Tube Amplifier Chassis Grounding question

[occam25]

Hello,
I am building my own tube amp based on the fender Princeton reverb schematics and I have a doubt about grounding.

I want to follow the grounding methodology shown in this diagram, that looks simple and nice:
http://el34world.com/charts/images/ChassisGrounds.png

Regarding that diagram, I understand that all jacks are not isolated so they are grounded by being in contact with the chassis (maybe the speaker jacks it the diagram are isolated so they need a wire to connect their ground to the main power transformer ground bolt), is that correct?

If so, wouldn't it be better to use isolated jacks and ground them with a wire so you can have control of the return path instead of let it go freely through the chassis? would that help in any way to reduce ground loops and noise?

Thanks!
Javi

[bnwitt]

Are yo using a Fender or Hoffman layout.  Whichever one just follow the particular grounding pattern on the layout. 

[shooter]

the link you pointed to works!
the BIG take-way, keep the preamp grounds separate from the power amp section.

speaker jacks need to be grounded if you have NFB, otherwise there is no need.
Isolated jacks are typically plastic, guitar players are typically hard on jacks, metal ones with good star washers wins.

at the top of this page you'll see a search type in grounding n watch your head spin n brain swell

[SoundmasterG]

It depends on the ground scheme that you are using. If you are using the Hoffman scheme, then it has particular points of the circuit grounding in particular places so the currents from each will not interfere with each other. This method does not require isolated jacks. I personally (and many others) don't agree with grounding to a transformer bolt...you should ground to a separate chassis bolt...but it will function if you ground to a transformer bolt....it just isn't best practices from an engineering and safety standpoint.


Another approach is to use Kevin O' Connor's galactic ground system or Merlin's similar system and these require isolated jacks. They have localized ground nodes that all sum at one point, but in order to accomplish that, the isolated jacks are needed and you can't use can caps unless you parallel the sections or unless the can cap has individual grounds.


Note1: You don't want to mix and match ground schemes. Stick with one and implement it completely and correctly.


Note2: In all cases the safety ground should ground to the chassis with it's own bolt within an inch or so of the AC line coming into the chassis and nothing else should ground at that same point. It should also be wired up with some slack so that in case the cord got pulled out, the last thing that would be contacting the chassis would be that safety ground wire.

Greg

[PRR]

Welcome.

Your cited diagram is Hoffman style.

There are many-many ways to do it. Pick one known-good plan and follow it *exactly*.

> maybe the speaker jacks

In Hoffman, those are also bare-metal to chassis. This is not a "ground loop" because it is isolated by the OT.

The speaker jacks do not "need" a ground to work. They need ground for any NFB to work, and they NEED a ground in-case the OT shorts and puts B+ onto the speaker winding (if grounded, the OT will burn/melt and save you from shock).

[92Volts]

I think in theory that "should" help, yes.

Other grounding schemes recommend the main/central ground should be physically close or connected to the input jack so any ground path through the chassis is similar to the wired ground path.

The actual effect depends a lot on the layout of course. Non-isolated jacks are used a lot because the standard metal jacks and cheap, decent, and small. Some cheap plastic ones can be bulky (or non-standard sizes) and don't feel as solid.

[occam25]

Thanks a lot for the answers!

I am not using any specific layout, I just took a fender Princeton reverb schematic, made some improvements like adding a mid control, a bright switch, a NFB switch, a better bias control and PT... and then I designed a PCB to build it.

The first version sounds awesome now, but at the beginning I had some ground loop issues that I had to solve. Now, I am doing a second version of the PCB and I want to rethink the grounding scheme to avoid that kind of problems.

Ok, I will use Hoffman grounding scheme. It is simple and, as 92Volts says, non-isolated jacks are cheaper and more robust. It will also simplify the PCB.

I am going to put a choke also. I know chokes are usually used in bigger amps, but B voltage feeds the reverb transformer directly and it is not fully filtered so, when I turn the reverb up I get some noise. Nothing important, but I think I can make it much quieter. I am gonna use 47uF, 100uF, 22uF and 22uF for A, B, C and D caps respectively and a 10H-50mA choke (between A and B). Doing some tests, with those values I get 52mV of ripple voltage instead of the 404mV I have now with original fender filtering. -> please, comment if you think I am crazy!

Javi

[bnwitt]

and then I designed a PCB to build it.
Javi

Aaack!  the P word

[mresistor]

Aaack!  the P word

Double Aaaack!! 

[pompeiisneaks]

Oh come now, PCB's in and of themselves aren't horrible, just most people design them terribly, with the cheapest components so the traces never last for a resoldering etc.  I'd wager there are still some pretty amazing early generation Mesa amps out there on PCB's that are outstanding, I just think Mesa got a bit too busy packing too much into their amps for their own good now so they're a nightmare to work on, but their PCB design has always been rock solid. Fender and Marshall, OTOH have sucked for a long time ;)

~Phil

[occam25]

I see, PCBs are not welcomed in vintage world! I can understand that.. Ooops 
but pompeiisneaks is right, PCBs are only a problem when they are poorly designed and cheaply manufactured. If you do it right, taking care about track widths, clearance, placement, layout.. and you use a good manufacturer the results can be excellent and, when it is done, the build process is easier.

Say hello to XXI century! 

[sluckey]

the BIG take-way, keep the preamp grounds separate from the power amp section.

Absolutely!

I see, PCBs are not welcomed in vintage world! I can understand that.. Ooops 
but pompeiisneaks is right, PCBs are only a problem when they are poorly designed and cheaply manufactured. If you do it right, taking care about track widths, clearance, placement, layout.. and you use a good manufacturer the results can be excellent and, when it is done, the build process is easier.

I totally agree. But there's another side. Many people involved with tube type projects shy away from any pcb because they simply don't have the skills to properly work with pcbs. It's easy to destroy even a quality board with too much heat or lack of technique. Also, many who are involved with tubes love to tinker and modify stuff. PCB are not easy to modify. And most pcbs found in consumer equipment such as modern tube amps are cheaply built using poor quality materials that will fail frequently in a high heat tube circuit. High current heater circuits that are very reliable in old hand wired construction are ticking time bombs when strung through cheap molex connectors and thin pcb traces.

[mresistor]

I totally agree. But there's another side. Many people involved with tube type projects shy away from any pcb because they simply don't have the skills to properly work with pcbs. It's easy to destroy even a quality board with too much heat or lack of technique.

Additionally even if they have the skills they may lack the equipment such as an appropriate solder station like this.. https://www.paceworldwide.com/products/conductive-rework-systems/prc-2000/prc-2000th-through-hole-miniaturemicrominiature-electronic-repair. When we worked for uncle sugar he provided us with similar equipment and the essentials to go along with it.

[occam25]

the BIG take-way, keep the preamp grounds separate from the power amp section.

In the first page of Hoffman princeton reverb pdf http://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf the first triode of the V4 tube is powered with B+ and it's ground is the "preamp" ground while the second triode is powered with D+ and it's ground is the "power" ground... shouldn't it be the other way around? (the one with B+ grounded with "power" ground and the one with D+ grounded with "preamp" ground)

[shooter]

the first triode of the V4 tube

I'll take a stab, but......I could be wrong

V4A is the trem osc, you need a strong tube, probably higher plate volts than say a V1 tube, but it  will probably be sensitive to noise/AC ripple and should be grounded with the pre or on it's own "tap".  V4b is the PI, any "extra" noise/ripple AC will/should get canceled by the fact it's driving a PP PA.