX7 channel, even with the amp on "standby". If you lean over and listen to the PT you can hear it kind of "hummmmmm" all by itself, and placing a finger on it or on the chassis will let you feel a very slight vibration. Now, I've heard PTs that hum before, but I don't know that I've ever hear that hum get introduced into the amp.

So I removed all the 12AX7s in the preamp (all the tubes on the 12AX7 side) and the hum was still present. I also (don't know if this was a good idea...) took a wire and touched the PT ground point and then went around and touched all the other ground points in the amp (preamp and PI buss bars, filter cap grounds, cathode resistor and cap, etc...) to see if something noticeable happened, but nothing quieted down. Then I took a piece of buss wire and touched the back of the Master, Cut and Bass pots (the ones closest to the power section), and doing this caused a scratchy noise to be heard when contact was made. Interesting.

Then, I noticed that the Master Volume makes a couple of slight "pops" when swept through the range. These pops occur roughly at the points where the hum becomes audible, at about halfway and 3/4 of the way open (up). Strange. Don't know how to interpret that.

Plus, I noticed that the PT hum is pretty loud even unamplified...so much so that it causes the metal tranny cover to resonate somewhat with the vibrations of the PT just sitting there. You can feel the vibration if you touch the Master, Cut, or Bass pot...it is that noticeable. The hum that the PT makes sure sounds identical to the hum that I'm hearing coming through the speakers...makes me wonder...

Finally, I noticed that the hum becomes less noticeable when the amp is switched to "low power" (inserts a 121 ohm resistor between the recto and the 1st filter cap...see pics) and/or when the filtering switch is set to "high filtering" (switches in another 16uf cap for a total of 32 as the first cap stage off the recto). So, does this seem to point to the recto/filter caps/power tranny as being the cause of this, or should I still be looking at ground loops elsewhere?

Here are a few pictures of the chassis....the pots closest to the PT are the master, cut, and bass knob, looking left to right...

http://www.18watt.com/files/power-section.jpg

http://www.18watt.com/files/preamp-section.jpg

Please let me know if something pops into your minds regarding what this could be. I'm going to go through the troubleshooting suggestions here http://www.geofex.com/ampdbug/hum.htm and see if something works out, but for now I'm stumped... The hum isn't THAT bad, but it definitely sounds like it shouldn't be there. I've played many, many amps, and I know what a quiet one sounds like (or doesn't sound like).

Thanks!

[dehughes]

OKAY, new development: I've found that if I turn the EQ controls completely DOWN (OFF), the amp is DEAD QUIET even when the volumes are up full. INTERESTING. So the noise is then coming from somewhere in the eq network, right? Or is it just seeping into the EQ network as opposed to being generated in/by it? This seems to get me thinking that I need to look in the preamp/eq wiring, maybe from the pots to the components...

Whaddya think?

Thanks!

[davoux]

EQ stops Hum

I'm pretty much a newby so what I add may be worthless. But, on many amps if you turn all the EQs off, you have effectively turned the amp off. I don't know about your system, but the reason the hum disappears is that you have basically turned all amplification off. If that is true, then it does not point you to the EQ circuits.

Could you have a bad ground connection where the vibration from the trans vibrates the bad connection at the same frequency? You said you built this unit. Could it be that you need to change the orientation of the trans slightly or put magnetic shielding between it and other components. The other thing might be to move your star grounding slightly.

Sorry I can't help more. Happy solutions John

[dehughes]

I'm pretty much a newby so what I add may be worthless. But, on many amps if you turn all the EQs off, you have effectively turned the amp off. I don't know about your system, but the reason the hum disappears is that you have basically turned all amplification off. If that is true, then it does not point you to the EQ circuits.

Could you have a bad ground connection where the vibration from the trans vibrates the bad connection at the same frequency? You said you built this unit. Could it be that you need to change the orientation of the trans slightly or put magnetic shielding between it and other components. The other thing might be to move your star grounding slightly.

Sorry I can't help more. Happy solutions John

I didn't use star grounding....the pics will give you some idea of how I went about it. I used the chassis for a ground in most places, save for a buss bar for the preamp section. I copied the grounding scheme and layout from a Top Hat amp that was the quietest thing I've ever (not) heard, so I hoped that would transfer over to my amp. I guess not. Oh well.....live and learn.

So, the hum and noise is coming either from or before the tone stack. That narrows things down a bit. I've had several people say to look at the grounds, so....any specific suggestions as to where, exactly? Should I lift all the preamp grounds and just have them run over to the main (power tranny) ground?

[Enzo]

Isolate the problem. if you kill all the EQ, it means your hum is entering the system at or BEFORE that point. Same with any control. if the control affects the sound, then the source of the sound is right there or earlier in the amp. If I pull a tube and the sound goes, then the sound was coming from that tube or earlier. ANd so on. That tells you the hum is coming through the signal path.

Transformers hum often. They are layers of flat metal plates with an alternating magnetic field running through them. They are not perfect, so some of them will be able to flex a tiny bit. This makes them hum. The current is AC at 60Hz, so the humming will also be at 60Hz. ANy other frequency would be weird.

Electrical hum will also be 60Hz since it is the same 60Hz AC powering things. Power supply ripple will be 120Hz, since the full wave rectification results in twice as many bumps on the waveform. If your bias is half wave, then any ripple there will still be 60Hz. SO pretty much any and all humming coming from an amp will be 60Hz or one octave higher at 120Hz.

If there is hum in the preamp, have you tried other tubes? A spiral heater might hum less than a conventional one in a 12AX7.

RUn a ground wire across the rear covers of the controls. In a high gain circuit, something like the volume control does not need an ungrounded piece of metal an eighth of an inch away from the signal path acting like a hum antenna. Many controls are metal so the mounting bushing grounds the case cover, but not all. That is shielding.

Are the preamp tubes shielded? Are they near the PT?

[dehughes]

Isolate the problem. if you kill all the EQ, it means your hum is entering the system at or BEFORE that point. Same with any control. if the control affects the sound, then the source of the sound is right there or earlier in the amp. If I pull a tube and the sound goes, then the sound was coming from that tube or earlier. ANd so on. That tells you the hum is coming through the signal path.

Transformers hum often. They are layers of flat metal plates with an alternating magnetic field running through them. They are not perfect, so some of them will be able to flex a tiny bit. This makes them hum. The current is AC at 60Hz, so the humming will also be at 60Hz. ANy other frequency would be weird.

Electrical hum will also be 60Hz since it is the same 60Hz AC powering things. Power supply ripple will be 120Hz, since the full wave rectification results in twice as many bumps on the waveform. If your bias is half wave, then any ripple there will still be 60Hz. SO pretty much any and all humming coming from an amp will be 60Hz or one octave higher at 120Hz.

If there is hum in the preamp, have you tried other tubes? A spiral heater might hum less than a conventional one in a 12AX7.

RUn a ground wire across the rear covers of the controls. In a high gain circuit, something like the volume control does not need an ungrounded piece of metal an eighth of an inch away from the signal path acting like a hum antenna. Many controls are metal so the mounting bushing grounds the case cover, but not all. That is shielding.

Are the preamp tubes shielded? Are they near the PT?

Cool. I'll look into this all. I've not tried the tubes with their shields on yet, so I'll have to do that...same with sprial filiment preamp tubes. The preamp tubes are on the opposite end of the chassis from the PT (see the pics in the first post...). Good info on the PT....makes sense. I'm thinking that this is a layout/grounding issue so I'll get to messing with that...

THANKS MAN!

[dehughes]

Well, I tried isolating the OT jacks from the chassis and moving the ground around to see if I could get things quieter (just in case this was the trouble...), and wasn't able to find a better ground point that was quieter. So, I assume the OT ground isn't the trouble.

HOWEVER, I noticed that when I stretch out my fingers on my right hand and touch both the OT casing and the tube socket shield for V1, there is a POP and then a really loud hummmm/buzzzzz until I remove my hand from either the tube shield or the OT casing, or just touch another finger to the chassis. What's that about? Touching the OT casing and then the shield for V1 shouldn't introduce noise/hum/popping into the circuit, should it? I have the tube sockets isolated from the chassis with rubber washers....and none of the tube socket pins are touching either the chassis or anything connected to the chassis.

This gets me thinking that maybe all this noise has something to do with the preamp ground and/or V1's interaction with the OT...

Am I correct in this, or way off?

THANKS GUYS!

[dehughes]

I've done some tinkering and moving of grounds, and that doesn't seem to help anything. I do know that the problem is in the preamp somewhere as when I pull V1 and V2 the noise disappears, just like when I turn all the tone knobs down to zero. So, I guess I'll just take another look at the preamp wiring and see if something there jumps out at me...

[dehughes]

So, I've tried connecting the ground buss for the preamp to the ground for the filter caps on that stage (star-ish ground, a la Aiken, etc...) and this didn't help anything. I've also tried isolating the input jacks from ground and this didn't help anything either. I've tried poking and prodding various wires on the sockets and to the pots and haven't discovered anything either. One oddity, though:

I've found that this hum/hiss is present on also when the EF86 channel is turned up. This channel connects to the opposite side of the PI as the 12AX7 channel, and only has a volume and a 6-way rotary tone switch. What's more, when I removed the EF86 tube and turned up the volume pot for that channel, the low-level hum and a measure of hiss was produced! Weird? With the tube gone that channel should be dead, right? I found that removing V1 and V2 (leaving only the PI) on the 12AX7 side killed the tone pots and volume controls and the noise, so why wouldn't the EF86 side to the same when that tube was removed? Pulling out the PI tube killed all preamp noise for both channels, so it is definitely getting in before or at the PI.

Also, before I pulled the PI, but with V1 and V2 pulled, the 12AX7 side exhibited some odd behavior. With all the tone controls off, and the master and preamp gain at noon, bumping up the mid control produced some pops...odd. What's more, with the PI still in there and the tone controls up at about noon, I noticed that the master volume when turned all the way off or all the way on was very quiet, EXCEPT that the sweep in between off and on produced a fairly loud hum!?! Weird. Man, this thing is getting more funky every time I look into stuff....and I haven't really changed anything yet!

Finally, I noticed that when I reduced the voltage to the amp as a whole (switches in a 121ohm resistor before the 1st filter cap...), things are much quieter, but with the resistor out of the circuit (standard AC30 voltages...) the amp produces more low-level hum. This makes me think that something from the power supply is getting into the preamp, and when the power supply is running hotter then there is "more" seeping into the preamp, causing hum and hiss....

So, that's that. I'm done tinkering at this point as I'm totally clueless as to what's up. Maybe I'll get back to it later in the week...eesh.

Thanks for all your help so far, though.