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Power Ground

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4/11/2004 5:24 AM

Power Ground

Hello everyone,

I've started building my 5E3 head and since I parted everything out a little at a time, I will not be using a brass plate that would normally come with Bruce's kit. (Bruce was nice enough to send me his board, thanks, Bruce.) So I was thinking about having two different ground points, a power ground and a preamp ground. For the power ground I was thinking about star grounding the Red/Yellow center tap, the green yellow center tap with a 100ohm resistor at the end, The orange lead (Shield-Ground) and the ground wire coming off the board at the first filter cap. What do you guys think ? (this is only my second build so I need all the help I can get) For the preamp ground I was thinking about using the hoffman method of the pot/buss wire system (Hey Barry do you have any pictures of the hoffman buss wire ground? I thought I saw a thread where you say that is the method you use. By the way I'm just about a half hour away from you in San Gabriel, Ca)Any thoughts or ideas would be great.

4/13/2004 7:56 PM
MasMusica	Anyone? please help.

4/13/2004 10:04 PM

Mas,
if you have a heater center tap there is no need to create an artificial ground. Don't use that 100 ohm resistor on the heater center tap to ground, just go straight to ground. If you wanted to create the artifical ground instead of using the center tap(not necessary)you would put a 100 ohm resistor from each winding tap to ground and wirenut the center tap wire. You should really have three chassis grounding points. A preamp ground, a power section ground and a separate power cord ground (as Bruce mentioned earlier the separate power cord ground is a UL requirement). My experience says don't use any of the transformer bolts for anything but the power cord ground.

4/13/2004 11:23 PM
Al Lang	I grounded my PT ground wire along with power chord green grounding wire. Is this a problem? They just happen to be close to the stud on my chassis. Al

4/14/2004 12:16 AM
bnwitt	Al, that's not a problem at all unless you want a U.L rating, and I doubt you are selling amps so the point is moot. Bruce is the expert on that U.L. stuff though. I have been following his advise for quite some time as I do sell my amps.

4/13/2004 11:00 PM

Thank you for your reply. I understood or at least I thought I understood that a power transformer with a center tap has the potential to short out the heater winding, so as a safety precaution it is suggested to put a 47 to 100 ohm resistor at the end of the center tap lead, so that if it shorted out, the resistor would act as a fuse? I just want to be sure that I don't fry my power transformer and have to buy a new one. I'm fairly new at all of this and I've seen post where some guys opt not to use the center tap to ground and will create an artificial center tap using 2 100 ohm resistors, I guess just looking at it from a practical sense if a transformer has the center tap on it why do some guys opt not to use it. Thank you for your patience, i look forward to your reply.

4/14/2004 12:14 AM

Mas,
All PT heater windings have the potential to burn if a Tube shorts and causes excessive current on the heater circuit. The whole thing with the artificial ground, is that it is suppose to allow the two 100 ohm resistors to act as fuses and protect the heater winding of your PT in the event of a tube shorting as you say the single resistor is suppose to do, but most people will tell you that depending on the type of resistor used (cc metal film etc) it might not open in time to save the PT heater winding anyway. Putting a single 47 to 100 ohm resistor on the center tap would theoretically do the same thing I guess, but for a maybe result, I would just ground the center tap and be done with it. If you really wanted to protect the heater winding, you would need to use a real fuse (designed to open on current excess) with the proper rating to prevent the current in the heater winding from exceeding the rated level and frying.

The real point of the two resistor set up is to produce a ground reference for the heater winding when the PT does not have a center tap and thus produce hum reduction.

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