3 vs 2 prong power cord when restoring old tube gear??

"Ground loops". I see this thrown over this forum over and over. Ground is supposed to be bonded on every piece of equipment for obvious safety reasons. What is not supposed to be connected on ground is "objectionable current", induced by a defective piece of equipment that leaks current from mains to the ground and that's what 90% of the time people say it is "ground loop". It's not!

When that hum/noise is heard, the gear pieces have to be unplugged one by one until that noise stops. Then that piece should be checked for isolation (resistance) between the mains terminals and chassis/ground and the leakage fixed.
 
I have all my rebuilt tube amps and preamps wired with 3-prong cords. There are no hum issues at all. BUT, don't try connecting them to a home theatre system that has a cable TV connection. The house Earth and the cable Earth are usually spread way apart from each other. This is what creates the ground loop.
With 300-450v. in these devices., I feel better knowing I'm protected against a catastrophic failure to chassis.
 
I feel that there is a topsy-turvy approach here somewhere to 'ground' loops'. Sure, they can occur - with injudicious wiring. The cure for that is not to leave everything floating somewhere 'in space' as it were. Ground loops are avoided by grounding one point in a system, not by no ground (if I understand correctly). When there is more than one ground the solution is trivial: Remove one of the grounds (but not all of them).

I also read about the so-called 'death capacitor'; not sure of the origin of that unfortunate term. A capacitor across the mains is simply part of a filter system to reduce h.f. interference lurking on the mains. Not always effective, but then also not a bank-breaking measure. (All providing the correctly rated capacitor is used, of course - a.c. rated, that is.)
 
I think the "death cap" filter as it's known, had a leg to ground or chassis. So when they failed, or at least shorted, they energized the chassis. The X1Y2 caps go across line as filters, and fail "open" So no harm to the user.
 
yeah, mains to chassis capacitor with no grounded cord makes for a very unhappy day if it shorts. Its a bigger problem in a guitar amp for example where you might be essentially holding one side of a mains feed in your hand and touch something that is grounded.

An across the line cap isn't a death cap, though if they explode you might soil your pants. I've had a couple things come to me with those caps completely destroyed for whatever reason.
 
rudedogg: studying your replies and the schematic in question, I see you are right. Across the line at the AC inputs or across the Black - Black PT wires is essentially the same thing- buckers in series do not change that scheme.

Anyone: Do you have a recommendation for a specific uf value and rating of safety cap for tube gear?

Thanks again to all for the very informative expansion to my knowledge base!
 
triode17 said:
... The house Earth and the cable Earth are usually spread way apart from each other.
...
Click to expand...

While it is true the CATV drop sometimes isn't properly bonded due to faulty/negligent installation, at least in the USA, the incoming CATV feed is supposed to be bonded to the main earth wire of the house per National Electric Code.
 
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whoaru99 said:
incoming CATV feed is supposed to be bonded
Click to expand...

All grounds are supposed to be bonded, telephone, CATV, etc. Usually its just tied to the service entrance ground. If there are multiple ground rods, that gets into heavy bonding connections between them. Not having them bonded gets into potential difference in voltages in case of a lightning strike.
 
whoaru99 said:
While it is true the CATV drop sometimes isn't properly bonded due to faulty/negligent installation, at least in the USA, the incoming CATV feed is supposed to be bonded to the main earth wire of the house per National Electric Code.
Click to expand...
The key word being "SUPPOSED" to be bonded to the main Earth wire of the house. I can remember when cable first came around and they had their own technicians. They did good, clean installations. Then, years later, they instead hired sub-contractors to do their work and these guys stunk! THey were all lazy and would install whatever, wherever to save the company (time) money. I lived in two different areas and they both had separated Earth grounds and associated 60 Hz hums.
 
I installed a grounded 3-wire on my amp and removed the D-cap. I only use it with a turntable so ground loops are not a factor and I found a sharp reduction in hum by doing this.
KALAMAZOO: Did you add a ground the output side of your impedance matching transformer or was that the factory configuration?
 
"I also read about the so-called 'death capacitor'; not sure of the origin of that unfortunate term. A capacitor across the mains is simply part of a filter system to reduce h.f. interference lurking on the mains. Not always effective, but then also not a bank-breaking measure. (All providing the correctly rated capacitor is used, of course - a.c. rated, that is.)[/QUOTE]

Audiovet - I will explain the "Death capacitor" to you because you are from South Africa and it's American slang. This started to rear it's ugly head in the 60's, when guitar amps. were selling like mad and had a paper, film cap to a Ground switch. That switched the cap. from either the Mains to chassis or the Neutral to chassis, in order to eliminate noise. But those caps were notorious for shorting out, thus connecting the chassis and your guitar strings to either the Mains or Neutral (hopefully the latter). Some unfortunate guy went to play his electric guitar in a damp basement without any shoes on and ................you can guess the rest. Therefore the name, "death capacitor".
 
I knew I had these around somewhere. This is a tuner that was given to me some time ago.
Roasted across the line cap:

100_0046.JPG

The roasted line to chassis cap
100_0050.JPG

The paper cap that was the other line to chassis cap. No idea why it had two, but thats what it had. Double death? Killed by death?

100_0051.JPG

I don't honestly know what happened here. I suspect lightning strike, but it could have been any sort of power surge. Tuner actually works fine after replacing those parts. I changed it to a single across the line cap and got rid of the other stuff. Its a late 50s Bogen FM-50.
 
volvogv said:
I installed a grounded 3-wire on my amp and removed the D-cap. I only use it with a turntable so ground loops are not a factor and I found a sharp reduction in hum by doing this.
KALAMAZOO: Did you add a ground the output side of your impedance matching transformer or was that the factory configuration?
Click to expand...

sorry if I confused anyone by using DC Gillespie's schematic. I just used it to display the heater bucking scheme with the power transformer. The schematic beyond the PT is not my amp 196-00. It is Mr. Gillespie's Magnavox Amp 8600 mod schematic found elsewhere on this forum.
 
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When I restored my Sanui 1000A I replaced the 2 wire cord with a 3-wire one and have no trouble with ground loops or hum yet (increased safety was my concern).

Mark T. :music:
 
For all new builds. Not rebuilds, what do most do? Try 3 then go to 2 if hum says hello?
 
I have heard that some guitar amps have a ground lift switch,there must be a reason. I have used 2 and 3 wire cords, on the 3 wire cords, I have them go to a terminal strip, and then to chassis ground, with instructions to disconnect it if a hum problem exists.
 
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