Mechanical/magnetic power transformer hum

YouTube · Post by **xtian** » Fri Jan 31, 2014 5:57 pm

I have a friend's 1981 Jim Kelly FACS head. It has a high level of hum. No so bad that it would prevent you from playing with a live band, but very obvious and obnoxious when playing alone.

The power transformer is clearly to blame. I removed ALL the tubes, and hooked up a speaker. Even in standby, you can hear the hum in the speaker.

But before I report to the owner, I want to rule out filter caps. During my test, above, the output transformer's primary's CT is connected to HV. Is there any chance bad filter caps could inject ripple into the OT via its CT?

Reeltarded · Post by **Reeltarded** » Fri Jan 31, 2014 7:05 pm

I remember some of them doing that.. IN 1981.

M Fowler · Post by **M Fowler** » Fri Jan 31, 2014 7:36 pm

How about the bias cap? if this amp is fixed bias.

Jana · Post by **Jana** » Fri Jan 31, 2014 7:48 pm

It's probably the orientation of the transformer. Marshall goofed with the JTM45 reissue and had the transformer oriented wrong. I have spun a few of those 90 degrees and no more hum.

Just because it's factory built doesn't mean they did it right.

Milkmansound · Post by **Milkmansound** » Fri Jan 31, 2014 7:50 pm

Jana wrote:Just because it's factory built doesn't mean they did it right.

insert snarky CBS Fender comments

YouTube · Post by **xtian** » Fri Jan 31, 2014 7:56 pm

Considered the orientation, but looks correct. However, must acknowledge that I can hear that damn PT humming like a bad fluorescent lamp ballast even with no speaker plugged in. Made sure nuts are tight. I can feel the vibration with my fingers.

YouTube · Post by **xtian** » Fri Jan 31, 2014 7:57 pm

@Mark: I think we can rule out the bias cap, since this problem exists with NO power tubes installed. Agree?

M Fowler · Post by **M Fowler** » Fri Jan 31, 2014 8:12 pm

xtian wrote:@Mark: I think we can rule out the bias cap, since this problem exists with NO power tubes installed. Agree?

Yes it would definitely show up, so rule that out.

I like what Jana said didn't know about that

But sure worth a try huh!

Baxtercat · Post by **Baxtercat** » Sun Feb 09, 2014 4:02 pm

Did you get anywhere w/ this Xtian?
I was just coming on here to ask the same thing.
I've got the same symptoms w/ an old Bell 35 watt PA amp [cathode biased].
Speaker hums with no tubes in the amp.

What would cause this, a short in the PT? or in a filter cap?

[edit, I'll try moving the OT. It is kinda close to the PT.]

YouTube · Post by **xtian** » Sun Feb 09, 2014 5:38 pm

No, didn't get anywhere. Owner did not want to pursue.

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Mechanical/magnetic power transformer hum

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