Replacing power cord with new polarized cord, what is best orientation?

Can you explain how to build a shorting plug for the jack, or point me to a good resource. I'll search as well. Would the shorting plug just be a brief install, or do you leave it in for a while?

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Pick up a bag of plastic RCA's at RatScat, tie the signal and ground together, fill the void with silicone, clay, goober, whatever and plug em in. Or, you can get em already made for a lot or a little $. Just leave em in unless/until you need to use the jack(s).

notdigital said:

Pick up a bag of plastic RCA's at RatScat, tie the signal and ground together, fill the void with silicone, clay, goober, whatever and plug em in. Or, you can get em already made for a lot or a little $. Just leave em in unless/until you need to use the jack(s).

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I have built up two versions of the shorting plugs. What will work where is determined by trial and error.

The first is a dead short as above, the second has a 1k, 1/8w resistor between plus and ground.

I always try the resistor version first (safest) and if it doesn't do the trick I use the dead short version.

drewbolce said:

Never, ever ground a Fisher chassis or other two prong corded unit from this era. It will set up a ground loop and hum like a banshee.

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^^^^^^
This.

-D

drewbolce said:

Never, ever ground a Fisher chassis or other two prong corded unit from this era. It will set up a ground loop and hum like a banshee.

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Thanks Drew, glad I asked!
Glenn

So, for those with experience installing this "3 way" X/Y capacitor setup, looking at my before and after photos: looks fine?


Like I said, I'll probably undo one or a couple of them tonight as a test to see if they are involved in generating the faint "buzz". ...or not.

Thanks again,
Al

OK, well I ultimately removed all three X/Y capacitors and the thermistor. For all steps I also reversed the plug several times.

I first removed the new line-to-chassis capacitor, which didn't appreciably change the volume of "buzz" regardless of plug orientation.

I then removed the across-the-line capacitor, and that seemed to slightly reduce the "buzz", but it could have been my imagination.

I then removed the thermistor, which made no change, which was expected.

I then removed the last line-to-chassis capacitor, which was in the original location. To my surprise it at first didn't make any difference, but when I flipped the plug around I soon found that while the "buzz" was still there, it was down in volume by a good third I'd estimate. Keep in mind, my ear is essentially IN the cone of a little 4 inch speaker to really hear it.

So, interestingly when ALL X/Y "AC filtering" capacitors are removed, it is actually a little less "buzz". Weird.

Keep in mind this "buzz" is really light, and probably was there before(I'm sure it was as nothing else changed in the circuit) but now I'm hyper-sensitive since I made a change.

My last test is to use another old 1KV ceramic .01uf capacitor on the original line-to-chassis and see what happens. If the "buzz" really goes away (or down), then these Vishay X/Y capacitors perhaps aren't a good fit. I'll do that tonight.

Any thoughts? Maybe I am making a mountain out of a molehill and these old amps will always have some AC "buzz" at very low volume as opposed to near absolute silence like a good modern SS unit (like my Marantz AV receiver)? That's fine, but just looking to fix it if not normal.

Al

So after some advice from folks, I used a shorted plug in the input jack and viola, no buzz at all. So, what would that seem to point to as a cause for the buzz?

I've tried two different 12ax7 and two different rectifier tubes with the same buzz resluts. I have two old, but strong, GE EL84 tubes that tested very well. Unfortunately I don't have any other EL84s to swap to see if they are the culprits.

And on a lark, I did confirm that grounding the chassis does indeed make it even worse as predicted earlier in the thread.


Thanks,
Al

I just went and read over the whole thread again. I think you're chasing gremlins that are harder to find because it's over the net Al. A slight bit of buzz/hum with no source connected is pretty much normal for 50+ year old gear.


My own 30A was tempermental as to buzz/hum after an overhaul. I spent almost a week tracking down one on the one in my 560 companion. Turns out I had moved a wire when I replaced a cap/resistor and hadn't moved it back. It was too close to one of the B+ leads. locate all the B+ leads and make sure they are somewhat isolated from the signal path. Gonna be one of those trial and error things. A WOODEN tool to move the leads around while it's on, works well.

Plug your preamp into the 30A. It shouldn't buzz. If it does, add a "ground" wire between them (like you'd do with a phono and a receiver) and check again.

I don't think rolling tubes isn't going to change things. If they've all tested good, they shouldn't cause problems. IME 98% of problems in a tube amp/preamp/tuner is caused by something OTHER THAN Tubes. The other two percent are either caused by a tube primarily, or something else causes a tube to act up.

Larry

Thanks, much appreciated. I am sure you are right about some buzz being reasonable.

I am curious though why the suggested test of shorting the input eliminates all the buzz entirely though. Can someone explain why that would happen, and what it may indicate?

Thanks
Al

Here are some thoughts I received from a local technican that has helped me before:

The buzz is being picked up in the input stage. if it's got disc caps for coupling, could be there. Or the foil side of a cap is not connected to the high impedance side of the coupling circuit. I'm assuming the noise is 60 cycle which is what would be picked up. Might have to shield the heater supply lines, the ac could be coupling from there. As you can see, it could be a lot of things that are causing that noise.

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Along with the suggestion above about checking routing of the B+ wiring, after reading the thoughts above and looking at the photos I've posted of the wiring in the amplifier, does anyone have any thoughts if any of these might be a good path to investigate based on the work you see in the photos and if the photos give you any insight on how likely some may be, or what to check?

Thanks,
Al

It may indicate that the shorting wiper in the selection knob is either dirty or worn.

little-al said:

Thanks, much appreciated. I am sure you are right about some buzz being reasonable.

I am curious though why the suggested test of shorting the input eliminates all the buzz entirely though. Can someone explain why that would happen, and what it may indicate?

Thanks
Al

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Years ago I lived in a 6th floor apt in NYC where all the telephone, TV, and (early) internet cabling ran past my window. Needless to say, It mucked up my TV reception AND, more importantly, my audio rig. After having many bottles of beer trying to figure it all out one evening, I noticed that when I moved some components around, the buzz coming out of my speakers would change character. Hmmmmmmm.

Did some research at AA and learnt about shorting plugs. Went to that Ratty Place, bought a bag of cheap RCA's, and shorted every INPUT I had. Virtually eliminated the buzz. I say virtually because, like yourself, I experienced a very faint hum still. I narrowed it down to my TV cable (my audio was connected to my video) and picked up an in-line isolation transformer that connected to the incoming cable before the cable box. Dead quiet.

So, long story to say: EMI and RFI sneak into systems via the exposed RCA jacks, (they and the internal wiring sometimes act as antenna.) Shorting them via dead shorts or via a resistor blocks these stray signals from being introduced.

EDIT: There is a difference between stray interference and noise caused by ground loops. I had both. The isolation tranny cured the ground loop buzz. It cost $10 if I recall; there are more expensive alternatives just like more expensive shorting caps if you want to give your money away

audiodon said:

It may indicate that the shorting wiper in the selection knob is either dirty or worn.

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Apologies, but what would be the "selection knob" in this case? I think I know what you are referring to, but don't think it is applicable in this case?

This amplifier only has one adjustment, which is a potentiometer for the initial 12AX7 gain. It really never moves and is left "full on". And moving it doesn't impact the "buzz". The 30-A is not an integrated amplifier with a full preamplifier and input selection, it is simply a mono amplifier.

Appreciate all the responses. I think this may end up being a very useful thread for others looking to repair their power input sections and tracing down AC interference.

Thanks,
Al

Hum solved and notes about filter caps

Some additional information in another thread I had going about the 30-A, its hum problem, and when/where to add additional filter caps:

http://www.audiokarma.org/forums/showpost.php?p=6255188&postcount=19


http://www.audiokarma.org/forums/showpost.php?p=6256649&postcount=22


Regards and thanks to all that have been of assistance!

Al