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...

[img2136]http://www.18watt.com/files/power-section.jpg[/img]

[img2136]http://www.18watt.com/files/preamp-section.jpg[/img]

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!

[drz400]

If the power transformer hums and you are USA I would return the transformer and ask for a replacement. Europe does have some issues with 50cycles but USA should be pretty good.

IF you are sure it isnt your grounding scheme then I would look at the Power tranny. But personally I'm a huge believer in not using the chassis as a ground path. Any weirdness in volume pots steers to bad ground scheme

FYI I know a guy who had a new Vox and his power tranny hummed, he sent it back for a replacement and his replacement also hums. Usually this is went they skimp on the turns ratio, or bad/loose laminations, not the correct steel, also a Hum band helps a lot

[dehughes]

If the power transformer hums and you are USA I would return the transformer and ask for a replacement. Europe does have some issues with 50cycles but USA should be pretty good.

IF you are sure it isnt your grounding scheme then I would look at the Power tranny. But personally I'm a huge believer in not using the chassis as a ground path. Any weirdness in volume pots steers to bad ground scheme

FYI I know a guy who had a new Vox and his power tranny hummed, he sent it back for a replacement and his replacement also hums. Usually this is went they skimp on the turns ratio, or bad/loose laminations, not the correct steel, also a Hum band helps a lot

Thanks for the reply! This is a brand new Mercury Magnetics MMP-V30W Woden clone tranny, so while what you're saying makes sense I find it doubtful that the PT is bad. Being as this is a clone, maybe the original Woden trannies hummed like this? I mean, those guys a Mercury Magnetics are pretty neurotic in regards to making their clone trannies as exact as possible. Is it possible that's just the way the tranny is?

Also, being that when I increase the filtering in the power supply the hum lessens, could this just be a power supply "oddity" that is natural? Again, the hum isn't THAT bad, but I've played many a nice amp that doesn't have this hum so I'm inclined to think it is not necessary....

Whaddya think?

Thanks!

[jimipage]

dehughes, is your chassis aluminum or steel?

a thought occured to me when you mentioned the difference when switching down to lower power: in Dan Torres' book he recommends adding an extra filter cap before the rest of the PS caps to help quiet the hum in single-ended amps. I'm not sure if that would help in a p/p situation, and that might be a big pain to install in your already-immaculate layout, but that could be the answer.

just trying to help. take care

[dehughes]

dehughes, is your chassis aluminum or steel?

a thought occured to me when you mentioned the difference when switching down to lower power: in Dan Torres' book he recommends adding an extra filter cap before the rest of the PS caps to help quiet the hum in single-ended amps. I'm not sure if that would help in a p/p situation, and that might be a big pain to install in your already-immaculate layout, but that could be the answer.

just trying to help. take care

Jimipage,

The chassis is aluminum...which I've heard changes things if you're copying a layout (i.e., this layout is copied from a King Royale, so being as the King Royale uses a steel chassis, this aluminum chassis may well change the interaction of the layout). So, I'm kinda stuck figuring things out from scratch, in a sense....which is good as I'll learn a lot, but bad in that I have a lot of work to do.

Regarding the power supply caps....when I switch an extra 16uf cap on top of the 16uf cap that is already the 1st cap off the recto, the hum does drop down somewhat. However, this also changes the feel/tone of the power section, which is good in some ways... I'd rather deal with hum (if necessary) and have the amp feeling like I want it to, instead of having a super tight, quiet power section that is too stiff.

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!

[Bob-I]

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!

I'm not sure exactly what type of preamp you have, but if it's close to a Fender/Marshall/Vox tone stack, then turning the tones to 0 loads down the ckt enough that no sound can get through the tone stack.

What this means is that your hum is either IN or BEFORE the tone stack, but not after.

[drz400]

[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!

It is your grounding scheme
Follow the guidlines from Kevin OConnor in his book or from Randall Aiken
Use a main ground buss with individual stars going to the buss, only ground the chassis to the circuit at one place. Decoupling PS caps should ground to their stage, Aiken has some good tech tips on his site all about proper grounds. Oconnor spells it out a little better, Remember... ground isnt ground, it carries signal

[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!

I'm not sure exactly what type of preamp you have, but if it's close to a Fender/Marshall/Vox tone stack, then turning the tones to 0 loads down the ckt enough that no sound can get through the tone stack.

What this means is that your hum is either IN or BEFORE the tone stack, but not after.

It is a Vox-ish tone stack. Vox with a midrange pot instead of the fixed resistor...

Exactly what I had hoped. COOL! I'm narrowing it down......

THANKS!

[dehughes]

[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!

It is your grounding scheme
Follow the guidlines from Kevin OConnor in his book or from Randall Aiken
Use a main ground buss with individual stars going to the buss, only ground the chassis to the circuit at one place. Decoupling PS caps should ground to their stage, Aiken has some good tech tips on his site all about proper grounds. Oconnor spells it out a little better, Remember... ground isnt ground, it carries signal

Okay. I've heard some people swear by star grounding, and other say not to use it. Cogent arguements for both....

I have one Kevin O'Connor book...I'll have to see which one it is and what he says.... Probably star grounding, right?

So, does this mean that I'll need to insulate the jacks in some way? I've always been unclear about what to do with pots and jacks, as to whether or not they can touch (ground to) the chassis...

[drz400]

[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!

It is your grounding scheme
Follow the guidlines from Kevin OConnor in his book or from Randall Aiken
Use a main ground buss with individual stars going to the buss, only ground the chassis to the circuit at one place. Decoupling PS caps should ground to their stage, Aiken has some good tech tips on his site all about proper grounds. Oconnor spells it out a little better, Remember... ground isnt ground, it carries signal

Okay. I've heard some people swear by star grounding, and other say not to use it. Cogent arguements for both....

I have one Kevin O'Connor book...I'll have to see which one it is and what he says.... Probably star grounding, right?

So, does this mean that I'll need to insulate the jacks in some way? I've always been unclear about what to do with pots and jacks, as to whether or not they can touch (ground to) the chassis...

IMO star grounding as a complete scheme star only works if you ground to 3 points of the chassis. If you take EVERYTHING to one point you will most likely have plenty of buzz, hum. Never use 1 star alone in my experience, you can do it in low gain amps but not high gain. Also I will never use the chassis as a ground, you just dont have control.

Get yourself TUT Volume 3, it is spelled out clearly and works 100% of the time for a dead quiet amp. I rewire Marshalls all the time this way and it makes them silent. The problem with ground currents is that you will never isolate them, you just think you do. You track it down to one stage but you will never get rid of it till you correct all the grounds.

Do a buss bar, support it with standoffs, keep each stage decoupling cap and related grounds together then run them to the buss. So Volume ground goes to the decoupling cap ground along with the stage grounds it is feeding, these go to the buss as one wire from the cap, return the OT ground to the speaker jack and from there to the phase inverter ground node. (or you can also use the main filter cap grounds if there is no feedback) keep all jacks isolated from the chassis with isolation washers or use the Cliff jacks, run your chassis ground from the input jack soldered or bolted to chassis. Dont use the back of the pot as a ground, the pot case gets grounded to chassis not circuit Aiken also covers this info on his website. It works, really works! and if you float the CT of your heater it will be a silent amp when it comes to any buzz or hum, there will be no weird interaction in the volumes either like more quiet at one spot.

IF you do it any other way you could get mixed results depending on the chassis size and material as well as your circuit. Take control of the grounds!

[dehughes]

It is your grounding scheme
Follow the guidlines from Kevin OConnor in his book or from Randall Aiken
Use a main ground buss with individual stars going to the buss, only ground the chassis to the circuit at one place. Decoupling PS caps should ground to their stage, Aiken has some good tech tips on his site all about proper grounds. Oconnor spells it out a little better, Remember... ground isnt ground, it carries signal

Okay. I've heard some people swear by star grounding, and other say not to use it. Cogent arguements for both....

I have one Kevin O'Connor book...I'll have to see which one it is and what he says.... Probably star grounding, right?

So, does this mean that I'll need to insulate the jacks in some way? I've always been unclear about what to do with pots and jacks, as to whether or not they can touch (ground to) the chassis...

IMO star grounding as a complete scheme star only works if you ground to 3 points of the chassis. If you take EVERYTHING to one point you will most likely have plenty of buzz, hum. Never use 1 star alone in my experience, you can do it in low gain amps but not high gain. Also I will never use the chassis as a ground, you just dont have control.

Get yourself TUT Volume 3, it is spelled out clearly and works 100% of the time for a dead quiet amp. I rewire Marshalls all the time this way and it makes them silent. The problem with ground currents is that you will never isolate them, you just think you do. You track it down to one stage but you will never get rid of it till you correct all the grounds.

Do a buss bar, support it with standoffs, keep each stage decoupling cap and related grounds together then run them to the buss. So Volume ground goes to the decoupling cap ground along with the stage grounds it is feeding, these go to the buss as one wire from the cap, return the OT ground to the speaker jack and from there to the phase inverter ground node. (or you can also use the main filter cap grounds if there is no feedback) keep all jacks isolated from the chassis with isolation washers or use the Cliff jacks, run your chassis ground from the input jack soldered or bolted to chassis. Dont use the back of the pot as a ground, the pot case gets grounded to chassis not circuit Aiken also covers this info on his website. It works, really works! and if you float the CT of your heater it will be a silent amp when it comes to any buzz or hum, there will be no weird interaction in the volumes either like more quiet at one spot.

IF you do it any other way you could get mixed results depending on the chassis size and material as well as your circuit. Take control of the grounds!

Alrighty...I'll archive this post of yours and get to tinkering... A couple of questions:

1) What value of resistor (watts and ohms) should I use to raise the heater CT from ground?

2) I have a thicker, plexi-type faceplate so I'm not sure that the jacks can be isolated from the chassis. I'll have to check...

3) Well, I think that's all...I'll have to go get that book as I'm not sure I'm visualizing things correctly (that's what I get for staring at amp chassis pics instead of reading...).

THANKS MAN! You're a super help. Please post any other stray ideas if you get them, and I'll post back when I make some great discovery/breakthrough.

[drz400]

Start with a 330K or 470K 1W to the DC (on top), 100K/1W and a 10uf/100V Electrolytic from the middle Junction to ground. Shoot for around 75VDC no higher than 80.

It is worth a try to just try that but I have found that in many cases if the whole grounding path isnt really correct then it can make thing worse.

The Cliff jacks work fine with Plexi, you might have to get get long shaft Switchcraft jacks if trying to use them.

I'll see if I can hook up my scanner

[dehughes]

Start with a 330K or 470K 1W to the DC (on top), 100K/1W and a 10uf/100V Electrolytic from the middle Junction to ground. Shoot for around 75VDC no higher than 80.

It is worth a try to just try that but I have found that in many cases if the whole grounding path isnt really correct then it can make thing worse.

The Cliff jacks work fine with Plexi, you might have to get get long shaft Switchcraft jacks if trying to use them.

I'll see if I can hook up my scanner

Sweet. Thanks. I'll need clarification on the heater resistor/cap wiring. Never done that before and still don't understand it. I'm kinda dense with new things...

I'll look into those value resistors though, and possibly some long shaft Switchcraft jacks, though I think there might not be enough room for those in my build (see the pics for examples of my lack of forethought...).

Thanks!

[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!

[drz400]

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!

A) You should solder a piece of desolder braid from the tube socket to chassis, otherwise the socket shield can be an antena
B) The only way to figure out the ground path is to completely rebuild it correctly, the ground path needs as much thought as the whole circuit layout