is this JTM45 layout missing a ground?

[mikepukmel]

Going through this layout: the input jacks, preamp cathodes, vol control, are all tied to a 'star' like ground on the back of vol pot 2. Then, a wire is run from the back of vol pot 2 to the ganged 32uf/32uf filter cap. So, at this point, the "ground" of the preamp section is floating with respect to the chassis.



Knowing the company, this has to be by design? But don't you need to have that common "A" lug of the 32/32 cap can also tied to the chassis someplace (preferably far from where the power transformer is grounded).

I look up the mounting for these caps and that A lug is not tied to the case its just the common of the 2 x 32 caps.

all of the voltages in the preamp will be with respect to the common side of the 32/32?

Thanks
MP

[Dave H]

Isn't it grounded by being soldered to the back of the vol pots?

[Chuck H]

Only if the volume pots are making good contact with the chassis. I've seen some amps do this. I guess it's not much different than a ground bolt for aluminum chassis.?. Except in that case you would use a toothy washer and some anti ox grease. I wouldn't rely on pot to chassis contact alone, but I just looked at the Ceriatone layout and it's the same thing, except they run one long buss across all the panel controls. So it could be a nod to authenticity.?. I'd install a ground right near the input. But only with the TAD layout. If you installed a ground with the full buss wire across all the pots you might end up with a loop.

[Dave H]

Only if the volume pots are making good contact with the chassis.

That's true. I've fixed quite a few amps just by tightening the pot nuts.

[mikepukmel]

Thanks everyone.

I thought that many of these aftermarket kit amps modeled after Marshalls anyway, had plastic spacers on all of the pots and input jacks to keep them isolated from the chassis. If they are relying on the mechanical connection between the front of the pot casing and ALL of those ground connections . . . If that's not the case, then why would they make a star ground on the back of the vol pot? its fairly ugly looking and wouldn't it be a pain to solder all of those wires onto the back of a pot, Wouldn't it be better to route all of the ground wires, and the wire leading to the common lead of the filter caps, to a "star" on the chassis, the position of the chassis chosen to minimize the total wire length of all the ground wires?

[mikepukmel]

Only if the volume pots are making good contact with the chassis. I've seen some amps do this. I guess it's not much different than a ground bolt for aluminum chassis.?. Except in that case you would use a toothy washer and some anti ox grease. I wouldn't rely on pot to chassis contact alone, but I just looked at the Ceriatone layout and it's the same thing, except they run one long buss across all the panel controls. So it could be a nod to authenticity.?. I'd install a ground right near the input. But only with the TAD layout. If you installed a ground with the full buss wire across all the pots you might end up with a loop.

Thanks Chuck. I found some photos of a very good looking build on a facebook amp builders page, of this amp. It looks like they ran a bus wire across all the pots, and had the bus end at this pot, the far right vol control (vewed from the underside) the one with all of the grounds connected, but I didn't see any wire leading to something firmly connected to the chassis. (will check on that). Will ask the builder as well.

I could not quite make out the cereatone layouts since one at least had a whole bunch of wires in green that just had text next to it "to star".

[mikepukmel]

So one reason for asking all this is I want to build a JTM45 type amp. There are folks out there who make steel chassis, and I can screw and solder a couple or 3 lugs down to the chassis in 2 places, one near the PT for power supply grounds, and one near the input jacks for the preamp/low signal grounds. No wiring on the back of pots that might come lose. But there are a ton more suppliers of aluminum chassis, and they seem to be cheaper.

Is it worth it to buy the steel chassis if only for the ease of making more permanent tie points on the chassis?

[Leo_Gnardo]

Wouldn't it be better to route all of the ground wires, and the wire leading to the common lead of the filter caps, to a "star" on the chassis, the position of the chassis chosen to minimize the total wire length of all the ground wires?

Yes, that. Or do like Randall, that stick of thick copper wire in his Deluxe Reverb photo makes a dandy ground bus. In Marshalls one thing I long ago learned, to not do like them, is ground the bias supply on the circuit/controls ground bus. Fly a lead from the bias supply ground over to the general power supply chassis ground & that's one less source for buzz in your audio.

[Chuck H]

I actually use three ground points. One near the input for the preamp and it's HV filters, one near center for the PI, reverb (if present), trem (if preamp type), presence and associated HV filters, and a third at the other end of the chassis for transformer CT's and finals, main filter and screen filter, trem (if bias vary) power tube cathodes, bias supply, etc. I know this isn't ideal. I've been told that a single star nearer the input for everything is actually best. I started doing it like this and it's worked so I haven't changed it. Oh, no daisy chains! Everything grounded has it's own ground lead to the specified point. Even if two eyelets side by side are to be grounded I don't just share the eyelet and run one ground lead. I run one for each. This can get pretty ugly unless you plan for it, and it's not necessary in all circumstances. There are indeed places where it's ok to share a ground. But unless you know for sure, individual grounds is a safe and sure thing.

EDIT: Oh, and you can do A LOT better than Marshall by not using the pot backs on a buss. That basically means you're grounded at six or seven places along the front of the chassis and who knows which may, or may not have good contact. Even then, if you provide a dedicated ground point for the buss you have ensured a ground, but you've also added another ground point on a buss where all grounds are shared. Little loops are possible. Skip the buss and tacking to the pot cases and use one point and individual grounds. You're Marshall will hum and buzz less than any other.

[Dave H]

In Marshalls one thing I long ago learned, to not do like them is ground the bias supply on the circuit/controls ground bus.

I'd connect the bias supply ground directly to the reservoir cap -ve terminal. I think the CT of the high voltage winding should also be connected directly to cap -ve.

[mikepukmel]

Thanks everyone, reading and re-reading. So, the layout posted above sure isn't as good as it can be re grounding. Maybe they were thinking about easy to build, for their kits, rather than the most quiet. I like quiet!

[mikepukmel]

Yes, that. Or do like Randall, that stick of thick copper wire in his Deluxe Reverb photo makes a dandy ground bus. In Marshalls one thing I long ago learned, to not do like them, is ground the bias supply on the circuit/controls ground bus. Fly a lead from the bias supply ground over to the general power supply chassis ground & that's one less source for buzz in your audio.

Leo, I read this schematic too long, my eyes are tired. Any everything else! They have the ground side of their bias supply wired to the back of the presence pot, and the back of the presence, bass, mid, treble are wired together with bus wire. I had thought that most Marshall kits had those little isolater spacers between the pots and chassis, and also the marshall input jacks were also isolated from teh chassis. But this build, they must be using the connection between the pot and chassis for electrical connections since all those wires go to the back of the pots? It sounds like this layout has many, many of the things you guys said not to do across many posts. Maybe the worst thing is to grund the bias to the back of the presence pot. You're recommending that wire goes to the lug they have near the corner of teh PT and above the big PS caps?

So, in my next build, that will be something like this thing, dosen't matter if the pots are not isolated as long as I don't use the back of the pot to ground stuff to?

[alexradium]

Real Marshall has a wire from the pot bus to the preamp filter cap, which is tied to chassis, like all other caps.

[mikepukmel]

Thanks Alex.

[mikepukmel]

To this layout:
1) Remove the wire going from the bias ckt to the back of the presence pot, instead run the bias 'ground' wire to the star near the power supply.
2) Remove all wiring from the back of the pots, and run them on a little bus bar, hanging off the pot side of the turret/eyelet board.
2b) Run all the 'ground' wires in the preamp to the bus bar. Wire the bus bar to the preamp - cap lead. (OK leave the big bus wire, that prevents the pots from spinning around if the nut loosens, but don't use it as a "ground". )
3) Remove the wire going from the preamp cap negative lead to the back of the vol pot.
4) Then, run the preamp cap negative lead to the star over by the power supply.
5) Move the grid stops to the preamp socket pins, then run shielded wire to the input jacks (grounding on one side of the shielded wire)

Did I miss anything? The kit I got has cliff jacks for in and out so are isolated from the chassis.

Should V2 (second stage pre) and V3 (PI) have grid stops on pin 2 and 7? Is the size of the input grid stop good enough?

OOps, can't believe I missed it: add a resistor between the output tube cathodes and ground to assist measuring bias. Run the other end of the 'bias measure assist' resistor to the start near the PT.