3 channel preamp hum issues

[Captain chunkulus]

The filter cap grounds are locally decoupled. In other words they sit on the board and are grounded right at the stage they are supplying power for. There is one ground going from the input all the way to the output in a straight line.the power rail is in red if you look at the layout I posted and the grounds are in green.

[Willabe]

Yes, I saw that, but there's more grounds. There's a -lot- more to that preamp then just that main board.

Each set of controls, volume/tone stack, should not be tied/daisy chained together to 1 wire and then brought over to the main board ground buss.

Each set of controls should be brought over separate to the tube ground on the board it feeds. So there should be 3 separate ground wires, 1 from each set of controls, coming over to the board and going over to their own ground star grouping. This is part of keeping the channels grounding separate, so no ground loop between channels.

And each relay ac signal ground should be brought over separately to the tube it comes from and grounded to that tubes ground star. They shouldn't be grounded just anywhere on the ground buss. They should each stay with the ground star of the tube they are dealing with.

What about those 4 B+ filter caps on the separate board on the back of the chassis? What are those caps for, what do they feed/supply? They need to be grounded to the ground star that they feed B+ to.

The 1 B+ filter cap should be grounded to the negative end of the FWB rectifier and nothing else, let it form it's own little loop. Then you bring a buss wire from that point to the next B+ filter cap ground. Very important.

All these things need to be dealt with separately, and need to be going to their own separate ground star. You have 3 separate channels, each channel needs their grounds to stay separate from the other 2 channels. Then you string the ground stars to a buss that ends at 1 single chassis ground connection. It's gonna be a little tricky to do, but it can be done.

You quoted Merlin on grounding. He clearly shows grouping all the grounds from 1 tube together, a ground star, then a buss wire over to the next ground star.

With your preamp, lets take a tube with controls as an example.

12AX7 grounds;

1. Grid R (= volume control)
2. Tone controls
3. K R (cathode resistor)
4. K bypass cap
5. Relay ac signal turn off (ac signal grounded, which is part of the grid circuit)
6. B+ filter cap node that feeds this tube

Those 6 tie together to form a ground star, nothing else but those 6 wires/component leads at that star. Then buss wire over to the next star.

By doing this you make a ground loop that is small and only loops around itself. And this eliminates other possibly ground loops from being formed.

It's a 'wired ground', not 'random grounds'. It controls the ground loops from modulating each other, other parts of the circuit. 

[Captain chunkulus]

Okay so first of all, sorry. I have redone the grounds since the first pictures of the chassis. I got rid of the heavy ground wire on the pot grounds. The gain pot grounds for each Channel are grounded to the cathode cap they go to. The 3 different sets of EQ are grounded to a single point which is the cathode of v7. I was unaware that the separate eqs would  be grounded to the cathode of the last stage for each channel. I though since they all ended at v7 I should ground them there. So, I need to ground the eq fir say channel 1 to the cathode follower  of that stage before it hits the last cathode follower  buffer output stage and so on? Also, those caps on the back are the power supply for the preamp. I have the bridge rectifier then two 100uf 350v f&t caps with 220k blender resister tied directly to the bridge rectifier. Then that resavoir cap minus gets grounded to the chassis near the power transformer. The two 100uf caps with bleaders after the choke coil are grounded to the cathode of the last cathode follower. The relay power supply grounds  are tied directly to the reservoir capacitor and Bridge rectifier circuit on the chassis at the same point at the reservoir capacitor is grounded. The signal grounds from the relay circuits are grounded at the point in the circuit their signal is coming from. So let's just say where the two High Gain channels are, where they meet to hit the output stage when one of those channels is grounded that ground gets grounded to the cathode of its cathode follower. I hope this makes sense.

[Willabe]

I though since they all ended at v7 I should ground them there. So, I need to ground the eq fir say channel 1 to the cathode follower  of that stage before it hits the last cathode follower  buffer output stage and so on?

Since V7 has it's own K R and a coupling cap in series just before it, I would try grounding those 3 TS's and 3 volume controls back to the previous tube's K ground.

From V4b/V5b/V6b to V7a you have 3 volume pots, 3 tone stacks and 3 relay's.

And ALL of that is part of V7a's grid circuit.

That's a lot of grid wire, the grid is very sensitive and can act like an antenna and pick up all kinds of stray signals. They get injected into the grid along with the ac signal and get amplified.

You might also need some of the grid wires to be shielded wire.

With all that said, go with what Sluckey is telling you to do. (I would.)

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]

Here's the most recent schematic. Edited to see if I got the star grounding right? also, tried rewiring the relay power like silver gun said and it smoked my rectifier. i wire it exactly like in the picture.

[Willabe]

Yes, that's what I meant for moving the TS/VC grounds back to the stage before.

Did it kill any of the buzz/hum?

I  *think*  Sluckey want's you to try just running V7 to see if the PT is causing the buzz/hum, to close to V7?

DL/Pete, brought it up early in the thread. You can try a shield on that tube. It might lessen the buzz/hum, if it does lessen it, then the proximity of PT to V7 is causing the problem.

I'd still try what Sluckey asked you to try, even if you try popping a tube shield on for quick test. (Do what Sluckey asked with the tube shield off.)   

[PRR]

> i wire it exactly like in the picture.

Not sure how literal to take this. But the round-the-bush path of the relay first filter cap return to rectifier WILL hum/buzz. That wire is full of crap. It must go only from rectifier to cap. THEN take amplifier grounds from first cap.

[Captain chunkulus]

> i wire it exactly like in the picture.

Not sure how literal to take this. But the round-the-bush path of the relay first filter cap return to rectifier WILL hum/buzz. That wire is full of crap. It must go only from rectifier to cap. THEN take amplifier grounds from first cap.

So, the center tap of the relay transformer to the first filter cap in the supply for the relays. then from the last filter cap to the ht power supply reservoir cap ground? Then I make my switching connection for the relays to that point as well? so, that the relays make their switching connection there? Does that make sense?

[SILVERGUN]

... also, tried rewiring the relay power like silver gun said and it smoked my rectifier. i wire it exactly like in the picture.

"rectifier" or regulator?

[Captain chunkulus]

... also, tried rewiring the relay power like silver gun said and it smoked my rectifier. i wire it exactly like in the picture.

"rectifier" or regulator?

Both actually.  torched the two diodes as well as the 5v regulator. Saw smoke as it happened. Anyway, im going to build another relay power supply with a full wave bridge. so, can I just wire it just like hoffmans circuit and just use the center tap to ground? or does it need to be floated keeping the center tap un connected?

[SILVERGUN]

... also, tried rewiring the relay power like silver gun said and it smoked my rectifier. i wire it exactly like in the picture.

"rectifier" or regulator?

Both actually.  torched the two diodes as well as the 5v regulator. Saw smoke as it happened. Anyway, im going to build another relay power supply with a full wave bridge. so, can I just wire it just like hoffmans circuit and just use the center tap to ground? or does it need to be floated keeping the center tap un connected?

If you follow PRRs drawing you don't have to switch to FWB.

But, if you are going to use a FWB rectifier you do not want to attach the center tap of that secondary winding to anything. Just tape it off (or attach it to an unused point on your board)and do not use. The DC negative end of your FWB will become your new ground connection for your relay power supply.

Once you do that there is no reason why you can't just float the relay supply.

What is the AC voltage of that secondary winding that is supplying your relay power?
I ask because I thought I remembered you saying that is was 28V, and if so, after rectification you are exceeding the max. V rating of the 7805

[Captain chunkulus]

Yeah, 28v. I do however have a 9v non center tapped transformer I can use.

"Once you do that there is no reason why you can't just float the relay supply."

So don't ground anything in the relay power supply even the bridge? Float the bridge ground as well?

[shooter]

read this link
http://el34world.com/projects/relay_switch.htm

close to the bottom Doug has in BOLD
......do not use chassis for DC minus, just run 2 wires from bridge to relay (paraphrased )

[Willabe]

So, the center tap of the relay transformer to the first filter cap in the supply for the relays. then from the last filter cap to the ht power supply reservoir cap ground? Then I make my switching connection for the relays to that point as well? so, that the relays make their switching connection there? Does that make sense?

Yes.

I should have picked up on this before.

PT CT's should always go directly to the 1st filter caps ground end. Like I described for the B+ power supply.

In the 1st drawing there's 2 paths to ground in the circuit, 2 ground paths = ground loop = buzz/hum. In the 2nd (PRR edited) drawing, there's only 1 path to ground for the circuit.

[Captain chunkulus]

So in this picture it shows that nothing is connected to ground, correct? The minus just goes to the minus of the diodes on the top of the relays or the switch that is making or breaking the connection with the relays thus switching the relay on or off? So the transformer has a floated ground correct and the bridge is basically also floated? I know all this seems obvious to you guys who have been doing this a long time. However I'm a bit confused about this I look at the drawing and it appears that there's a minus but no ground. So is the minus the ground and you just make her break the connection with the diode on the relays with a switch to ground? Or does the switch connect to the minus on the power supply and all the grounds are floated including the the Transformer and the rectifier and the voltage regulator and the capacitors? Cuz that's what it looks like from the diagram.

[SILVERGUN]

Your confusion is coming from your definition and usage of the term ground.
Ground can also be referred to as common and in some instances the term can help us clarify a mis-conception.
The term "common" refers to a common connection for a circuit, and is usually therefore also know as "circuit common".
It is most widely used as a way to represent 0 volts in reference to some positive voltage.
In our case the positive voltage is +5VDC

When we look at this relay power DC circuit and see a (+) and a (-) it is assumed that we know what we are looking at and will assign 0V to the (-) and +5V to the (+)

Floating this supply ensures that we don't have any interaction with any other circuit commons inside the amp. Switching the (-) path to the relay coil is a simple way of enabling the relay. This switching has nothing to do with the audio signal or it's common connection.

All we need to do is get a switched +5VDC to our relay coil to enable it. There is no need to share another ciruit common to accomplish this so it makes sense to just keep it separate. (even though it could work the other way). In your case this makes even more sense to try because you are experiencing a hum that is usually associated with a grounding issue.

By keeping your "relay circuit common" separated from your "signal path common" you are eliminating one variable that could be causing your issue.

[sluckey]

There's absolutely nothing wrong with using that 28V PT just as it is with that conventional 2 diode full wave conventional rectifier. You get exactly the same dcv as you will by switching over to a FWB and only using half the PT. Nothing gained at all.

[SILVERGUN]

Thanks for the correction sluckey.
I got rid of the post to avoid adding confusion.

[Captain chunkulus]

So, I built a new power supply for my relay circuit using a 9 volt non Center tapped Transformer using the Hoffman relay schematic. Works fine but did not add or subtract from the hum issues that I have had since the beginning. So then, I decided to just try star grounding the whole preamp. What I ended up with is exactly the same thing the same hum. If I remove the input tube and both of the High Gain channels first tube the hum goes away. Seems to be affected by the gain pots of those channels. When I turn the gain down hunter gatherer of those channels and their volumes up there is no hum it's just associated with those three tubes the input tube which if I pull it the hum is still there. However when I pulled the two tubes for the high gain the hum goes away like I said. Star grounding made no difference at all.

[vladm]

Sorry if I'm offtopic, but allow me to give you some advices/opinions.

I don't know if other colleagues suggested you this, but have you tried to rectify the heaters voltage?
In my Soldano Sp77 clone I supplied the heaters by using a full wave rectifier with a 6800uf filter capacitor. You have 7 tubes, so you better use a chassis-mounted 25A bridge rectifier.
Also, the connection between input jack and first triode of V1 needs to be short as possible and using shielded cable. You can solder the 33k gride resistor directly to the socket pin of the tube.
From what I see in the pictures and video, your input jack wire goes to the turret board and then to the grid with a simple wire; please correct me if I'm wrong.
Signal wires between relays and turret board/pots panel also needs to be shielded.

[Willabe]

Also, the connection between input jack and first triode of V1 needs to be short as possible and using shielded cable. You can solder the 33k gride resistor directly to the socket pin of the tube.
From what I see in the pictures and video, your input jack wire goes to the turret board and then to the grid with a simple wire; please correct me if I'm wrong.
Signal wires between relays and turret board/pots panel also needs to be shielded.

Yes.

Grid wires can pick up all kinds of stray signal noise.

Compounds the problem with hi-gain preamps.

I do see some shielded wire runs in the 1st pic you posted. You might need a few more in places. The tubes you pulled yesterday and the amp stopped buzzing, those grids and relay signal wires if more than a few inches might need to be shielded.   

You only connected 1 end of the shield to ground right, not both shield ends?

I'd do that before rectifying the heaters. That can have it's own problems introduced.   

[Captain chunkulus]

Sorry if I'm offtopic, but allow me to give you some advices/opinions.

I don't know if other colleagues suggested you this, but have you tried to rectify the heaters voltage?
In my Soldano Sp77 clone I supplied the heaters by using a full wave rectifier with a 6800uf filter capacitor. You have 7 tubes, so you better use a chassis-mounted 25A bridge rectifier.
Also, the connection between input jack and first triode of V1 needs to be short as possible and using shielded cable. You can solder the 33k gride resistor directly to the socket pin of the tube.
From what I see in the pictures and video, your input jack wire goes to the turret board and then to the grid with a simple wire; please correct me if I'm wrong.
Signal wires between relays and turret board/pots panel also needs to be shielded.

I do have shielded wire going to the input of the first tube. I have a 33 K resistor going from terminal  strip directly to the input of the first tube. The two relays that are going to The High Gain channels are strictly grounding relays. All they do when switched on is ground the inputs of those tubes so the clean Channel doesn't have crosstalk. They carry no signal really except to ground the signal out. The three relay board does carry signal but it's just a ground out the output of the two high gain channels. I could use shielded wire on those those runs. And maybe I need to use shielded wire on the two tubes that seem to be causing the noise problems to the grids cuz I currently I'm not.

[Willabe]

The two relays that are going to The High Gain channels are strictly grounding relays. All they do when switched on is ground the inputs of those tubes so the clean Channel doesn't have crosstalk. They carry no signal really except to ground the signal out.

The inputs are the grid. Your grounding the grid. That grid grounding wire is an antenna when it's not grounded, doesn't have to carry signal from 1 point to another. The longer it is, the larger the antenna, picks up more. Should be shielded.

The three relay board does carry signal but it's just a ground out the output of the two high gain channels. I could use shielded wire on those those runs.

After the tone stacks volume controls, those 3 wire runs go to the grid of V7, that is all antenna. Should be shielded.

And maybe I need to use shielded wire on the two tubes that seem to be causing the noise problems to the grids cuz I currently I'm not.

That's a pretty good clue there.

Start with those grid wires 1st. Install the shielded wire 1 run at a time and test for noise reduction. That way you will hear/know where the problem was and how much it was reduced. It might take several shielded wire runs to get it all.   

[Willabe]

If the wires with the red arrows are grid/relay ground wires, they need to be shielded. Those are long wire runs, hi-gain makes it worse. Two of those white wires on the board go to blue wires that go to the tube, very long. The 3rd white wire goes to a brown/orange wire to a tube, also pretty long run.

Don't use those turrets/eyelets on the board for the shielded wire runs. Run the shielded wire in 1 piece keeping them as short as possible with in reason. You might be able to use the turret as a tie point for a zip tie?   

Most, if not all, of any grid/relay ground wires in the yellow circle are long enough to need shielded wire.   

[Willabe]

What is the white wire under the board, red arrow.

What are the 3 zip ties holding under the board, yellow circle?

[Captain chunkulus]

"What is the white wire under the board, red arrow"

If you look at the original layout, it show that those wires run under the board to the tone controls. I had asked at one point if having  the tone controls so close to the gain controls might be the culprit.  However. The white wires from that end if the board carry the signal back to the tone controls. I did try shielding the 2nd tubes grid runs and it made no difference.

[Captain chunkulus]

Okay, I think I've hit rock bottom on this thing. I've tried everything I can think of. Shielded grid wires, shielded all long runs, changed tubes, star grounded the preamp, buss grounded it, with relays, without relays, moved the transformer, absolutely nothing changes the hum. It is just there no matter what I do. I'm at a complete loss for what to even try next. The Transformer itself hums but so do others I have in other amps I own but they don't make the amp hum. I'm all out of ideas. I can't even get one single channel to work  without hum. You guys got any thing else I could try? Thanks.

[shooter]

The Transformer itself hums

my last build had a hummer PS, nothing "fixed" it, swapped it out, walla
that hummer is now breadboard ONLY. 

[sluckey]

Disconnect the power cord green wire from chassis. Still hum?

What is the output plugged into?

Plug a headphone into the output jack. Still hum?

[Captain chunkulus]

Disconnect the power cord green wire from chassis. Still hum?

What is the output plugged into?

Plug a headphone into the output jack. Still hum?

Tried unhook ing earth ground, no difference.  Tried head phones,  no difference. It's plugged into an amp front but, I have an ebtech  hum eleminator as well. Tried it both ways, same hum. So, I don't think it's hum between the two pieces of gear.

[sluckey]

Try connecting the output to a power amp. Any better?

[Captain chunkulus]

Okay, just made an interesting discovery. Coming off the input stages, I decided to try an experiment. So coming off the coupling cap where it has a 2meg to ground, it connects to all three gain pot circuits. I lifted the coupling caps on all three, one at a time. Here's the weird. I'm still getting guitar signal on all three  channels. No distortion but clean on all three. Way less on the clean channel but pretty loud on both distortion channels. This should have killed all the signal entirely because nothing is connected to push signal down the line.

[Willabe]

This area? What coupling caps?

[Captain chunkulus]

This area? What coupling caps?

Yes, that's it. If I lift the 1000pf on both channels as well as the 470k on the clean, there should be no signal at all and yet, I have signal. If I pull v-1 tube all signal goes away. I'm scratching my head. Also, the gain pots still work as well. More like boosting the gain but not distorting it.

[Willabe]

Has to be wired wrong.

This area here, red square?

[Captain chunkulus]

Has to be wired wrong.

This area here, red square?

I hear ya, most definitely something  is up. However, I have continuity tested the connections from the .022 off the first plate and after breaking all the connections to the gain pots for all three channels. After  the .022uf it should have broken the connection.  Yet, I still have signal and no continuity between the .022uf cap and any of the gain pots.

[Willabe]

I think this is his most updated schematic?

http://el34world.com/Forum/index.php?action=dlattach;topic=25122.0;attach=79549

[Willabe]

I don't know how but you may have a 2nd path through the relays?

One or more relay signal path may be wired wrong?

[Willabe]

I would try breaking/disconnecting ALL the relay signal wires, input/output. Then jumper the outputs with a gator clip jumper wire, 1 at a time and see if that kills the signal. 

If it does, it's the relays miswired, giving secondary signal path. If not, then the preamp R/C filters are miswired. 

[Willabe]

Have to go out shopping for food with my wife.

I think you might get it fixed now.     

[Captain chunkulus]

Have to go out shopping for food with my wife.

I think you might get it fixed now.     

Not so much. Lol. I've removed every connection from the board going to tone controls volume controls gain pots relay switching everything nothing Remains. The only thing that remains is the input jack connected to the V1 tube and the output and yet I still have Guitar signal passing.

[shooter]

Not so much

what do you have for test equipment?
scope
listening amp
signal source

[Captain chunkulus]

Head phones, guitar amps. No scope unfortunately. 

[Willabe]

Did you disconnect the 3 wires going from R-1/R-2/R-3 to the .22uF cap feeding V7?

If not, just disconnect either end of that .22uF cap going into V7.

[Captain chunkulus]

I did. I decided to try an experiment and just hook up one channel, no relays just straight connection. Same hum. I think I'm going to buy another transformer and try that. I'll keep you guys posted.

[Willabe]

Was there still signal at the output with V7 .22uF cap lifted?

[shooter]

I think I'm going to buy

just straight connection. Same hum

1ST take to neighbors house AND different Headset, way cheaper than shipping for a 15min test

[vladm]

You can create an audio probe as in the attached picture and connect it to the different inputs of the stages of the preamp.
Maybe you detect if there are any "noisy" stages in your build.
Regarding the tubes, they're new? you're sure abot their good functionality?

[Captain chunkulus]

You can create an audio probe as in the attached picture and connect it to the different inputs of the stages of the preamp.
Maybe you detect if there are any "noisy" stages in your build.
Regarding the tubes, they're new? you're sure abot their good functionality?

I'm going to give this a try and see what I come up with. On a different note, i rewired one single channel of the High Gain. Still has unbelievable amounts of hum. As well as parasitic oscillation now. Shielding from the input and output of a gain pot makes no difference whatsoever to the hum or to the oscillation. It's a complete mystery to me. I've built other preamps that work just fine. It's like this preamp is possessed. LOL

[vladm]

You can create an audio probe as in the attached picture and connect it to the different inputs of the stages of the preamp.
Maybe you detect if there are any "noisy" stages in your build.
Regarding the tubes, they're new? you're sure abot their good functionality?

How do you use the probe? It may seem obvious to some but it's eluding me. Thanks.

You connect the guitar in the jack, and the capacitor terminal "To circuit (test)" you connect it to the input grid or to the grid stopper resistors from different stages as in the attached picture.