Where to ground the first filter cap?

[wsscott]

I recently read an article on grounding by Aiken Amplification, and am now confused on what I think should be a simple task in building a tube amp.  And that is where to physically ground the first filter cap.

I'm assuming when he says the "first filter cap", he's literally talking about the very first filter cap, ie. the reservoir cap.  The article says when using a ground buss (assume the buss is connected to ground at the input jack), to ground the PT's secondary's CT directly to the ground lead of the first filter cap-not to anywhere else on the ground buss or you will get 120hz hum.  I'm assuming that ground lead is then connected directly to the buss. Or maybe I'm wrong about grounding it using the buss.

So is connecting the CT lead to the filter cap's ground lead vs. connecting the CT lead directly to the ground buss an important distinction, or am I just reading the comment too literally?

And, as a follow up, is there any problem with connecting all of the remaining filter cap ground leads to the buss at different locations along the buss?

Thanks.

[dmp]

This is Merlin's chapter on grounding: https://www.valvewizard.co.uk/Grounding.pdf
Fig 15.10
The transformer CT and the filter cap ground lead connect together to the ground bus (I usually use heavy gauge, ~10awg, copper wire for the bus). Then filter caps for stages connect near the tube cathode connections. Finally the bus connects to the input jack. Grounding is not a simple task - it has caused many headaches. A quiet amp can be hard to achieve.

[shooter]

Did Aiken have a schematic example?


the 1st filter cap is the dirty AC, you want it and the PT/rec grounded "Away from" everyone else.


this is a snippet from a 4MB PDF the title is in the middle, hunt it up, read it, understand it, and you'll be the best ground-gremlin hunter in your state 

[wsscott]

Thanks guys. This is very helpful.

No, Aiken didn’t show a diagram or other type of schematic in the discussion.

Happy New Year too!

[HotBluePlates]

... I'm assuming when he says the "first filter cap", he's literally talking about the very first filter cap, ie. the reservoir cap.  The article says when using a ground buss (assume the buss is connected to ground at the input jack), to ground the PT's secondary's CT directly to the ground lead of the first filter cap-not to anywhere else on the ground buss ...

Assume the "Ground Buss" is a thick copper bar.

Connect the Power Transformer Secondary Center-tap to the copper bar.
Connect the 1st Filter Cap Negative lead to the same spot on the copper bar (right beside the CT).
Connect the copper bar to Ground (doesn't really matter what part of the bar gets grounded).


Aiken's point was to have the current-path of the power transformer and filter-cap make the smallest possible loop:
  PT Secondary Winding-End -> Filter Cap Positive -> Filter Cap Negative -> PT Secondary Center-Tap -> PT Secondary Winding-End -> Repeat

Do this for your entire power supply (make the current-loop as short and direct as possible), and you will avoid most hum/oscillation/strange-interactions.

[wsscott]

If you were going to build an amp from scratch, based on an existing schematic like a Fender, Magnatone, etc.,  that you might modify some, would it be a best practice to install the filter caps on the eyelet/turret board as close as possible to the component that it serves first, rather than having all the filter caps in a doghouse, or all at one end of the circuit board, or in a cap can?  This might result in the leads connecting between the filter caps and balancing resistors being longer than if all of the caps were next to each other, or maybe not.  Seems like it might be better for a filter cap to be closer to its associated component connection rather than having a longer flying lead wire.

Would the type of grounding system to be used, ie. buss bar or a single or multiple star system, have any impact on the filter cap location?

Thanks.

[shooter]

Aiken's point was to have the current-path of the power transformer and filter-cap make the smallest possible loop:

I typically put the caps "onboard" vs doghouse.  that's not just to keep the loops small, it's easier to measure/troubleshoot when "B+" is located close to the circuit it feeds, otherwise you're eyes get all cris-crossy following mystery wires back to the source 

[wsscott]

Yea, that's nice looking, and easily accessible.  I like that layout.

What amp is it?

[shooter]

my last amp, a tinker's special 


search me n "Christian Delight" to find the build thread


it's a steal from Steve that drew up a cleaner version of a Silvertone 1482 for Platefire I believe

[AlNewman]

The funny thing that Fender did with caps in the doghouse was install the brass plate, which although it allowed crosstalk between the chassis and circuit, it created a 'buswire' between power amp and input. 

If you look at Fender's layout, all the solder connections to the brass plate are at convenient points in the circuit.  Basically, you could follow their layout with isolated pots and board mounted filter caps, and stay true to a modern grounding system if you position the ground points where they are on the brass plate.

I see a lot of people not having success with buswires, because they don't follow what Fender laid out 60 years ago. 
Maybe with a high gain amp, with several different channels and gain stages the brass plate (chassis) would no longer serve it's purpose, but on the other hand you would have to position everything absolutely perfect if a buswire was your only path to ground.

Of course, reading and learning from people smarter than us, it makes complete sense to separate the different stages of filtering along a linear path.

[jjasilli]

See:


1.  Merlin's website on grounding.


2.  psionic audio's youtube series on grounding (who acknowledges Merlin):

&t=29s

[Willabe]

If you were going to build an amp from scratch, based on an existing schematic like a Fender, Magnatone, etc.,  that you might modify some, would it be a best practice to install the filter caps on the eyelet/turret board as close as possible to the component that it serves first, rather than having all the filter caps in a doghouse, or all at one end of the circuit board, or in a cap can?

Yes.

Read Merlin's grounding, he has drawings in there.

Seems like it might be better for a filter cap to be closer to its associated component connection rather than having a longer flying lead wire.

Yes.

Read Merlin's grounding. HBP's small loops.

Would the type of grounding system to be used, ie. buss bar or a single or multiple star system, have any impact on the filter cap location?

Multi star system.

Read Merlin's grounding.

Akin is the same, so is Kevin O'Connor London Power TUT books.

[Willabe]

The 1st filter cap/PT CT loop has the largest charging pulses in the amp. So it's the dirtiest connection in the amp.

Like already said, connect the PT CT directly to the 1st filter caps ground lead. NOTHING else there. This lets the cap's charging pulses flow in that small loop by themselves. They will not modulate any other circuit in the amp by doing this. The charging pulses are isolated from the other circuitry.

Then you run a single wire from the PT CT and 1st filter caps ground lead connection as a buss over to the next ground star with it's filter cap located at the circuitry it feeds, ie, make a filter cap ground star. You go from 1 ground star to the next ground star to the next, etc.

This way each filter cap has it's own small loop ground star that's charging current's are isolated from the other star's. 

[pdf64]

... Then you run a single wire from the PT CT and 1st filter caps ground lead connection as a buss over to the next ground star with it's filter cap located at the circuitry it feeds, ie, make a filter cap ground star. You go from 1 ground star to the next ground star to the next, etc.

This way each filter cap has it's own small loop ground star that's charging current's are isolated from the other star's.

That's a great description of the multiple star ground system associated with Merlin, but both the accompanying images in the post seem to show chassis connections at the star points, such that the chassis, rather than a wire / bus, links the stars together.
Merlin advises only one connection between the multiple star ground system and the chassis, made very close to the input socket.

[Willabe]

That's a great description of the multiple star ground system associated with Merlin, but both the accompanying images in the post seem to show chassis connections at the star points, such that the chassis, rather than a wire / bus, links the stars together.
Merlin advises only one connection between the multiple star ground system and the chassis, made very close to the input socket.

Nope, look again, only 1 chassis ground connection.

PDF, as many times as I've posted about Merlin's multi star grounding and helped people redo their grounding, I'm surprised you would say this. Guess my drawings not as clear as I thought? But it wouldn't make any sense to talk about a multi star ground and then post drawings using random chassis grounding.   

In the schematic look at the 1st filter cap's ground lead, it's labeled as chassis ground.

I tried to show the individual stars by group number. Follow the PT power supply filter cap series string buss ground wire. They correspond to the circuit ground star component grouping above the power supply filter cap series string. It's the best I could think off. Maybe I should have turned the arrows the other way? 

In the schematic, the circuit ground star component grouping for 2, 3 and 4 are not going to the chassis. Those are supposed to be arrows pointing to the ground star number, not actually showing a chassis ground. I just labeled them that way to line them up with the numbered power supply filter cap ground leads.

In the layout, it shows 1st filter cap chassis ground at a PT bolt. I should have labeled the input and output jacks as isolated. But you can see the jack ground lug on both of them has a ground wire that goes back to the circuit ground star related to them. (I need to fix the PT chassis bolt ground in the layout. I never use a PT bolt for chassis ground. I use a dedicated bolt for the chassis ground.)

On any of my builds, I use a single chassis circuit ground connection. But I think in many amps using 1 chassis ground for the power tubes ground star and a 2nd for the preamp ground star is fine too.