Aaaaannd it's on... (amp build)

[Surgeon]

Re: Aaaaannd it's on... (amp build)

Thanks for your interest in my problem fellas... I highly appreciate it.

Here are the schematic and layouts I'm using. Note that I've corrected a few mistakes (confirmed afterwards by the folks on the weber board).

Schematic

Layout

Have you been able to isolate the hum to any part of the amp by any chance.....pre or power.?
best

Pre-amp... removing v2 (or v3) cures it. Removing v1 has no effect.

Are the heater lines running parallel to anything that carries signal?

Not really, as you'll see on the pics below, the wires from the PT go to the first power-tube then branch out from there: 1- other powertubes, 2- terminal strip where they again branch out to the pilot light and pre-amp tubes.

What kind of hum are you getting? And under what conditions?

You may want to try some elevating heaters and trying different valves.

It's constant, only gets louder with the master and it's "tonality" is affected by the tone controls (I'm not certain but it does sound like 60 cycle hum to me) I max both pre-amp volume and master volume and it doesn't get out of control so that's why I'm ruling out oscillation. Note that it doesn't change anything to have a guitar plugged in or not.

I've tried known good preamp valves in there but it didn't change anything.

The heaters, as you'll see in the pics below are as nested in the corners of the chassis as I could... I assumed that was the ideal spot for them...

I've taken the amp out yesterday and played it in the garage. While the hum is not a product of my imagination (and none of my other amps do anything similar), it's still not that bad. It's easy to over-estimate it when the master is maxed and you're sitting 6 inches from the speaker but when you're not sitting next to it, it's not that bad...

I'd look for a dry joint.

I've looked through and through, maybe I need to reflow a few more...

pics

Overall guts:

V2 and v3:

V2 close-up:

v3 close-up:

None of the pins touch each other (verified like a zillion times...)

Thanks in advance for any help/ideas you guys may come up with...

 

[GilmourD]

Re: Aaaaannd it's on... (amp build)

Using a chopstick or something non-conducting move those yellow and green lines around. It sounds to me like one of the heater wires between the preamp tubes is inducing current in one of the wires that carry signal. Since the heaters are AC in that, there's your 60Hz hum from somewhere after V1 to V2.

 

[Surgeon]

Re: Aaaaannd it's on... (amp build)

Using a chopstick or something non-conducting move those yellow and green lines around. It sounds to me like one of the heater wires between the preamp tubes is inducing current in one of the wires that carry signal. Since the heaters are AC in that, there's your 60Hz hum from somewhere after V1 to V2.

You just described what I spent the last 2 days doing...

 

[zzmoore]

Re: Aaaaannd it's on... (amp build)

It is Real Hard to hear 60Hz vs 120Hz...even when you inject it straight into the amp.
But...if 60 cycle is what you have been spending a lot of time searching for, I would say it is time to lift the heater supply, and run the amp off of DC Heat, At least from the PI back, and see if that cleans up your noise.

Let me ask about your ground scheme.
That is a Ground Buss I assume:
1. Grounded at just one end.?
2. Separate power amp B+ ground from preamp B+ ground.
3. Try running the PI B+ with the power tubes B+ ground.
Just some suggestions.....try them if you like.
good luck

 

[zzmoore]

Re: Aaaaannd it's on... (amp build)

Depth perception is real tricky.....But it looks like, in your last pic, the jumper to connect the PI Cats...looks like THAT jumper gets Real Close, almost touching the 4-5 heater connection. Probably just looks that way in the picture.?
best

 

[Surgeon]

Re: Aaaaannd it's on... (amp build)

It is Real Hard to hear 60Hz vs 120Hz...even when you inject it straight into the amp.
But...if 60 cycle is what you have been spending a lot of time searching for, I would say it is time to lift the heater supply, and run the amp off of DC Heat, At least from the PI back, and see if that cleans up your noise.

Let me ask about your ground scheme.
That is a Ground Buss I assume:
1. Grounded at just one end.?
2. Separate power amp B+ ground from preamp B+ ground.
3. Try running the PI B+ with the power tubes B+ ground.
Just some suggestions.....try them if you like.
good luck

At this point, I'll try anything really. So first: thank you for the suggestions, they are more than welcome. It's getting frustrating to have an amp that worked from the first flick of the switch but that has a hum that I can't diagnose... plus it does sound pretty damned good already.

About the 60 cycle hum: I'm far from being a specialist so it could very well be something else...It's constant and not a high-pitch squeal... that's all I know... It is definitely audible but related to the master volume. When I try to diagnose it I max the master. It is heard almost all the time but it's a matter of context: when I'm alone in the studio it seems quite loud. When I play guitar a few feet from the speaker (and I definitely don't dime the amp) it's not a big deal, I can't actually really hear it when I don't play if I'm not at ear-shattering levels (that a LOUD amp). It's definitely something I don't have in any other amp so I want to get rid of it but it's never as annoying as it is when it's quiet, no instrument plugged in and all tone controls and master dimed.

1- Yes, the buss is only grounded at one point (close to the input jacks)... I've tried switching the grounding point to a PT bolt... not much of an improvement if there was one.

2- If I understand you correctly: yes. Meaning that the buss bar is my preamp ground and is bolted to the chassis close to the input. All that is power-amp related (as described in Weber's layout) is grounded separately to a separate star-ground at a pt-bolt. Basically a two star-ground strategy.

3- Again, if I understand this one correctly (and if you don't mind having a look at the schematic and layout I've linked to in order to follow me), you mean the ground point that goes from R46 (4.7k) right? I'll give this one a shot tomorrow night.

Thank you very much for taking the time to suggest some actions...

Depth perception is real tricky.....But it looks like, in your last pic, the jumper to connect the PI Cats...looks like THAT jumper gets Real Close, almost touching the 4-5 heater connection. Probably just looks that way in the picture.?
best

Yeah, trick of the picture... the jumper is actually raised in the center. They're not even close. Still, I already cleaned up that big solder glob... so they're quite far at the moment.

 

[Surgeon]

Re: Aaaaannd it's on... (amp build)

You may want to try some elevating heaters and trying different valves.

As I'm quite a newby with amp building and wire-routing: would you mind elaborating on this?

 

[zzmoore]

Re: Aaaaannd it's on... (amp build)

You are very good.! I am out of Ideas.

But not R46. I was thinking the Ground for C9. Sometimes I have had trouble if I mix the PI B+ Ground with the ground for the preamp B+ which is C26 on your schem.
Since that time, I have always tried to include the PI B+ with the power amp B+
Christ...never mind. I did not see the can caps. You are not going to have that option. No worries.

Believe me...no expertise involved. It has been my experience (including me) that people cannot distinguish 60 from 120. You might be able to hear it, if you were switching a tone generator, but even then, they sound pretty similar. What I was getting at, is that I can never tell.
A few weeks ago I was sure I had 60Hz noise, but when I measured it, it was 120Hz. My tech says guys get THAT guess wrong all the time.

I still think it is a 120Hz problem, but I could be wrong. If you can get DC to the heaters, that will tell you for sure. EL34 draw over 1 Amp for heat, I think.
But if you have a small DC supply (even a wall wart that is 6-7 Volts) you can hook that up at the PI heaters (V! 2 and3) and see if that gives you any 60 cycle relief.

Toe-Knee has some good points as well.
It could be a tube giving you 60Hz trouble...internal from its filament. A tube change would show...maybe you tried that already.?
Not sure what you would use for DC Elevation in that amp, but he can answer that for you.
good luck

 

[Surgeon]

Re: Aaaaannd it's on... (amp build)

You are very good.!

I appreciate the compliment but at this moment I do not feel particularily good. The rush of having the amp work at the first flick of the switch has been replaced with frustration of failure to remove the damn hum...but thanks!

Believe me...no expertise involved. It has been my experience (including me) that people cannot distinguish 60 from 120. You might be able to hear it, if you were switching a tone generator, but even then, they sound pretty similar. What I was getting at, is that I can never tell.
A few weeks ago I was sure I had 60Hz noise, but when I measured it, it was 120Hz. My tech says guys get THAT guess wrong all the time.

Then let's assume that I too have it wrong...I've listened to samples on youtube (yeah I know... but it's the best I have right now), and while they are distinguishable, I'm pretty certain that I couldn't tell one from the other without having heard the other before and after and all that...

I still think it is a 120Hz problem, but I could be wrong. If you can get DC to the heaters, that will tell you for sure. EL34 draw over 1 Amp for heat, I think.
But if you have a small DC supply (even a wall wart that is 6-7 Volts) you can hook that up at the PI heaters (V! 2 and3) and see if that gives you any 60 cycle relief.

So a simple 9v dc supply (for pedals for example) would do the trick? Just hook it to the filament heaters and listen? Please do correct me if I misunderstand what you're suggesting...

Toe-Knee has some good points as well.
It could be a tube giving you 60Hz trouble...internal from its filament. A tube change would show...maybe you tried that already.?
Not sure what you would use for DC Elevation in that amp, but he can answer that for you.
good luck

Yeah, I tried changing the power tubes (removed the 4, replaced with 2 new ones but didn't change a thing). As I said, removing v2 and v3 did remove most of the hum so I replaced each one in turn with a good 12ax7 to no avail. Maybe I should replace all of the pre tubes and check, I must admit I haven't tried this.

Thanks again!

 

[VanDerchuck]

Re: Aaaaannd it's on... (amp build)

I used Pano Tuner, an iPhone tuning ap, to distinguish hums before. It works really well.

 

[GoldenVulture]

Re: Aaaaannd it's on... (amp build)

Originally Posted by GoldenVulture I'd look for a dry joint.
I've looked through and through, maybe I need to reflow a few more...

I'd do that to joins like the one to the upper right of the socket bolt on the left, and maybe the one to the right of it.

The Amp is looking pretty good BTW.

 

[Surgeon]

Re: Aaaaannd it's on... (amp build)

I'd do that to joins like the one to the upper right of the socket bolt on the left, and maybe the one to the right of it.

The Amp is looking pretty good BTW.

Thank you, I did reflow all of them and remove the extra solder globs that looked so aweful. But that didn't cure anything... thanks again for the pointers!

 

[crusty philtrum]

Re: Aaaaannd it's on... (amp build)

If it was me, I'd lift the centre-tap of the heater winding and reference the two outer ends to ground via a pair of 100 Ohm resistors rather than via the centre tap (insulate the c/t wire and secure it out of harm's way).

V2b is a cathode follower and has 170 VDC on it's cathode, greatly exceeding the 100 Volt heater to cathode rating for the 12AX7. Some quick research online illustrates that many peole have found certain braands and models of valves (tubes) do not fare well in that position.

Following on from the first paragraph, I'd also look at referencing the heaters to a voltage higher than ground. But obviously the voltage figure would need to work for both the cathode follower stage as well as the other stages with their much lower cathode voltages. Off the top of my head, I'd guess 70-80 Volts might be the approximate area to begin with.

However I'd check for DC leakage across the interstage coupling caps first.

 

[zzmoore]

Re: Aaaaannd it's on... (amp build)

I appreciate the compliment but at this moment I do not feel particularily good. The rush of having the amp work at the first flick of the switch has been replaced with frustration of failure to remove the damn hum...but thanks!




Then let's assume that I too have it wrong...I've listened to samples on youtube (yeah I know... but it's the best I have right now), and while they are distinguishable, I'm pretty certain that I couldn't tell one from the other without having heard the other before and after and all that...



So a simple 9v dc supply (for pedals for example) would do the trick? Just hook it to the filament heaters and listen? Please do correct me if I misunderstand what you're suggesting...



Yeah, I tried changing the power tubes (removed the 4, replaced with 2 new ones but didn't change a thing). As I said, removing v2 and v3 did remove most of the hum so I replaced each one in turn with a good 12ax7 to no avail. Maybe I should replace all of the pre tubes and check, I must admit I haven't tried this.

Thanks again!

I am not sure I would put 9 Volts on the heaters.

Did not look at the layout...does the PT have a center tap for the heater winding.?
You could try lifting that, and making a virtual center tap. It could be anywhere...but most people use 2x 50-100 Ohm resistors from the pilot light supply (assuming that is 6 volts). The value is not really important. Getting 2 that are close in value is more important. Just a 100 Ohm resistor from each end of the pilot light to ground.
Seems like if that were the problem, you would have unbalanced 60Hz hum everywhere (not just V2 V3), but it is worth a try.
I am just curious, the way things are right now...what numbers do you get with your meter on VAC, one probe on ground, and the other probe to Pin 4/5 or Pin 9 of V2 and V3.....do you get close to 3.5VAC, or do you get something else.?

good luck

 

[Surgeon]

Re: Aaaaannd it's on... (amp build)

If it was me, I'd lift the centre-tap of the heater winding and reference the two outer ends to ground via a pair of 100 Ohm resistors rather than via the centre tap (insulate the c/t wire and secure it out of harm's way).

V2b is a cathode follower and has 170 VDC on it's cathode, greatly exceeding the 100 Volt heater to cathode rating for the 12AX7. Some quick research online illustrates that many peole have found certain braands and models of valves (tubes) do not fare well in that position.

Following on from the first paragraph, I'd also look at referencing the heaters to a voltage higher than ground. But obviously the voltage figure would need to work for both the cathode follower stage as well as the other stages with their much lower cathode voltages. Off the top of my head, I'd guess 70-80 Volts might be the approximate area to begin with.

However I'd check for DC leakage across the interstage coupling caps first.

I am not sure I would put 9 Volts on the heaters.

Did not look at the layout...does the PT have a center tap for the heater winding.?
You could try lifting that, and making a virtual center tap. It could be anywhere...but most people use 2x 50-100 Ohm resistors from the pilot light supply (assuming that is 6 volts). The value is not really important. Getting 2 that are close in value is more important. Just a 100 Ohm resistor from each end of the pilot light to ground.
Seems like if that were the problem, you would have unbalanced 60Hz hum everywhere (not just V2 V3), but it is worth a try.
I am just curious, the way things are right now...what numbers do you get with your meter on VAC, one probe on ground, and the other probe to Pin 4/5 or Pin 9 of V2 and V3.....do you get close to 3.5VAC, or do you get something else.?

good luck

Thank you both, I have some thinking to do as some of what you suggested is a bit over what I can do/comprehend right now.

 

[zzmoore]

Re: Aaaaannd it's on... (amp build)

Oh...I see crusty has addressed elevator heaters also...something to consider.
His idea of a coupling cap leaking is a smart possibility also.
good luck
I meant to say...if you have 9 VDC, you can use some diodes (3-5) in series.
That will get you down to somewhere near 6.5 VDC. Or you can use a dropping resistor in series also.
Just something to your supply closer to 6.5-7 VDC.
Hope some of that other stuff works and you do not have to go that route.
best

 

[Surgeon]

Re: Aaaaannd it's on... (amp build)

I used Pano Tuner, an iPhone tuning ap, to distinguish hums before. It works really well.

So, I did this with the tuner on my ipod touch and it turns out it's 120hz, which makes my heaters not the culprit... most likely a bad filter cap or something else???

 

[crusty philtrum]

Re: Aaaaannd it's on... (amp build)

Measure for DC Volts leaking through C17, C18, C1, C2. Maybe C16. They will all have high DC voltages on one side, and should have no DC voltage on the other side. Many brand new coupling caps can have enough leakage to cause problems.

 

[Surgeon]

Re: Aaaaannd it's on... (amp build)

Measure for DC Volts leaking through C17, C18, C1, C2. Maybe C16. They will all have high DC voltages on one side, and should have no DC voltage on the other side. Many brand new coupling caps can have enough leakage to cause problems.

Alright then, thank you again. Your cap numbering: based on the schematic I provided or some other? Just so it's easier for me to locate...

BTW: you called it almost from the start... you rock! But instead of saluting I thank you.

 

[VanDerchuck]

Re: Aaaaannd it's on... (amp build)

More than likely it's from the schem. But it's a good chance to take voltages at all the points as well. And it's good that it isn't the heaters cause now you can cross it off.