bus bar and single point ground

[Geezer]

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Funkalicousgroove
Junior tube assistant
Posts: 38
(4/5/04 2:30 pm)
  bus bar and single point ground
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 I am building my first amp using a bus-bar to a single ground point. I have always used a star ground in the past but I've heard that a bus bar can reduce noise greatly. Here is the question: Every point of ground with the exception of the main Ground out of the wall is connected to the busbar, all the pots, cathodes, and filter caps as well as the bias supply. Is there any reason I Shouldn't connect the power transformer center tap and shield to the bus bar?? To avoid the question, the main ground is hooked to one Lug of the PT and the busbar is connected to the next lug.
 
idiotequed  
Hey get your own solder
Posts: 227
(4/5/04 7:36 pm)
  re:
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 what about the potentiometers and jacks? are they isolated from the chassis and connected to the busbar, or are they grounded to the chassis directly?
 
PHATamps
I only work on tube amps
Posts: 432
(4/5/04 9:46 pm)
  Re: re:
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 I've always had best results using Doug's advice, with 3 ground points on the chassis:

1) AC ground from the wall, usually at the nearest PT lug to where the cord enters, sometimes with a dedicated stud.

2) Power amp ground: PT center tap(s); heater reference resistors (if used); plate, screen and PI filters; bias supply (bias feed resistors in cathode bias); power tube cathodes. If your speaker jacks are isolated, ground them here also.

3) Preamp ground buss: all pots and input jacks; all preamp cathode networks; preamp filter(s). (To be really anal, ground the Presence pot at the power amp ground.) The buss wire is soldered to the back of the pots, which are bolted to the chassis -- no need to connect the buss bar to a stud or PT lug.

One caveat: when using an all-in-one cap can, where the preamp and power amp filters are grounded together, you might need to extend the preamp buss and connect it to the power amp ground. This solved a problem I had with one build, but in others using a cap can it wasn't an issue.
 
Funkalicousgroove
Junior tube assistant
Posts: 39
(4/5/04 11:34 pm)
  busbar setup
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 All of the pots are connected to the chassis, not isolated. The bus bar is setup is not connected to the body of any of the pots except for the bias control. I only have 2 points of ground: 1st one is main ac ground, PT center tap, pt shield, fillament reference. 2nd is everything else. I did not isolate any of the pots or jacks from the chassis. the busbar grounds to the chassis at the back of the bias control and at one of the PT lugs. I wanted to have as few points of ground as possible to avoid oscillation and ground loops. The amp is a bit noisy as it sits. The archetecture of the amp is basically ab763 with a few mods, a PPI master and a presence control, I know my lead dress is good so I'm guessing I have a ground loop-Any ideas?  
 
bnwitt
Senior tube assistant
Posts: 318
(4/6/04 9:56 am)
  Re: busbar setup
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 FWIW, I have stopped using the PT bolts as the power ground point due to hum. I have found that a separate drilled chassis ground with ring terminals and bolt, to be much quieter even at a mininimum of a half of an inch away from the PT bolt. I would use separate chassis grounds for the preamp section (including the pots), the power section , and the power cord ground(U.L. requirement)
Barry
 
Casey4s
Senior tube assistant
Posts: 158
(4/6/04 10:58 am)
  Re: busbar setup
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 I use a pair of buss bars, the bars are connected to each other with 18 GA wire.

Part of my last job was engineering grounding systems for telephone equipment and entire swith buildings. I use the Bell system principals when grounding my amps.

There are "providers" and "Absorbers" in a ground system, and the potential of each provider is evaluated for how it's placed on the bars.

...just my two cents....
 
Ritchie200
Senior tube assistant
Posts: 180
(4/6/04 12:42 pm)
  Re: busbar setup
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 Casey4s,
Would you care to expand with an explanation of providers and absorbers and how you adapt this theory to a high gain circuit? The reason I ask is that you must have had some success with this principle. I am just guessing, but I would assume that telephone equipment is pretty forgiving?
Thanks,
Jim
 
Casey4s
Senior tube assistant
Posts: 159
(4/6/04 1:06 pm)
  Re: busbar setup
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 Most of the extra attention to the grounding of Tele equipment came about when we went from mechanical to DIGITAL switching systems which is not very forgiving at all.

I'm in a hurry but I will elaborate on the info I posted earlier, later. (!?!)
 
Funkalicousgroove
Junior tube assistant
Posts: 40
(4/6/04 1:43 pm)
  Re: busbar setup
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 Yes, please elaborate, I'm assuming you're referring to reference points such as 100r's for the filament and the PT center tap, cathodes, bias supply ect vs circut grounds for the pots and such, but I'm always interested in a more technical explanation. As far as my problem goes I intend to separate my ground scheme into 3 points instead of two. I appreciate all the Help. I will also try drilling a separate hole for my main AC ground. I think my hum is caused by one of my preamp tubes after closer inspection and a bit of troubleshooting, One would hope that their Telefunkens wouldnt be noisy, alas I was wrong!!!
 
jbrew73
Junior tube assistant
Posts: 113
(4/6/04 8:30 pm)
  Re: busbar setup
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 i loosely follow dougs ground scheme. i think the key is the main ground and the preamp can vary slightly with good results. i have prototyped many different amp models in makeshift chassis' cake pans old stereo chassis' you name it i've used it. most have been built out of used parts (filter caps, resistors,signal caps, trannies). all of my amps, none higher gain than a marshall or vox, have been very quiet to my surprise. i always ground the center tap, power tube cathodes, heater tap or 100 ohm resistors to the transformer bolt and everything else wherever i can, the chassis, back of the pots, tube socket mounting screws. the only noise problems have been from wiring mistakes or something else that was hooked up wrong or whatever. i guess i've had dumb luck so far.
i do believe in Hoffman's law #1:
You have hooked up something wrong, or else the amp would be working!
 
Funkalicousgroove
Junior tube assistant
Posts: 41
(4/7/04 10:14 am)
  Re: busbar setup
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 Ok guys, I've narrowed my hum down to the reverb recovery circut using Gerald Weber's stage isolation method (I pulled preamp tubes one at a time until I found which one made it hum.) My noise seems to be coming from a 120 cycle source. One of the cathode bypass caps is a NON-polarized 25uf 50v that I used by mistake, I can't imagine this would cause a hum- I moved around my filament wires, that didn't help so I'm hoping it's that 1 cap-
 
GroundhogKen
Forum Moderator
Posts: 2342
(4/8/04 6:06 pm)
  Re: busbar setup
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 What type of amp is this? AB763?

Does the reverb pot affect the hum? ( Does the hum go away if reverb knob is off? )

Is the reverb return end of the tank near the power transformer?


Ken

Edited by: GroundhogKen at: 4/8/04 6:07 pm
 
Funkalicousgroove
Junior tube assistant
Posts: 45
(4/9/04 12:07 pm)
  Re: busbar setup
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 The amp is basically ab763 with 1 channel, no trem, and 3 knob reverb. It is scratch-built but it basically follows fender layout. there is a 3-knob reverb control(Dwell, mix, tone). I have yet to actually hook up a reverb tank, but a shorting plug makes the same HUMMMMMMM (Which does not go away, but the reverb mix knob makes it quite a bit worse). the only thing that stops the hum is removing the 12ax7 from the reverb recovery. I get the same hum using a jumper wire from the output of the .047uf after the 2nd gain stage to the .01uf at the input of the PI. I'm assuming that the issue is a ground. I talked to the local guru Lord Valve of NBS electronics who recently had a similar problem doing a bus-bar ground on a re-issue super reverb he converted to P-T-P. He suggested Isolating the input jack and the Reverb jacks with fiber washers. That sounds like quite a bit of work so I'm open to suggestion.    
 
bnwitt
Senior tube assistant
Posts: 323
(4/9/04 1:03 pm)
  Re: busbar setup
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 It's really not a good idea to crank up that amp without the reverb tank load on it.
 
Funkalicousgroove
Junior tube assistant
Posts: 47
(4/9/04 1:43 pm)
  Re: busbar setup
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 why??????????
 
bnwitt
Senior tube assistant
Posts: 325
(4/9/04 2:15 pm)
  Re: busbar setup
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[Geezer]

Re: busbar setup For the same reason it's not a good idea to crank up a tube amp without a speaker load on the OT. Pretty much the same set up with the tube on the primary side of the reverb transformer and the tank on the secondary. It will cause high voltages in that trannie that could short it out as I understand it.

Funkalicousgroove
Junior tube assistant
Posts: 48
(4/9/04 2:45 pm)
      Re: busbar setup gotcha!! I've only turned the verb up once but thanks for the warning!!! last thing I need is to wire in a new tf!!! otherwise All the knobs are down to 0 except my PPI master ala Ken Fischer.:beerchug

GroundhogKen
Forum Moderator
Posts: 2344
(4/9/04 9:20 pm)
      Re: busbar setup I made a little RCA plug with a 5 watt 10 ohm resistor soldered onto it. I plug it into the reverb send jack when I have a chassis powered up on the bench. It's the same idea as having a speaker load on the output. I don't know if the 12AT7 and the reverb transformer would really arc over like an unloaded output stage, but my little dummy load was simple to make so what the heck.

A shorted RCA plug is handy for the tremolo footswitch jack as well. You can plug it into the reverb return to see if it squelches the hum.


Ken

LooseChange
Senior tube assistant
Posts: 235
(4/10/04 7:11 am)
      Re: busbar setup When I built my AB763, I had the same problem. Also, I have noticed this problem crops up every once in a while and is in other threads on this forum. My first attempt was adding some grounding directly from the ground on the jacks to the cathode bias ground of that stage. It worked pretty well. If you are using the Bus bar this should have been natural. My amp is now grounded completely with a Buss-bar and ONE connection to the chassis. Everything is isolated from the chassis except for the pots for shielding purposes. This amp IS quiet. Dan
(BTW, how do you spell BussssBarrr??)

Funkalicousgroove
Junior tube assistant
Posts: 53
(4/11/04 3:32 am)
      Re: busbar setup Taking the advise not to blow up my reverb tranny, I hooked it up to the tank with no change in hummmm. I think I'll try isolating the input jack and the reverb in/out from the chassis. One friend said that he had to isolate all of his rca jacks and connect them together and then run one wire to the buss bar. He also said that he had to take a jumper lead and connect one end to the rca bar and then run the other end up and down the buss bar to find a spot that didn't hum. I read on another thread that someone used a 1M and a .0022uf cap in series on the reverb driver between the plate and grid to create a local feed-back loop and solved his seemingly similar hummm. Thats the beauty of what we do, a perfect marriage of math, physics, and VOODOO!:potion

LooseChange
Senior tube assistant
Posts: 238
(4/11/04 8:38 am)
      Re: busbar setup Hey Funk...,
Take my advice, connect the ground from the isolated reverb jacks to the cathode bias ground of the reverb recovery stage.

Funkalicousgroove
Junior tube assistant
Posts: 54
(4/12/04 10:32 am)
      Re: busbar setup I'll try that and post the results, although I put the jacks in the air away from the chassis and ran jumper wires to the buss bar with no effect on hummm.

Funkalicousgroove
Junior tube assistant
Posts: 55
(4/12/04 2:24 pm)
      Re: busbar setup In reply to loose change, all of my preamp cathodes are connected together at the ground point on the board, and then one bar connects that point to the bus bar. Should I connect that point to the reverb ground or should I just connect the reverb ground straight to the main buss bar?

LooseChange
Senior tube assistant
Posts: 239
(4/12/04 5:26 pm)
      Re: busbar setup I would go with the exact location of the Cathode Bias for that stage. If they are all together in "big lump" that's were I'd put it.

Funkalicousgroove
Junior tube assistant
Posts: 60
(4/12/04 5:45 pm)
      Re: busbar setup I'll let you know......cross your fingers!:flush

Funkalicousgroove
Junior tube assistant
Posts: 66
(4/16/04 9:56 am)
      Re: bus bar and single point ground ok, I've re-configured my ground scheme, preamp goes to the bus-bar, bias, power section, and PT grounds all go to the same pt lug. The main wall ground is connected to the chassis on it's own lug. The darn thing still hummmmmmms. When I remove the v1 it still hums so I know the problem is not in the pre-amp. The problem stops when I pull the PI so it must be the reverb or the PI. When I pull the reverb return tube the problem stops. I took a jumper wire and by-passed the reverb and I get the same hummmmm. could the PI input cap (.022uf) be the culprit?