Brass grounding plate

[soundmasterg]

Thanks for all your help guys so glad I found this forum and that people are so willing to offer such solid advice really helpful things are starting to make sense now
Scott

You're welcome! Keep practicing and reading and learning and you'll get better.

Greg

[Jarvini]

The only bit I don't understand is this:-
If I isolate all the jacks and pots ( I take it switches too) from the chassis then the pots shells will not be earthed do I have to run a wire from the pots shell to the ground bus as well also the output jacks will not be earthed so these will also have to be attached to the ground bus which will mean running the ground bus round to the back of the amp as well like a large horseshoe or can these be grounded elsewhere and as I am using 4/8/16 outputs should the ground be from the first output jack (4 ) or from the last (16)
I know the ac input has it's own ground and that the centre taps from the pt have there own near the pt but do all the others connect to the bus in succession as if it were the brass plate I.e input first large filter cap last
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[Dave H]

The only bit I don't understand is this:-
If I isolate all the jacks and pots ( I take it switches too) from the chassis then the pots shells will not be earthed

The input and output jacks can be isolated but the pots and switch shells should be earthed (bolted to chassis). It's as soundmasterg said in post #24

"I've never bothered to solder to the back of pots. I've done it in guitars but in the amps there is no point as if mounted correctly with the serrated washer and the nut, it will ground the case of the pot to the chassis."

[soundmasterg]

The only bit I don't understand is this:-
If I isolate all the jacks and pots ( I take it switches too) from the chassis then the pots shells will not be earthed do I have to run a wire from the pots shell to the ground bus as well also the output jacks will not be earthed so these will also have to be attached to the ground bus which will mean running the ground bus round to the back of the amp as well like a large horseshoe or can these be grounded elsewhere and as I am using 4/8/16 outputs should the ground be from the first output jack (4 ) or from the last (16)
I know the ac input has it's own ground and that the centre taps from the pt have there own near the pt but do all the others connect to the bus in succession as if it were the brass plate I.e input first large filter cap last
��

The jacks should be isolated from the chassis with a ground buss as the signal coming into the amp is bouncing around ground, up above it and down below it, just like a sine wave on a scope. If you want to avoid noise and hum, then isolating the jacks helps you towards that goal when using a ground buss that only contacts the chassis at one point. The pots and jacks can ground with bolting them to the chassis with serrated washers without issue as there is no signal on the cases of the pots and switches. Make sure there is a good mechanical connection and that is all you need. Some of the pot terminals are grounded depending on their location in the circuit, but that is a different thing entirely than the case of the pot. Just ground those at the appropriate point along the buss.

Regarding the buss grounding to the chassis, many say to ground at the input because you can then shunt any noise on the input to ground right away before it enters the circuitry in the amp, however I have instead grounded the buss at the main cap ground successfully without any issues and a very quiet amp in terms of hum and noise level. Each preamp stage and the phase inverter stage had their filter caps mounted right near the stage with a local ground node for that stage, and then each of those nodes grounded to the buss in order. I used isolated input and output jacks, and the ground for the input grounded at the beginning of the buss and the output jacks grounded where the buss contacted the main cap, and that is also the only place where the buss grounded to the chassis. The amp also had a safety AC ground point that was a different place from that buss ground. I grounded at the main cap in this amp for two reasons. The first is that an audio engineer friend of mine who has been an engineer for 50 years told me it is fine to ground there, and the second is that a Conn organ amp that I have modified grounded there with a buss and it was very quiet. I believe in Merlin's system he does the same thing essentially as far as local cap nodes, grounding to the buss in order, isolated jacks, etc., but grounds at the input instead. He also grounds the main cap to the buss rather than having a separate ground. I'd have to read his book again or look at his site again to be sure but I believe that is the case.

As you can see there are many ways to do the same thing, which is to minimize ground loops and hum and noise, and several of them work well. The key is to use a ground 'system' and to stick to it fully in your build. I've experimented with different methods of construction in the amps I've made and with each one I refine what I am doing and how I do it. The first amp I modified was a Silvertone 1484, which is a horrible layout from the factory. I isolated the grounds on that one in order to get some control over where I was grounding, but I didn't use isolated jacks. I also upped the gain and modified channels, added Fender reverb, etc., and with that amp I discovered there is only so much you can do to correct poor layout choices like putting the power transformer in the middle of the chassis right behind the inputs to channel 2 like they did! That amp becomes very quiet if I carefully stick some Mu-metal between the input to channel 2 and the power transformer, but Mu-metal is very expensive. Later I modified a rebuilt a Sunn 200S and used complete ground buss, jack isolation, and a circuit ground lift switch as Kevin O'Connor recommends. That one works well and is very quiet and can be used in combination with other amps at the same time with an overall lower hum level due to the addition of the ground lift switch. That one was a LOT of work to accomplish as modifying an amp to have a true isolated ground is harder if the amp was already made with random chassis grounding than if you build it from scratch with proper grounding from the beginning. Later I made myself a true PTP amp using a gutted Bogen chassis and transformers with my own circuit and layout. True PTP requires careful layout before the fact so you can plan everything including the grounds. That amp suffers from the Bogen transformer placement (right next to each other with nothing turned so there is a little more hum than I would like) but overall it works well and many people have asked me to build them one.... maybe someday I will but I'm still experimenting for now. I also built a Vox AC100 clone with an added higher gain channel, using turrets and a board and a isolated buss that grounds at the main filter cap and isolated jacks. This amp's hum level and noise is VERY low. I've also modified some vintage amps optimizing things but have kept the random chassis grounding and many of these are quiet too. So I guess in summary, it helps to build, no matter what design you are using, and you can get the hands-on learning started sooner.

Greg

[Jarvini]

Thanks guys it's a bit of minefield the subject of grounding I'll draw out a schematic showing what I have planned using all the information I have digested and post it here to make it easier to see and if you good fellows fancy it tell me what your opinions are?

Thanks
Scott

[soundmasterg]

Thanks guys it's a bit of minefield the subject of grounding I'll draw out a schematic showing what I have planned using all the information I have digested and post it here to make it easier to see and if you good fellows fancy it tell me what your opinions are?

Thanks
Scott

Pictures are always helpful!

Greg