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Old 13th April 2008, 03:31 AM   #1
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Default 60 hz hum, what causes those?

Hello all,

As you can see, this is my first post here...

I found this old Masco Model 375 5 inch tape deck (Google's never heard of it, so yeah). I saw it had 6V6GT power tubes in it and I always read they are decent tubes. So I ripped the tape deck part off and I'm running it with some of my keyboards for the fun of it, it has a "warmer" sound than my workhorse solid state amp It also has a 5Y3 rectifier, and 6SC7/6SJ7 (one of each) preamp tubes.

Now, I have a feeling somebody has discussed this here but I really can't find any solid answer. It has a 60 hz hum that comes out of the speakers, and it's very audible when you turn the volume up to "half way." Since I didn't like pay for this or anything (trashpicked) I'm not really worried about it; but what causes those very audible 60 hz hums? Are they easy to fix?

I've searched, but I couldn't locate a specific answer. I've read they're ground loop problems (but this thing doesn't even have a ground prong to begin with), I've read it's your psu (transformer?) going bad, and other such things.

Thanks, and sorry if I'm being an annoyance..I can include pictures of the unit if anybody wants to see.
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Old 13th April 2008, 04:06 AM   #2
Colt45 is offline Colt45  Serbia
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on antiques, it's often crappy power supply filtering... small caps, and no choke.

That would make it 120hz though
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Old 13th April 2008, 05:02 AM   #3
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How did you connect the keyboards? Maybe you have a ground/hum loop, or inadequate shielding.
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Old 13th April 2008, 05:11 AM   #4
m6tt is offline m6tt  United States
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definitely a ground loop. Just connect the inner conductor, and disconnect the shield from the wire you're using at one end of the wire (leave it on the wire though, and grounded at only one side).
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Old 13th April 2008, 07:23 AM   #5
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Quote:
Originally posted by m6tt
definitely a ground loop. Just connect the inner conductor, and disconnect the shield from the wire you're using at one end of the wire (leave it on the wire though, and grounded at only one side).
If a ground loop is what I think it is; I don't think it's that, I disconnected everything from it and plugged it into it's own outlet alone, and it still buzzed..
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Old 14th April 2008, 04:43 AM   #6
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Default Buzz..

Most likely a dried up power supply capacitor. Yes, some of the old tube gear had poor supplies but in the so called HiFi camp the designs were better at supply "regulation".
It's easy & fairly inexpensive to swap out the 40+ year old cap.
___________________________________Rick........
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Old 15th April 2008, 01:58 AM   #7
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Default Re: Buzz..

Quote:
Originally posted by Richard Ellis
Most likely a dried up power supply capacitor. Yes, some of the old tube gear had poor supplies but in the so called HiFi camp the designs were better at supply "regulation".
I didn't know capacitors dried up, I suppose that's the problem. It's strange though because the humming only becomes audible once you get up to "4" on the volume knob; then it starts to buzz really really loud at "5" and it oscillates, squeals and other nasty things. I think that's a bad preamp tube, as I pulled one of the preamp tubes and it stopped.

Thanks for the help guys.
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Old 15th April 2008, 02:28 AM   #8
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Excessively high input impedance can cause all the stuff you just mentioned, too. Back in those days input impedances were often 470k ohms or higher. You can replace the control grid resistor on the first stage with one around 68k ohms and see if that helps any. Modern source equipment won't mind the lower input impedance.

Wade
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