Added Ground - Now Get Major "Hum"

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A friend asked me to repair his 1960's "Paul" tube guitar amp made by Pine Electronics in Canada. The power cord cap was missing and I decided to replace the whole cord with a grounded cord. I connected the ground to the chassis. I now have major hum. I used a cord adapter to lift the ground and the hum stops so I know it has something to do with the ground. Is there something I can do to get rid of the hum AND have the safety of a grounded amp?
Thanks.
 
Adding the grounded power cord was the responsible thing to do. Now you must remove all the ground loops. You need to create a ground bus and go through the amp looking for points where the chassis was used as ground and rewire them so that they go to the bus. There are lots of grounding techniques and "how to" articles on proper grounding; you might consult them. The ground bus should connect to the chassis at one point; that point being where you installed the green wire from the power cord. Make certain the input jack is isolated from the chassis.
 
First, make sure your outlets are wired correctly. basic outlet testers are available at most hardware stores or departments for under $10. They have three lights on them, and a small chart to show the combination of lit and unlit you want. they will identify things like ground and neutral reversed. Which can cause hum.

If it used to have a two wire power cord, did it have a ground selector switch? And by extension, was ther a cap from the neutral wire to chassis? If so, remove that cap. We often call it the "death cap".

With due respect to Palustris above, you should not have to rewire all the internal grounds when earthing the chassis. Replacing old two wire cords with grounded ones is done all the time, and I have never faced the need to completely rework the amp circuit grounds.
 
Do you have any other electronic components in the system that also have grounded line cords? If so, you may have a ground loop between components. This is a common problem in hi-fi systems, with many interconnected devices with more than one grounded line cord (and some times a cable TV ground can add to the troubles too.) The rule-of-thumb is to have only one component with a ground connection, and all others are floating ground.

If you have only one "known" ground connection, look for other "accidental" grounds, like a metal chassis touching a water pipe, radiator, etc.
 
Thanks Enzo. I confirmed that the outlet is wired correctly using an outlet tester. The amp did not have a ground selector or polarity switch. From what I can see there is no cap from the neutral to chassis. I can't seem to find the exact schematic for it. The manufacturer was Pepco (Pine Electronics Products Co) out of Montreal. They made several branded amps. I remember them as "Paul". This is a Model 201. I've attached a pic of the layout - I know that's kinda hard to decipher but I'm hoping you might be able to see something obvious.
Thanks again.
 

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Do you have any other electronic components in the system that also have grounded line cords? If so, you may have a ground loop between components. This is a common problem in hi-fi systems, with many interconnected devices with more than one grounded line cord (and some times a cable TV ground can add to the troubles too.) The rule-of-thumb is to have only one component with a ground connection, and all others are floating ground.

If you have only one "known" ground connection, look for other "accidental" grounds, like a metal chassis touching a water pipe, radiator, etc.

Thanks. I have tried plugging in different outlets in different buildings. My other amps including a 1965 Fender tube amp do not have "hum" problems.
 
I found a thread from a guy who has modded the same model amp. He replaced the transformer and rectifier. I've attached his description. I'm not really prepared to get into that big a mod - I don't think the amp is worth keeping. However, I'd be curious to know if making the mods is the only solution to the hum problem. His mod may also be a clue as to why the amp has major hum.
Thanks.
 

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Actually, you just shouldn't use that without an isolation transformer. In the old days of two wire, you could plug into the socket with the plug turned either way. usually you positioned it for the least hum, and that usually also meant the least shock when you touched it and something else. That is why older equipment usually had a ground selector switch. once your stage was set up, you went around and got all your ground switches to the best position.

Sometimes turning the plug over or flipping the switch wasn't enough. sometimes you couldn't get it right and had to flip the plug over AND flip the switch.

It was a fact of life on stage, you got shocks. Especially singing, the PA and the guitar amp were not in agreement, you hold the guitar strings, which were grounded to your amp, and touched your lips to the microphone, and WHAM, you burned your lips. I recall one night my guitarist raised his guitar up while playing, and his high E string touched the microphone, and a big spark, and his E string melted in two.

it was dangerous. On this forum, if someone starts a thread about a hot chassis amp, they usually lock the thread. SO I urge you not to use it in its natural state. With your three wire plug, the only hope you have would be MAYBE to reverse the hot and neutral wires in your plug.

But an isolation transformer is not real expensive, and if you like the amp otherwise, that is the way to go about it.

Here is a thread about your amp model on another forum:
Pepco Paul Combo - Anyone have a schematic or more info?

Are your tubes a 35W4, a 50C5, and a 12AX7? There is a link on one post with a schematic that will be close to yours, if so.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Jay,
Get a used 1:1 isolation transformer. Surplus stores often have them.

This amp sounds like it would be a great little practice amp. But using an isolation transformer is non-negotiable. You must isolate that amplifier from the mains. Make certain that everything, including the heater string is isolated.

If you do go for a transformer and it has to be the $100 one, look at it as a personal safety cost. Besides, you can use that transformer in another amp if you find one better. I don't know about you, but I couldn't find any tube amps for less than $150 when I was looking for my daughter.

If you aren't going to use an isolation transformer, remove the power cord and write on the chassis that it is a live chassis as a warning for anyone else.

:cop:
I'm sorry, but I have to lock this thread because it deals with a dangerous situation.
:cop:
-Chris
 
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