Alembic F2B clone hum & noise

[EfarmosmenosStokos]

Hello everyone,

I am a newbie around here and I have some questions for you regarding the Alembic F2B.
I recently built a clone following the Moosapotamus schematic. There are attached pictures below.

I managed to make it work quite well, ~280VDC B+, it looks just as I wanted, and it has a heavy duty switch just for the CLUNK sound when it is turned on.

The problem I have is noise and hum. It is quieter than expected and playable, but I believe I must have made a couple of mistakes with the cable topology.
First off, when I turn the Master volume down, the output goes completely silent as it should, but when the pot gets to the last degree of rotation,  suddenly get very loud hum through the output. I opened the pot up and cleaned it properly with spray, but the problem persists, so I guess it has to do with grounding? A ground loop perhaps?
Secondly, while fiddling with the cables, I found that moving the shielded input and output cables around lessened the hum, so I would like your comments on what I could have done better.

I used shielded cable grounded on one side for input resistor to tube, tube to master pot, and output resistor to output. I ran the twisted filament wire away from the audio section, but it does come close to the tube and the output jack. The power circuit board is next to the transformer and the audio circuit board is between the tube and the pots.

Please, have a look at the pictures and tell me anything you think I have done wrong.

Thank you all

[antonis]

First - and most important - of all...

You MUST explain to the forum members what is the exact meaning of your nickname in Greek...

[JFace]

Sometimes grounding can be a bit finnicky. Nice and clean layout, good job.

A few things to try:

• Try a different volume pot, yours could be faulty
• When trying a different pot, keep it away from the AC mains switch and see if that makes a difference
• Disconnect your star ground from chassis ground (leave the AC safety ground connected!). Try connecting the chassis ground to different locations of the audio ground to find the least amount of hum. Start with the input jack ground. I made a custom tube DI and had to do exactly this to find the ideal audio ground location to connect to chassis.
• If you have enough VA on the filament side, you could try rectifying the AC and get roughly 6VDC. If it's well filtered, it may help reduce noise
• If you have a spare 12AX7 on hand (a good idea), try swapping it out to see if the one installed is noisy

[antonis]

Propably the twisted pairs of cables are for AC current, so we better take them out of account..

[JFace]

Is that an XLR output? If so, I don't think the stock circuit allows for a balanced output. What are you connecting this to when you test?

You will probably get more feedback from diyaudio.com. Check out the instruments and amps forum.

[PRR]

> anything you think I have done wrong.

_I_ would not put the power transformer and wiring in the same small box as the sensitive guitar amplifier circuits.

I think the originals were in a much wider box and with the transformer outside the box.

[duck_arse]

a cloth covered box! stitched edges? excellent, I'm not the only one, then.

I have nothing useful, carry on.

[EfarmosmenosStokos]

You MUST explain to the forum members what is the exact meaning of your nickname in Greek

Applied Stucco 

Sometimes grounding can be a bit finnicky. Nice and clean layout, good job.

A few things to try:

• Try a different volume pot, yours could be faulty
• When trying a different pot, keep it away from the AC mains switch and see if that makes a difference
• Disconnect your star ground from chassis ground (leave the AC safety ground connected!). Try connecting the chassis ground to different locations of the audio ground to find the least amount of hum. Start with the input jack ground. I made a custom tube DI and had to do exactly this to find the ideal audio ground location to connect to chassis.
• If you have enough VA on the filament side, you could try rectifying the AC and get roughly 6VDC. If it's well filtered, it may help reduce noise
• If you have a spare 12AX7 on hand (a good idea), try swapping it out to see if the one installed is noisy

I tested the volume pot, I cleaned it and tested again. It was working perfectly. I also shorted the two lugs and got the same thing.
The problem with the pot noise is that I only get it at the very last degree of rotation. There I get ~3Ohms. A degree or two to the right (more than 4ohms) and the noise is gone. As you continue turning, you get more noise again and the level raises as expected.

I will try changing the star ground location for less hum.

I have tried switching tubes, nothing of importance to note.

Propably the twisted pairs of cables are for AC current, so we better take them out of account..

Would you care to elaborate? I am indeed afraid the filament may be too close to the tube or the output jack. Any opinions on that? When connecting cables to a tube, do the grid and filament cables have to be as far away from each other?

Is that an XLR output? If so, I don't think the stock circuit allows for a balanced output. What are you connecting this to when you test?

You will probably get more feedback from diyaudio.com. Check out the instruments and amps forum.

No XLR, straight to jack out. When connected to an amp, the noise is tolerable. When playing through a soundcard and headphones it can get tiring.
I might have to visit diyaudio at some point...

_I_ would not put the power transformer and wiring in the same small box as the sensitive guitar amplifier circuits.

Unfortunately, that cannot be changed at this point. I would, however, like to hear if you have any suggestions for improvement, or any technical advice on the matter.
I plan on buying a book on tube amps at some point.

Thanks for all the replies!

[PRR]

> when the pot gets to the last degree of rotation,  suddenly get very loud hum through the output.

Can't say without a schematic. (I don't recall an MV on an Alembic? Maybe I'm just old.) But, I would take the ground off the bottom of the MV pot and connect it anywhere else. Probably to the ground of whatever comes *after* the MV pot (a good general rule).

I also noticed this:



The HV rectifier is right next to the tonestack caps. One has HUGE AC spikes, the other is very sensitive.

While I hate to disrupt your *beautiful* wiring, I think this is A Problem. I would at least unscrew the rectifier-cap board, lift it up away from the tone-caps as far as the wires will reach, with a block of wood or old paperback book to support it without short or shock, and see if the hum/buzz changes. If worth the re-work, that board could be turned-around so the dirty (AC) end is to the back panel, the clean (filtered DC) end is nearer the tonestack.

Building and de-noising a guitar amp is a little like laying out a home in the woods. Here is the drinking well. Here is the garden. Here is the septic pit, the manure pile, here's where we drain dirty oil from the tractor. Some things must be kept *apart*.

[EfarmosmenosStokos]

Can't say without a schematic. (I don't recall an MV on an Alembic? Maybe I'm just old.) But, I would take the ground off the bottom of the MV pot and connect it anywhere else. Probably to the ground of whatever comes *after* the MV pot (a good general rule).

Here is the schematic, I used different power supply:
http://www.geofex.com/FX_images/alembpre.gif
Also some more pics someone had requested:
http://imgur.com/a/5pVKM#2

Thank you for the clear answers.  On DIYaudio someone gave me the same advice on the grounding issue. What is the reason behind that?

About the hum and moving things around, that would be quite difficult at this stage, I'm afraid.
Would it help if a placed a grounded metal sheet of some sort between the power supply and the audio circuit?

[EfarmosmenosStokos]

I attached the ground lug of the volume pot to the star ground on the chassis (closest ground), instead of the ground lug of the Mid pot where it used to be.
The noise I used to get has stopped completely.

However, I still have hum. It's little enough for live playing, but too much for recording. I'll have to find some way to make it quieter. At some point in the future

Thanks for the help!

[R.G.]

Hum creeps in by many paths. Fixing one uncovers the next layer.

Here is a subtle one to check.  The (-) side of the power supply rectifiers must go ONLY to the (-) terminal of the first filter cap, and no where else. Not the case, not signal ground, not any part of other grounds.

The reason is that big currents flow in these wires, and the pulses happen at AC mains frequency. The voltages may be only millivolts, but they will always be there unless you force the pulses to flow only in and out of the first filter cap (-).  The first filter cap (-) may then be taken to other ground points by a separate wire, as this wire carries only the power and signal to the circuit, not the charging pulses.

Another subtle one is that solid state rectifiers can sometimes cause bursts of RF at the AC mains frequency. This can be picked up by the circuit as "hum". The cure is to either use fast recovery diodes for the rectifier diodes, like the UF4007, not the uN4007, or to use R-C snubbing networks across the power rectifiers.

[EfarmosmenosStokos]

Thanks for the reply

So, if I am getting this right:
The plus point of the rectifier bridge goes to the plus point of the first cap, and through the resistor to the plus of the second cap.
The minus point goes to the minus point of the first cap. That's it; no ground connection. The minus point of the second cap is attached to the third cap and to the grounded chassis.
Is this correct?

Could you recommend a book/pdf/site/forum where I could find more details such as these for building (pre)amps?

[davent]

Valve Wizard's chapter on grounding from his book on valve preamps for guitars and basses. Valve Wizard posts as Merlinb here and on other forums.

http://www.valvewizard.co.uk/Grounding.pdf

Correction, the grounding pdf is not from his preamp book, perhaps Chapter 15 from the Power Supply book he wrote, do not have that one.
dave

[R.G.]

Thanks for the reply

So, if I am getting this right:
The plus point of the rectifier bridge goes to the plus point of the first cap, and through the resistor to the plus of the second cap.
The minus point goes to the minus point of the first cap. That's it; no ground connection.

That is correct. The critical thing is that no wire from chassis or signal ground touch the CT wire (if you're using a FWCT) or the negative terminal of a FWB (if you're using that).  From the negative terminal of the first filter cap, you can run wires to other grounds. You can use the first filter cap negative as a star ground itself, or run one or more wires to other ground points. But the wire from the negative side of the rectifiers must be all by itself.

The minus point of the second cap is attached to the third cap and to the grounded chassis.
Is this correct?

After the first filter cap negative, you have more options, at least as regards hum. It's good to either make the entire grounding system come back to one point, whether that's the negative of one of the filter caps, or just a place you have determined "that's my One True Ground".  You can do a buss of "ground" with a heavy wire running around the chassis, as Fender does in many amps. That's fine, but every buss system is a special case.  

Well, really every grounding system is a special case. This is because wires are not truly "short circuits" but are actually low-value resistors. When we think of the lines in schematics or wires in a chassis, we always think the voltage is the same on both ends. That's a close enough approximation in most cases, but where there are high currents or lots of voltage gain (or both!), the tiny voltages that arise from the resistor nature of the wires gets amplified up.

The entire trick about grounding is to think about the circuit enough to know all of the currents that flow in the wires, and when that will make a difference to the circuit's operation. That sounds disastrously complicated, but in most cases wires connect things that are much more resistive than they are, or where the currents are so low the wire resistance doesn't matter. If a wire is in series with a 1K, or even a 1 ohm resistor and the wire is 10 milliohms, the wire mostly doesn't matter. If the wire is 10 milliohms and 10A pulses are flowing through it, as can easily happen in a rectifier setup with big caps, the wire now causes 100mV pulses, and that's easy to hear if the circuit input node is being waved around by that 100mV.

Could you recommend a book/pdf/site/forum where I could find more details such as these for building (pre)amps?

I wish I could give you a simple, comprehensive reference, but there's not a good, simple, and easy to read reference for this that I'm aware of.

The bible, such as it is, is Henry Ott's "Noise Reduction Techniques in Electronic Systems", but this is a professional level text, and is not all that accessible for beginners. It covers much more than grounding and very little in the way of building techniques. It takes a good background to understand what he's talking about.

I've posted a lot of what little I know over in the Music Electronics Forum when the topic of grounding and noise came up. It's a big area to learn, as it encompasses grounding, leakage, capacitive signal transfer, magnetic loop transfer and radio emissions. Worse yet, the answers are different for different frequency ranges. Star grounding tends to work more reliably for DC and low frequency (audio) circuits, but in RF and ****RF**** circuits, ground planing and transmission line techniques are mandatory.

Valve Wizard's chapter on grounding from his book on valve preamps for guitars and basses. Valve Wizard posts as Merlinb here and on other forums.

http://www.valvewizard.co.uk/Grounding.pdf

I haven't read that, may be a good guide.

[R.G.]

Actually, I take part of that back. There's a decent explanation of grounding issues and many other considerations in "The Circuit Designer's Companion" by Tim Williams.

[JFace]

Could you recommend a book/pdf/site/forum where I could find more details such as these for building (pre)amps?

I have Merlin Blencowe's Designing Tube Preamps for Guitar and Bass. I highly recommend this book. It is the right balance between theory/application. The book is easy to read, and inexpensive. He does not, however, talk very much about power supplies. You can check out his website for a lot of this information: http://www.valvewizard.co.uk/