Some grounding questions

[KGW]

That looks weird, I'm not sure what I am looking at.

You should have one main buss like a real piece of buss wire suspended off the chassis and only grounded to chassis at 1 place.
Each stage should have it's local star consisting of the supply decoupling filter cap with lets say the volume pot ground and the cathode ground that the volume pot is feeding and any grid resistors etc, these go to the filter cap ground and then 1 wire to the buss, take each stage like this. OT ground goes to the speaker jack ground, then 1 wire to the phase inverter ground and 1 wire to the buss, all jack isolated, You need TUT3 it is spelled out

Did my drawing show up? It was there when I posted, gone earlier, and now it is back.

Actually, you don't want a single buss going to a single point. I actually tried at least 5 different grounding schemes on a DIY 50W Marshall 1987. The best one turned out to be 2 ground busses: 1) PSU, Power Tubes and OT secondary ground -- the buss was grounded to the chassis at the first filter cap, and 2) Preamp -- grounded to the chassis near the input jacks. It is important to have star #1 and star #2 grounded to the chassis at opposite ends (Aiken elaborates on this).

Aiken suggests 3 stars, with the additional star for the PI and NFB. That is what I have shown in my drawing. In addition, I have shielded filaments (as in TUT1). All jacks are isolated (though my drawing does not show this).

And, yes, I do need to get TUT3 (and 2, 4, 5).

[drz400]

I finished re-wiring my other clone that didn't sound right. First filter cap ground next to but not on the CT of PT, common OT to isolated speaker jack and then to the same point as the filter ground, with the PI ground on top. Instant mojo, changed the character in a way I would have not thought possible. Of course in this build I could not have picked a worse ground scheme using a non-isolated jack and the whole chassis as a loop for the feedback. Doh! FYI this is a sort of clone of the "Blue" channel of a Bogner XTC in a 50-watt version. Sounds very close to the real thing.

What Bogner schematic did you use? Got it handy?

[bueller]

DRZ400, it's the one available on Schematic Heaven. It shows four gain stages which is the Red channel. To get the Blue channel eliminate the third stage, and to get the "plexi" mode eliminate the second and the third. All the values are correct except the bright cap on the gain pot is switchable 500p or 5000p not the 1000p shown and there is a 1000p across the 100K to ground in the voltage divider past the gain pot. I use a fixed resistor and cap on stage two and eliminate the "structure" switch as it sound best with it on. Basically this is a hot rodded JCM 800 with the option to switch to the model 1987 with a master volume.

For the power amp I just use about 375 VDC plate volts (a kin to using a variac at 90 volts on a stock 475VDC Marshall except the heater voltage is correct), 30 mF first filter in a 4x4 bank, NFB of 100K, and cathode biasing of two EL34 at 0.063mA each - fan cooled. The final touch was getting the grounding correct which makes this very reponsive and feels like the note is being produced before you pick it. Hard to describe. Just makes you want to play and play. I leave the XTC at home any more....

[drz400]

Actually, you don't want a single buss going to a single point. .

Sorry I got you mixed up with the original poster
If it works for you then cool but......
I DO want a single buss that goes to the chassis at one place, like the highway being the buss, each town goes to a star and has one road going to the highway. The traffic flows one direction and it is hard for someone to go the wrong way sort of kind of .
This has always created a 100% hum free amp for me and I have tried 3 stars to chassis many times, better than Fender or Marshall but not 100% to me.
All you are doing is taking your local stars to the buss instead of the chassis, the chassis is unpredictable, you dont know where the currents are going to run. Ask Randall, he will tell you there is nothing wrong with stars going to a buss. I have tried 3 stars many times to the chassis, works OK but not as quiet or predictable in my experience, I still say each star of each stage connected to the buss is the best I have ever heard with high gain circuits and this is what Kevin OConnor says is the best he calls it galactic ground, you probably will agree once you envision what I'm talking about. The way Fender and Marshall ground amps is the worst, doesnt work when you get high gain going.

1 path to chassis ground, make your ground predictable and keep currents in each stage local to the star 1 line from star goes to ground buss and take it to the input to eliminate RF from cabling or take it from the middle of the buss to chassis and use RF filter of .01 and 51ohm from input ground to chassis ground, keep 3rd pin ground away from audio ground, float your heaters to 75VDC or use regulated DC heaters. Do this and all hum will be a thing of the past, it has been for me. When I use this method with DC heaters on preamp tubes I have absolutely not one trace of hum even with very high gain circuits at any setting of the levels

Also this way you can have a lift switch to help in elminating ground loops when other gear is used

[straycat]

What grounding changes would you make to this layout.I have clarostat pots in my build and buss wire on pots is not an option.

[drz400]

What grounding changes would you make to this layout.I have clarostat pots in my build and buss wire on pots is not an option.

I wouldnt do a Buss on the back of the pots anyway, support it on some standoffs or attach to the board, IMO the buss should be separate from the chassis and only grounded where you need it to be, input jack and chassis at the jack is good. Third pin AC of course goes to chassis.
I would remove those grounds from chassis and tie them to the buss at the point that they decouple so the filter caps should not all be tied together Just run the first bank to the same place as the PT ground and then go from the filter cap to the buss (begining of it)
Each stage that is decoupled from the supply with a filter cap that should be your star point, run the cathode and any grid grounds feeding that point to the star, then go to buss, keep tem all in order on the buss and run the power tibe grounds to that main filter ground, run the OT ground to the speaker jack as you have it and then run the ground to the same point on the buss as the Phase inverter ground (your presence pot ground) That is my last opinion for a while. Too much other work to do
Later

[KGW]

If it works for you then cool but......

Thanks. I will get TUT3 soon. Until then, I'll reserve comment. I do know that my 1987 was the most hum and buzz free with separate preamp and power busses (grounded at opposite ends of the chassis). My newly completed Express has 3 busses grounded at 3 points on the chassis. It is also hum and buzz free.

You can do a ground lift this way, but it gets more complicated. Of course Kevin O'Conner likes fancy switching.

Also, I have done a lot of DIY tube hifi gear (my whole system and more). Unfortunately, what works for hifi does not (always) work in guitar amps. I have star ground in all my hifi gear. The stereo preamp has each channel lifted 10 ohms from the power supply ground. This works very well. I tried a similar ground lift in a guitar amp and it hummed like hell.