Buss grounding scheme

[booj]

This is my take on a 2204 design. It had a low 60 HZ hum that got in the way if the amp was cranked when it had a star grounding scheme. When I put in the buss grounding I had a heck of a time trying to figure out how to draw it out so it would make some kind of sense. the BN designators mean "Buss Node". I made the high voltage connections nodal as well as the ground connections (on the drawing). No pics of the chassis. It ain't too pretty after 7 years of re-works. It does however work quite well. I'll probably build another one that's more perfect.
I'm still puzzled about how to design the "perfect" input circuit, since that seems to be so critical in terms of getting rid of the hum. I've got a Mesa Boogie in the shop. Maybe I'll take pictures and get a clue from that. Say what you will about Mesa's, but they don't seem to have any problems with hum.

[R.G.]

It had a low 60 HZ hum that got in the way if the amp was cranked when it had a star grounding scheme.

That means the star grounding system was not implemented correctly. They *are* a lot of work to do properly.

[I'm a beginner!]

That means the star grounding system was not implemented correctly. They *are* a lot of work to do properly.

It is possible that it was implemented incorrectly, but it is also possible that the problem was not the star ground. If for instance the instrument to be amplified was picking up a very low level 50 or 60hz hum from an external source - well, this being an amplifier that hum would be amplified. If it is a very low level then it may not be noticable until the amp is "cranked". Not saying the star ground was or wasn't implemented properly, just saying that the hum may have had a different cause.

[Raybob]

Sounds like a grounding problem in the instrument, if it's picking up a hum that's being amplified. No amp grounding scheme will cure that.

[R.G.]

It is possible that it was implemented incorrectly, but it is also possible that the problem was not the star ground. If for instance the instrument to be amplified was picking up a very low level 50 or 60hz hum from an external source - well, this being an amplifier that hum would be amplified. If it is a very low level then it may not be noticable until the amp is "cranked". Not saying the star ground was or wasn't implemented properly, just saying that the hum may have had a different cause.

You are right. There are many, many sources of hum, not just the grounding scheme. I was going on his note that it hummed with star, seemed to be cured with a buss system. It is entirely possible that a side effect of changing the grounding system cured an unrelated hum source. Methinks you're not such a beginner.

[booj]

R.G. , Raybob, I'm a beginner! - Thank you very much for your replies and interest! The star grounding scheme consisted of collecting all the different ground points from the circuit and wiring them to one spot, which was close to the P.T. The filament virtual ground point was close to the power tubes as the resistors were connected to one of the power tube sockets. Then the input jack ground was tied to the chassis close to the input jack. I should have pointed out that with no instrument plugged in and the amp cranked the hum was just too loud, in the order of almost 900 mv. Of course, with the volume and gain set for real world needs the amp was actually pretty quiet.
In this new version, I made sure that all the ground points were connected in order from the input jack ground to the final P.T. ground point. The only chassis connection is very close to the input jack, and the virtual filament ground is connected to the power tube cathodes. The amp is extremely quiet now with the majority of the idle noise being hiss. There still is a little trace of hum in the background when the amps master and preamp are turned all the way up, but dang! She's useable now.

[booj]

Guys, I'm a little dissapointed. 7 years of work to get an amp to sound good and all I get is I must have plugged in a hummy guitar or I just got a bad star ground system hooked up? Nobody even noticed the unusual cathode biasing system or made a comment about why I used those particular resistors for the screen supply, or why I have a rectifier emulator. Oh well, that wasn't the question. I've done star grounds out the wazoo on this thing and the bus ground worked like, right now. Nor was there any comment about my puzzlement about the "perfect input grounding system. c'mon peeps. Is this the best you can do? I'm really interested in this stuff. I guess after I crack open the Mesa and take pictures ......

[Enzo]

I have to admit, if you ask me about hum, I am not going to look at the screen resistors. And I don;t see an emulator, I just see two diodes. Hum comes from all over the place. You do want to minimize hum through the inpuit stage, but you also need to watch out for ungrounded pot bodies, distributed ground issues such as were discussed, radiated and coupled grounds, heater-cathode current hum, etc etc.

[trobbins]

booj, tracking down hum/noise performance issues (when hum is fairly low already) is going to need experience and tools. Not an easy way forward - as I'd be connecting up my soundcard spectrum analyser to start sleuthing, and making sure the power supply and heater and output high level signal, and input low level signal layouts were all ok (photos would assist others to check that aspect fairly quickly).

[Steve Conner]

Does the hum go away when you short the input jack out? It can be difficult to get rid of the last little bit.

[R.G.]

Yep. Especially when some tubes have hum from their heaters.

Some really-devious sources of hum can include wire-routing, connections of the split heaters on duotriodes, and magnetic hum induction. Incomplete shielding and non-ferrous chassis construction can let hum in because the magnetic field of the air around us can induce hum in a loop of signal wiring which couples to a stage or around several stages.

It is possible for an amp to hum with no tubes in it at all if you're unlucky about how the power transformer and output transformer are arranged. The OT can act like a pickup for the magnetic leakage of the PT and drive the speakers a little with that.

The hum monster has many teeth.

[Dave H]

...The star grounding scheme consisted of collecting all the different ground points from the circuit and wiring them to one spot, which was close to the P.T. ...Then the input jack ground was tied to the chassis close to the input jack.

That "one spot, which was close to the P.T." wasn't a chassis connection was it? It could well hum if connected like that as it's not a proper star ground scheme with the input jack ground also connected to chassis at a different point.

[loudthud]

Your grounding scheme looks about as good as it could get if I understand it correctly. For instance, there is a line connecting BN1 and BN2 in the preamp, and an additional connection in the power supply. Is this the way it's wired or is there just one wire connecting those two points?

First test. Turn up all the pots. Now flip the standby switch from Play to Standby. Does the hum stop immediately, or does it fade out as the caps discharge?

What pots change the volume or tone of the hum? Does removing V1 or V2 kill the hum?

Have you tried a hum balance pot?

A stack of spade lugs on a transformer bolt does not make a good star ground. The order you stack the lugs in will make a huge difference.

[booj]

Whoa whoa whoa! Boys! Now I'm embarassed! I DO NOT have ANY problems with hum! I'm trying to tell you that the hum problem was fixed when I switched over to the buss grounding system. You know, my literature teacher didn't like the way I wrote and now I'm beginning to understand why!
Enzo, I like your point about ungrounded pot bodies. Never thought too much about that. The layout as best as I could make it keeps radiated hum out of the picture for the most part, with high voltage high signal content plate resistors mounted by grounded parts whenever possible instead of sensitive input signal carrying parts. That also may address Mr. Trobbins point about layout. I don't put high current items anywhere close to signal carrying items. Steve, you're close to what I'm really curious about. Perfecting the input circuit. R.G., at one time I did experience the "no tubes still hums" phenomena, but I moved the trannys apart and that is not an issue. (but I know what it sounds like). Dave H, you may have hit the nail on the head. The original ground system before the star ground attempt was a typical Marshall-esque scheme using the chassis as a giant buss bar which actually worked just fine until I started trying to mod the amp and have a switchable gain change when I would plug into the "low" sensitivity input. that's when I moved all the grounds to one spot by the P.T. and had one ground connection by the insulated input jack. by the way, why is that bad? ( I know it is but why?). Loud thud, there is just one wire connecting BN1 and BN2. I put two drawings in, one of the power supply by itself, and one of the schematic to be able to show for one thing, the BN5 connection which is not shown in the schematic. Any residual hum I have now does indeed stop with V1 and V2 removed or when it's switched to standby or unplugged. The driver/power amp section is pretty much dead quiet. I haven't tried a hum balance pot. When the star ground was used I had one single washer with an extension for a hard-wire connection/doohikey thingy. What do you call those? Going to that was a soldered wire and all the grounds were soldered to that. I had no idea the way you would stack lug connectors at one point would make a difference. Yikes! For a good laugh, I will very soon post pictures and you all can fall about the floor holding your bellys whilst laughing so hard at how ugly this thing is inside. BUT... it does work. Maybe I'll post a video as well.

[Dave H]

...A stack of spade lugs on a transformer bolt does not make a good star ground. The order you stack the lugs in will make a huge difference.

A transformer bolt isn't ideal but why does the order matter LT?

I think he is saying his bus grounding is fine but the original star grounding was no good which was why I was looking for a fault with it.

I've just noticed that the OT secondary isn't grounded on the schematic. I don't know if that would cause hum but it can cause other problems.