3 channel preamp hum issues

[Captain chunkulus]

Okay, Im back from the last soldano ish thread. Decided to go a different route. so I'm posting these pics and layout hoping I'll learn something from you guys. All 3 channels work but I have 120hz hum that is LOUD as all get out. I'm figuring a grounding issue but I'm no expert by any means. all parts and relays are brand new as well as the tube sockets and wire. so if any of you guys have time to take a look and tell me where I might be missing it I would appreciate it.

[shooter]

I'm posting these pics and layout

probably what hung your thread
post a schematic and gut shot.

all 3 ch hummm?
does one do it more or less than the others?
does it change with gain/vol/tone knobs

[EL34]

Not sure why your original post went wacky?
I could see it in the database, but could not load it into a browser

[Captain chunkulus]

yes, all 3 channels hum. its affected by the gain pots and master volumes. clean channel has way less hum but its there. I'm using an elevated heaters supply. all preamp grounds are on a single buss, the tone controls are bussed and then grounded near the input as well as the preamp grounds. the power supply first filter cap in grounded near the power transformer. dropping resisters on on the bottom of the turret board and are 10k except the last one near the last filter cap which is 4.7k.

[Captain chunkulus]

more pics

[Captain chunkulus]

layout minus tone stacks.

[DummyLoad]

power transformer seems too close to that rightmost tube with the winding window pointing right at the tube. if you've eliminated all other possible sources of hum (hope you have shields for those tubes), try temporarily moving the PT outside of the chassis with clip leads to see if that helps. you could also try inserting a piece of sheet metal between the PT and the tube row to see if that improves hum, be sure sheet metal is grounded.

relay channel switching in high gain amps is nothing but trouble if not laid out very carefully. the soldano vactrol switching wil be much quieter. 


--pete

[Captain chunkulus]

power transformer seems too close to that rightmost tube with the winding window pointing right at the tube. if you've eliminated all other possible sources of hum (hope you have shields for those tubes), try temporarily moving the PT outside of the chassis with clip leads to see if that helps. you could also try inserting a piece of sheet metal between the PT and the tube row to see if that improves hum, be sure sheet metal is grounded.

relay channel switching in high gain amps is nothing but trouble if not laid out very carefully. the soldano vactrol switching wil be much quieter. 


--pete

I did finally find most of the hum. I grounded the power supply to the preamp buss right at the end on the last tube stages. What's left seems like I can kill it by running the first tube on filtered dc maybe. We'll  see. I've been thinking about the power transformer fir a while. Not really sure where to relocate it but may try that as well. For now it has a weird high pitch noise. It's not oscillation I don't think. But maybe I'll post a video later today.

[Captain chunkulus]

Okay, so here's where I'm at. Any ideas guys? I appreciate  your guys help so much. I have learned a lot here. I'm so grateful for the knowledge  you guys share. Here is a video of what I'm dealing with.

[sluckey]

Couple things to try...

Run a 18AWG wire from the power supply filter caps negative side directly to the chassis ground near the input jacks.

Disconnect the AC feed to the relay power supply. I know this will kill any switching. Just interested to see if that is contributing to the hum.

[sluckey]

yes.

[Captain chunkulus]

Couple things to try...

Run a 18AWG wire from the power supply filter caps negative side directly to the chassis ground near the input jacks.

Disconnect the AC feed to the relay power supply. I know this will kill any switching. Just interested to see if that is contributing to the hum.

Okay tried this, no difference in hum. the hum is affected by the gain controls and master volumes.

[Ritchie200]

Coming in late - but is there a schematic for this?  Or is it on another thread?  From your description I believe you mention hum is present on all channels?  Are you sure the unused channels are shunted to ground?

Jim

[Captain chunkulus]

Coming in late - but is there a schematic for this?  Or is it on another thread?  From your description I believe you mention hum is present on all channels?  Are you sure the unused channels are shunted to ground?

Jim

There is a layout I posted earlier in the thread. I do have a relay diagram.  The input and output of all channels  get shunted to ground when not in use.

[SILVERGUN]

In the video, you pointed out the green ground wire that is connected to the ground buss of your board and then terminates at the back of the amp with that cluster of other grounds.

Is that wire also connected to the chassis up near the input jack?

[Captain chunkulus]

In the video, you pointed out the green ground wire that is connected to the ground buss of your board and then terminates at the back of the amp with that cluster of other grounds.

Is that wire also connected to the chassis up near the input jack?

The power supply ground runs directly to the preamp ground on the side of the board. Then runs up near the input jack. So it's grounded right at the input. The preamp grounds are basically on a bus and grounded right at the input jack on the chassis. The power supply is grounded to that buzz at the very end last tube stage.

[SILVERGUN]

It looks like you have created a ground loop by having that "shared" buss connected to 2 points on the chassis.

[Captain chunkulus]

It looks like you have created a ground loop by having that "shared" buss connected to 2 points on the chassis.

Can you explain please? How so? The only connection to ground is at the input.

[SILVERGUN]

It looks like you have created a ground loop by having that "shared" buss connected to 2 points on the chassis.

Can you explain please? How so? The only connection to ground is at the input.

In the video it looks like you have the left side of the board buss grounded at the input and the right hand side of that same ground buss coming off of the right hand side of the board traveling to the back of the chassis and connected with the main cluster of ground connections.

[Captain chunkulus]

It looks like you have created a ground loop by having that "shared" buss connected to 2 points on the chassis.

Can you explain please? How so? The only connection to ground is at the input.

In the video it looks like you have the left side of the board buss grounded at the input and the right hand side of that same ground buss coming off of the right hand side of the board traveling to the back of the chassis and connected with the main cluster of ground connections.

There power supply ground isn't grounded before it gets to that point.  It may look that way but the end of the preamp ground on the right side just goes to the negative of the first filter cap.

[SILVERGUN]

There power supply ground isn't grounded before it gets to that point.  It may look that way but the end of the preamp ground on the right side just goes to the negative of the first filter cap.

There power supply ground isn't grounded before it gets to that point.
Which point?

That entire cluster at the rear of the amp is the most important ground connection in your amp and needs to be the most SOLID connection under the hood. If it is not super-duper grounded at that point it needs to be.

[Captain chunkulus]

So, ground the power supply separate from the preamp grounds? I've been  Reading merlins book and he says " The rest of the amplifier will then be connected directly to the terminals of the reservoir capacitor. Connections to any other points on this noisy current loop are not allowed, and no part of this network may be connected to the chassis if a quiet ground scheme is to be maintained! Some very sensitive preamplifiers may even shield this part of the power supply from the audio circuit with a metal bulkhead or screening can."

So now I'm confused. Do I ground the power  supply separate  from the preamp? Or, ground the power supply and the run the preamp /input ground wire to the power  supply ground?

[SILVERGUN]

So, ground the power supply separate from the preamp grounds? I've been  Reading merlins book and he says " The rest of the amplifier will then be connected directly to the terminals of the reservoir capacitor. Connections to any other points on this noisy current loop are not allowed, and no part of this network may be connected to the chassis if a quiet ground scheme is to be maintained! Some very sensitive preamplifiers may even shield this part of the power supply from the audio circuit with a metal bulkhead or screening can."

So now I'm confused. Do I ground the power  supply separate  from the preamp? Or, ground the power supply and the run the preamp /input ground wire to the power  supply ground?

As it is now, if you have all of those power supply grounds coming across your board and then the only actual ground connection is near the input jack, that is not a good idea, and could very well be the cause of your issue.

So, you will want to create a main power supply ground at the rear of the amp and attach that to the chassis....i.e.  main caps, rectifier

Then you can get rid of the green wire that comes off of the right side of the board and just make sure that the connection you have off of the left side, up near the input is solid.

This is still not an ideal grounding scheme for a ultra high gain monster, but may well get the monkey off your back.

[Captain chunkulus]

Thanks so much. What would you suggest as a grounding schem for something high gain like this ? If it were  you? I'm just trying to understand and learn stuff.

[shooter]

I like to think in Guitar amps we use 3 grounds

1st ground is power plug grounded where it enters amp

2nd ground is power supply ground.  PI, PA tubes, Rectifiers and associated caps. gets a bolt ground at the PT mounting bolt.

3rd ground is preamp ground, preamp tubes and filter caps for DC gets grounded at input jack

the chassis becomes the "buss" in which they all gets back to the beginning

[Captain chunkulus]

Thought I'd post this info maybe it will illuminate something I'm missing and BTW guys, I appreciate all your thoughts and expertise.
Ground reference for elevated heater to first filter cap
Ground from relay power supply
to first filter cap
Ground for the relays them selves to the switches then
To first filter cap
28v from center tapped secondary (used For relay power)
to first filter cap. also including a schematic for the circuit I'm using.
Second filter cap
To first filter cap
First  filter cap then goes to ground with one wire.
Preamp bus is grounded  near input jack on chassis lug
Relay signal grounds are grounded near input jack on chassis lug
Input grounded near input jack on chassis lug.
All tone control/gain pot ground on a buss wire also grounded on the ground lugs near the input jack.

Hums like crazy. Ground loop?

[sluckey]

Post your schematic in this thread.

[Captain chunkulus]

I haven't been working from a schematic per say but I'll try to drawn one up this evening and post it.

[Ritchie200]

More of a question for me...  Should the switching supply be floated?


Thanks!
Jim

[Captain chunkulus]

More of a question for me...  Should the switching supply be floated?


Thanks!
Jim

That's a good question.  I could rebuild the relay power supply with a bridge rectifier instead of what I did. Buthe I'm wondering if I could still use the 28v center tapped secondary by not connecting the center tap and doing an artificial center tapped with 2 100ohmy resisters? Then float the relays? However when I removed power to the relays, it defaults to the clean Channel and still hums. So I dunno.

[Ritchie200]

Haha!  Well I guess that answers my question! 


Have you tried removing tubes to see when it first appears?


Jim

[Captain chunkulus]

Haha!  Well I guess that answers my question! 


Have you tried removing tubes to see when it first appears?


Jim

I have, its all there all the way till i pull the last tube to varying degrees.

[Ritchie200]

Have you tried isolating each channel from the switching circuit - just to abso-positively-everafter rule out the switching circuit?  I just cant shake that... In any case you can check it off the troubleshooting list.


Jim

[sluckey]

An accurate schematic would be much more helpful than a layout.

[Captain chunkulus]

Fair enough.  I will try to find a way to draw one out.

[newguitarsmell]

Did you remove the ground loop and did that help?

[Captain chunkulus]

The forum is not letting me post jpeg images. The file isn't so big  so, I guess I'll try pdf for the schematic I did. Hopefully it will help.

[SILVERGUN]

Schematic looks good. (as a piece of art.  I haven't actually looked it over)

-Show us where the relay power supply comes from (i.e...separate transformer, filament winding, etc.?)
-And then show us how you have the filaments referenced to ground.
-And how you have the relay power supply grounded.

[sluckey]

Good schematic. I have a plan. DIVIDE AND CONQUER!

Break the circuit at the four points I've indicated.
Add a jumper (doesn't have to be shielded) between pot wiper and .22µF cap.

At this point the only things between the input and output are V1, V4, and V7. Does it hum?

[Captain chunkulus]

Her is an updated schematic with filament and relay ground.

Schematic looks good. (as a piece of art.  I haven't actually looked it over)

-Show us where the relay power supply comes from (i.e...separate transformer, filament winding, etc.?)
-And then show us how you have the filaments referenced to ground.
-And how you have the relay power supply grounded.

[Captain chunkulus]

Good schematic. I have a plan. DIVIDE AND CONQUER!

Break the circuit at the four points I've indicated.
Add a jumper (doesn't have to be shielded) between pot wiper and .22µF cap.

At this point the only things between the input and output are V1, V4, and V7. Does it hum?

I'll give it a try and let you know. I also posted a NEW updated schematic. All of the relay signal grounds get grounded at the preamp grounds and the relay power grounds are grounded at the main power supply first filter cap. all signal grounds are grounded near the input jack on the chassis.

[Captain chunkulus]

Good schematic. I have a plan. DIVIDE AND CONQUER!

Break the circuit at the four points I've indicated.
Add a jumper (doesn't have to be shielded) between pot wiper and .22µF cap.

At this point the only things between the input and output are V1, V4, and V7. Does it hum?

Tried this,worse hum and buzz on the clean channel.

[Captain chunkulus]

Found error on my schematic, it goes v3a and v2b. It should be v3b.

[SILVERGUN]

Here's what Ritchie200 was suggesting

[Captain chunkulus]

Here's what Ritchie200 was suggesting

Would I need to change the rectifier to a bridge rectifier? Also,  the center tap for the relay secondary on the trans former just goes to the floated ground on the relay power supply? Just want to make sure before I change stuff around.

[sluckey]

Good schematic. I have a plan. DIVIDE AND CONQUER!

Break the circuit at the four points I've indicated.
Add a jumper (doesn't have to be shielded) between pot wiper and .22µF cap.

At this point the only things between the input and output are V1, V4, and V7. Does it hum?

Tried this,worse hum and buzz on the clean channel.

So, you have bad hum even with this simple arrangement. I would leave it connected just like this and solve the hum issue. Once solved for this arrangment, the hum will probably be solved for the whole circuit.

[SILVERGUN]

Would I need to change the rectifier to a bridge rectifier? No
Also,  the center tap for the relay secondary on the trans former just goes to the floated ground on the relay power supply? Yes, as it appears in my little schemo snip

BUT, stick with sluckey's advice. Be thankful he is here. You have something mis-wired or improperly grounded and my wish for you is that you will eventually find it.

My original concern was that you had 5-6 ground wires connected to a terminal and then 1 wire to carry all of that over to your preamp board.
The fact that you thought that was a good idea leads me to believe that there might be another bad idea in there somewhere. 

Take sluckey's CAPS as gospel

[sluckey]

DIVIDE AND CONQUER AGAIN. Let's simplify even more. Remove the jumper between that pot wiper and that .22µF cap. Now the only thing in the circuit is V7. Does it hum?

[Captain chunkulus]

The fact that you thought that was a good idea leads me to believe that there might be another bad idea in there somewhere. 

Take sluckey's CAPS as gospel

I thought that since all of the the tone control terminate at the output stage  I needed to ground them all to that stages cathode resister.

[Willabe]

When I do a schematic and layout for an amp build, I include indicating where all the ground wires connect to each other and their B+ filter.

Below I attached a schemo and layout drawing of a build I did. It shows a wired ground star ground system.

I'm thinking where you have that group of B+ filter caps bunched and grounded together and then ran a single wire over and where you have your relay's grounded and relay ac signal grounds are goofed up. I think those relay ac signal grounds should go back to the tube stage ground star they belong to. And I'm thinking those relay switch grounds should not be grounded with the tube stage ac grounds. They should be grounded with the relay power supply.

No one can tell where you have all your grounds wired up and what their wired up to. If you had drawn out the ground stars then we could figure it out.

It's a fairly complicated build with 3 channels and all the relay switching. A lot of grounds to interact with each other as loops. Plenty of places for bug a boos to sneak in.