5F8A Ground scheme questions

[airistubolis]

Hello, This is my first post but i have been reading this and other websites for a few months now. I am an automotive Mechanical designer. I also do woodworking, RC cars and planes, High Amp Car Stereos and general mechanic.

So, I have been designing a version of the 5F8A and am trying to keep it as original as possible the way they were actually built but with applying latest learning and safety.

Grounding scheme. I was originally using Robs but i have questions.

Current design attached



Does it make sense to have the Bias Caps that look to be on the power side grounded with preamp or straight to main ground?


And also looking at this below, Is it ok to route the pot signal ground like it is here?


Lots of choices. lol.

Also, I noticed A LOT of 5F8A and 5F6A power tubes are grounded individually very close to the tubes. Any issue doing this as long as I do the preamp stuff?

[EL34]

The library is here
https://el34world.com/schematics.htm


This grounding scheme is tried and true for many years

https://el34world.com/charts/grounds.htm

[airistubolis]

Thank you and I have seen those. Are you advocating for the bus bar across the back of the pots? Doesn't that just ground things to chassis through the pot? Are those input jacks isolated from chassis? I thought the idea was to ground the preamp stuff separately through a singe self grounded input jack and everything else somewhere else?

[pdf64]

I don’t think that the bus bar is shown as being soldered to the pot cases. The note says it’s connected to the chassis at the input jack.

Lots of varieties of 0V arrangements can be used, almost invariably with good results.
However, when a problem arises, a single point connection between the circuit’s 0V common and the chassis, at the input jack, is simplest to fault find.

[sluckey]

Hoffman likes to solder the ground buss to the back of all pots. Like this PR example...

[acheld]

However, some of us have had trouble with soldering to the back of the pots -- IF you ever have trouble with one of them, you must remove them all to work on the system.  It's a pain.

Instead, you can run a buss bar near the pots, grounded at the input jacks, and run your grounds to the buss.

[PRR]

> Does it make sense to have the Bias Caps ...grounded with preamp or straight to main ground?

The bias supply has nothing to do with the preamps.

I'm not sure what "main ground" you see.

The bias has to be "relative" to power tubes cathodes. And in a push-pull amp is not at all fussy (crap on the bias cancels in the amplifier). So I'd tie it near power tube cathodes and not worry about it.

[Willabe]

More grounding scheme info. 

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

[pdf64]

Michael at Modulus has published a nice layout that has a single point connection between the amp’s 0V common and its chassis http://download855.mediafire.com/9cng2uo8xkng/arlsw02ata1j12m/Modulus+MR34+Layout.pdf
HT current loops are kept short and contained, thereby minimising interaction between them.
It’s for a highish gain Marshall type amp, which isn’t too far from your project.

[EL34]

However, some of us have had trouble with soldering to the back of the pots -- IF you ever have trouble with one of them, you must remove them all to work on the system.  It's a pain.

No you don't. You just clip the bus wire close to the solder blob and remove the pot. Easy
The shell of the pot is connected to ground and is a shield

[acheld]

Ya, ya, you're absolutely right.   

I would always make a hash of it when I did that, so I drifted away from that method.   Could probably do better now.

Soldering on the back of the pots does save space in the build, as compared to running a separate buss in between pots and the board.

Not to lose sight of the main message, the grounding scheme advised on this forum works great. 

[EL34]

I take a Dremel rasp bit and clean off a center circle area on the back of the pot where the solder blob will go.
Alpha pots have some sort of coating that solder does not like to stick to
And some pots have a greasy coating that must be cleaned off.

[airistubolis]

Thank you for all the amazing input. I have read / am reading over and over most of these articles.

It seams to me that, after looking at pictures of original 1959 Fender 5F8A, the grounds happen locally to the chassis all over the place and has a plate around the pots. Everybody seems to covet these originals. Anyway going down this road is one school of thought but has it owns perils. lol. I wonder if changing the way its grounded modifies the "tone" in any way other than less "noise"? I understand their are many benifets to having dedicated wired grounds to one or two locations.

Anyway, just trying to focus on schematic, knowing I am going to wire this as close to original as possible i.e. eyelet fiberboard, this is where I am with the schematic.

[pdf64]

...I am going to wire this as close to original as possible i.e. eyelet fiberboard...

I strongly urge you to reconsider that. A board substrate that’s a proper electrical insulator will avoid issues caused by unexpected circuit paths being created by the black vulcanised card.

[airistubolis]

...I am going to wire this as close to original as possible i.e. eyelet fiberboard...

I strongly urge you to reconsider that. A board substrate that’s a proper electrical insulator will avoid issues caused by unexpected circuit paths being created by the black vulcanised card.

I am going to have spacers of a good amount and an additional piece of material without holes between the chassis and actual circuits board. Is that not enough?

[sluckey]

The problem is that the board material itself is prone to become conductive, at least the stuff Fender used back then is.

[tubeswell]

Thank you for all the amazing input. I have read / am reading over and over most of these articles. ... this is where I am with the schematic.

If you follow Merlin Blencowe's recommendation, the signal ground would look more like the amended attachment, with only 1 chassis connection point at the input jack sleeve, and the power amp grounds all clustered together at the 'floating end' of this signal ground chain.(This minimises potential for ground loop hum). YMMV. The speaker ground can just ground at speaker jack chassis mounting point. Also, the presence ground is the return for the LTP, so it would ground at the same filter cap that the LTP takes its power supply from.

[adew]

Michael at Modulus has published a nice layout that has a single point connection between the amp’s 0V common and its chassis http://download855.mediafire.com/9cng2uo8xkng/arlsw02ata1j12m/Modulus+MR34+Layout.pdf
HT current loops are kept short and contained, thereby minimising interaction between them.
It’s for a highish gain Marshall type amp, which isn’t too far from your project.

I've used this type of scheme (actually, the Modulus JCM800 variant, which is effectively identical) with excellent results, and this is now my "go to" grounding method. Totally quiet.
Incidentally, the above link doesn't open for me.

[PRR]

> above link doesn't open for me.

Does for me; after a glitch. I didn't know MediaFire was still alive?