Another Amp Re-Grounded

gabasa

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I've had good success grounding my amps lately by using a lifted terminal strip and rewiring to make sure that no current can return to the PT via the chassis. This has been based on learning I have done with the guidance of @2L man. I just did this again, as part of an overall upgrade to my old 2X10 Bassman, which included PT and OT upgrades. Now, every amp I own is grounded this way.

Earlier this year, I realized that the way I was grounding my amps (based on typical layout drawings), the current through the preamp filter caps was returning to the HT center tap via the input jacks and chassis. Yuck!

Now, in this amp, the only two places that the electronic circuit touches the chassis is the green earth wire from my power cord and the input jacks. The terminal strip in the photo has no electronic contact with the chassis. Notice that the pots are grounded to the bus and not on the back of the pots. The speaker jack has isolation washers and a ground wire to the terminal strip. The power tubes also ground there.

I did something really cool with the bias pot that (check out the photo) that I've never done before. They only come in 10k but I wanted 25k, so I also purchased a 25k CTS pot and swapped wafers. Now it's perfect; I have a 25k CTS bias pot!

I've improved the noise level on a couple of amps by doing this, and pretty drastically in one. Just thought I'd share my experience, cheers.

Pots.jpeg
Grounding Strip.jpeg
IMG_7438.jpg
 

2L man

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Nice to hear what I have wrote has been useful :)

I hope someone who has been educated electronics technician/engineer will tell what are the correct english terms. I think ground-term is not used anymore for a component where flow secondary operative current?

I recall here Safety Earth was introduced 1960s and it came gradually and did divide and define Ground to SE and "secondary operation". Perhaps because our 220VAC Mains, back then, is about four times more dangerous than 110V was a main reason?

I remember discussion that it is a waste of wire when third Safety Earth wire must be installed to buildings between Outlets and Mains Board when previously it was connected to Neutral on power outlet. This does improve safety when this SE wire can still burn Mains Board fuse although Neutral wire has failed.

Nowadays 30mA leakage relays on Mains Table are mandatory in bathrooms and kitchen supplys and they protect already when failure happen "inside" electric appliance which has a SE, before it is appliance user who conduct the Mains current.

Instead of ground we use 0V and it can be defined as 0VDC or 0VAC.

Operative B+_ don't have misleading B = battery. Operative voltages are just + for positive voltage and - for negative voltage against 0V and there often follow a numerical reading.

However I see B+1, B+2, B+3, etc. good when they kind of define what stage they supply but there are triode power tubes which don't have Screens... But together B+ the "ground" should be called B0 and bias supply B-1. For Star-secondary B01, B02, B03, etc. should be used?
 

gabasa

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You mean you are using a single bus for ground of all ground connections back to the input jack?
Check out the second photo and you'll see a terminal strip bolted to the chassis beside the PT. Those terminals are all isolated from the chassis and all circuit ground connections go there:

- cap board ground
- preamp bus ground
- bias pot ground
- power tube ground
- speaker jack ground
- HT CT ground
- filament CT ground

Even though the input jacks are grounded to the chassis, the way that the amp is now wired means that no current can flow back to the HT CT via the inputs and/or chassis, because there is no path in that direction. The only way that electrons can get back to the CT is via the wires direct to that terminal strip.
 

cottontails1959

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... you'll see a terminal strip bolted to the chassis beside the PT. Those terminals are all isolated from the chassis and all circuit ground connections go there:

- cap board ground
- preamp bus ground
- bias pot ground
- power tube ground
- speaker jack ground
- HT CT ground
- filament CT ground
@gabasa - my apology for being pedantic ... simply put, the HT CT is the 0VDC reference ground; which itself is isolated from the safety earth. Only the input jacks have continuity to safety earth; and by extension, are isolated from the reference ground. Reading through @2L man posts, and now I get it. Cool!
 

gabasa

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@gabasa - my apology for being pedantic ... simply put, the HT CT is the 0VDC reference ground; which itself is isolated from the safety earth. Only the input jacks have continuity to safety earth; and by extension, are isolated from the reference ground. Reading through @2L man posts, and now I get it. Cool!
The input jacks are connected to both safety earth as well as 0VDC reference ground ... both.
Even so, there is no path in which the chassis can be used for return current.

What doesn't make sense to me, if anyone can help:

I have a 5E3 that had a pretty typical grounding scheme, 3-prong power cord correctly installed, and preamp filter cap grounds separated from power amp filter cap grounds. This ensured that the chassis was used for return current but was correctly wired as per "typical" wiring schemes.

This amp would shock me if I touched a mic at a certain local nightclub. However, after isolating the circuit grounds in this amp as I did in my posts above, the shocks didn't happen anymore. I don't understand why because the chassis was connected to earth in both cases.
 
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SerpentRuss

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The input jacks are connected to both safety earth as well as 0VDC reference ground ... both.
Even so, there is no path in which the chassis can be used for return current.

What doesn't make sense to me, if anyone can help:

I have a 5E3 that had a pretty typical grounding scheme, 3-prong power cord correctly installed, and preamp filter cap grounds separated from power amp filter cap grounds. This ensured that the chassis was used for return current but was correctly wired as per "typical" wiring schemes.

This amp would shock me if I touched a mic at a certain local nightclub. However, after isolating the circuit grounds in this amp as I did in my posts above, the shocks didn't happen anymore. I don't understand why because the chassis was connected to earth in both cases.
Just speculating here, in modern 3-wire AC wiring schemes (hot, neutral, ground), the neutral and ground are tied together at the distribution panel and ground is earthed with a grounding rod or a suitable substitute that meets code.

IF there is an outlet in this club that doesn't have three wires (a dedicated ground wire) it's entirely possible for hot and neutral to be swapped. Every piece of equipment will continue to work, but will have 120 VAC on the neutral instead of hot. Factor in equipment plugged into another outlet that IS properly grounded and wired, and you have 120 volt potential between the neutral of one outlet and the neutral of another.

It's hard to believe a situation like this exists in a commercial space, but I guess it could be possible. My suggestion is to buy a cheap outlet tester and keep it in your gig bag and test every outlet you use. It's also a good idea to check the AC voltage that's coming out of the outlet to make sure it's kosher. In many places like restaurants that have 3-phase service, 120 volts is being produced locally with transformers that are on site, or fed from 120/208 panels that may not have balanced feeds. 120 may be all over the place voltage wise.
 




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