Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MrBreeze on August 11, 2022, 08:36:57 pm
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Looking at wiring to make all the ground (common) connections. Trying to connect to only 3 locations on the chassis.
A) Power Cord Safety Ground. Should this be grounded to chassis with dedicated PT bolt? I had planned on this being the only thing grounded at this location.
B) Power Amp Side Common to Chassis. This is a chassis stud near the PT. Connections here made with bolted lugs.
C) Preamp Side Common to Chassis. This would be at the input jack(s) connection to the chassis. Other connections to this made with a bus wire.
Per the attached I have attempted to label on the schematic which location (B or C) all the “ground” locations should tie to.
Does this look correct?
Also which switchcraft jacks need to be isolated from the chassis? Since input jacks 1 and 2 are right next to each other do I need to isolate one or none? Does the speaker jack need to be isolated so it can be grounded to the power side location?
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Having the safety ground tied to a transformer chassi bolt is bad according to some people and it is better to have its own chassi bolt, especially if you are building a combo.
One thing to keep in mind also if you are using a cord and not a AC inlet is that the safety ground should be longer than the hot/neutral wires so if someone were to try rip it out the hot/neutral would loose their connection before the safety ground.
One thing i would change would be to also have the both center taps tied to their own bolts instead of the poweramps.
The rest of the poweramp / preamp grounding is more effected depending where you put the ground in terms of the signal flow.
Also all jacks should be insulated if you are having them grounded to the amp, otherwise they got two different chassi ground connections.
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Hoffman's grounding scheme. Newer copy of it in his Library. Grounds (el34world.com) (https://el34world.com/charts/grounds.htm)
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The 5F2A layout diagram is a good place to look
https://el34world.com/Hoffman/files/Hoffman_5F2A.pdf (https://el34world.com/Hoffman/files/Hoffman_5F2A.pdf)
It shows all the grounds
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I use Doug's grounding scheme exclusively -- it's simple, and it works.
On the other hand, I suspect the layout shown can be misleading to some. It was to me as a newb.
Some years ago, when I built my first amp, I thought the preamp bus bar ground symbol implied that it should be physically grounded to the chassis on the side closest to the power transformer. It was only later, after reading many posts and a couple of books, that I realized this was a "label" -- and not the place to physically ground the bus bar.
Fixing that error in that first amp solved a (admittedly minimal) hum . . .
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Not much to a 5F2A.
Two ground wires leave the board.
The pot bus ground goes through the chassis to the main chassis ground point
The PDF shows all that clearly
Forget all those colored marks on the schematic
The chassis is a ground wire
All the grounds bolted down to the chassis return to the main ground.
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Just to note that there’s no need or any technical advantage in using the chassis as a conductor in the circuit’s 0V common node.
And some potential for benefit in not doing so.
A single point reference connection between the 0V common node and the chassis is all that’s required.
That’s not to say that 2, several or multiple connections can’t work fine.
Rather that doing so doesn’t seem to be representative of best engineering practice.
Keeping the current confined within wires with soldered connections enables its route to be defined and so managed.
http://www.valvewizard.co.uk/Grounding.pdf
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Just to note that there’s no need or any technical advantage in using the chassis as a conductor in the circuit’s 0V common node.
And some potential for benefit in not doing so.
A single point reference connection between the 0V common node and the chassis is all that’s required.
That’s not to say that 2, several or multiple connections can’t work fine.
Rather that doing so doesn’t seem to be representative of best engineering practice.
Keeping the current confined within wires with soldered connections enables its route to be defined and so managed.
http://www.valvewizard.co.uk/Grounding.pdf (http://www.valvewizard.co.uk/Grounding.pdf)
Since he is installing one of my boards, I suggest he follows the simple tried and true grounding method as decribed in my install documents
It's the way Fender did it and it works just fine.
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Since he is installing one of my boards, I suggest he follows the simple tried and true grounding method as decribed in my install documents
It's the way Fender did it and it works just fine.
Thank you all for your assistance for me (a new amp builder)! After reviewing the comments and your latest grounding scheme, it looks like to me I need to modify the layout and turret board to have the last filter capacitor grounded with the pre amp side rather than how it shows as being with the other filter caps. Also, it looks like I need to islolate the speaker jack so it can be grounded at the power side chassis ground location. I plan on using the input jacks as the preamp ground location to the chassis.
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Since he is installing one of my boards, I suggest he follows the simple tried and true grounding method as decribed in my install documents
It's the way Fender did it and it works just fine.
Thank you all for your assistance for me (a new amp builder)! After reviewing the comments and your latest grounding scheme, it looks like to me I need to modify the layout and turret board to have the last filter capacitor grounded with the pre amp side rather than how it shows as being with the other filter caps. Also, it looks like I need to islolate the speaker jack so it can be grounded at the power side chassis ground location. I plan on using the input jacks as the preamp ground location to the chassis.
Stop trying to modify things!
Just wired up the board exactly like you see in the PDF document
I have no idea why you think you need to isolate jacks or move filter caps
This is the document layout you use to install a Hoffman 56F2A board
https://el34world.com/Hoffman/files/Hoffman_5F2A.pdf
Stop posting other pictures that have nothing to do with your 5F2A board