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January 27, 2023, 09:22:13 AM

Author Topic: Is mine working as expected?  (Read 119 times)

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Offline dstarr3

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Is mine working as expected?
« on: January 22, 2023, 04:05:51 PM »
So, this is my first proper tube amp. And because of the DIY nature of this, I just want to make sure that some of the things I'm experiencing are expected from tube amps, or if there is some improvement to be made in my assembly.

First, just for the sake of saying, all of my resistance and voltage checks passed first time during assembly. I initially had some crackling and popping issues, touched up the soldering on the 1/4" output jack, problem solved. And then it's been several months of very happy listening. This is the non-SB kit with RCA 6AS7G black plate and RCA 12AU7 clear top tubes, and the headphones I listen to on this amp are the HD6XX, Beyer 880 600-ohm, and an old AKG Sextett (midpro).

There are a couple things I'm experiencing that make me wonder if there are some imperfections in my assembly, or if this is just the nature of pure tube amps. The first thing is that, when powered on, I can hear the transformer quietly buzzing. It's not loud, nothing's overheating, it's just the kind of thing where I can only really tell because it's sitting right next to me on my desk and when my music drops out and I'm sitting in silence again, I'm reminded that I can hear it through my openbacks.

The second thing is that, I'm used to solid state or hybrid amps with perfectly black backgrounds. Y'know, no matter how sensitive the headphone is that's connected to it, if there's no music playing, there's no noise or hiss or hum whatsoever. That's not what I'm experiencing with my Crack. Or more specifically, I don't experience that with my HD6XX. My 600-ohm headphones, I get that black background, perfect silence at idle. The HD6XX being only 250-ohm, I assume it's sensitive enough to pick up some minor noise in the signal path somewhere that the 600-ohm headphones aren't. It's very subtle, again like the transformer buzz it's only detectable during otherwise silence. But it is there, and with my HD6XX, I do hear it in between music. It doesn't increase with volume or go away with temperature or any of that. It's constant and unchanging.

I picked up my RCA tubes in pairs, so I've tried it with the other pair I have, plus with the tubes provided with my kit, and changing tubes doesn't have any effect on the very-quiet-but-still-present hum.

Again, these are two very subtle issues, so subtle that it makes me think that maybe this is just what using tube amps is like. But I would like to research it in case my transformer should be perfectly silent and I should be able to achieve a perfectly black background with my HD6XX.

In short, if these issues mean I can improve my assembly somehow, I would like to. But if these things are normal or expected, I can tolerate them.

Thanks!

Offline Deluk

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Re: Is mine working as expected?
« Reply #1 on: January 22, 2023, 11:26:46 PM »
I'd wait for the experts, but you could check that the transformer fixing bolts are tightened down hard.

Offline Paul Joppa

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Re: Is mine working as expected?
« Reply #2 on: January 23, 2023, 04:06:51 AM »
It's possible that you have some "ground-loop" hum. Does the hum go away if you remove the input RCA plugs? (Try this with the volume control  all the way down.)
Paul Joppa

Online Paul Birkeland

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Re: Is mine working as expected?
« Reply #3 on: January 23, 2023, 04:27:05 AM »
Also if the transformer is buzzing and your DC voltages are all proper, I would tighten down the screws.  Loose screws can also make your amp noisy.
Paul "PB" Birkeland

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Offline dstarr3

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Re: Is mine working as expected?
« Reply #4 on: January 25, 2023, 06:59:10 AM »
It's possible that you have some "ground-loop" hum. Does the hum go away if you remove the input RCA plugs? (Try this with the volume control  all the way down.)

I don't think it'd be a ground loop hum, because all of my equipment is plugged into the same UPS. But yeah, tightening the transformer screws more did help a lot, although it's difficult because the lugs on the connected terminal strips start twisting a lot and if I tighten much more, things are going to bunch up and I'm going to start getting some shorts in places. So I can only tighten them so much. But that said, tightening as much as I'm comfortable, I've definitely reduced the hum by quite a lot.

Offline Paul Joppa

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Re: Is mine working as expected?
« Reply #5 on: January 25, 2023, 07:22:58 AM »
I don't think it'd be a ground loop hum, because all of my equipment is plugged into the same UPS. ...
Separated safety grounds are not the only source of what is usually called ground loop hum. That's why I suggested a test.
Paul Joppa

Offline Doc B.

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Re: Is mine working as expected?
« Reply #6 on: January 25, 2023, 04:59:35 PM »
Try grabbing the center lug of the terminalstrip that the screw goes thru with needle nose pliers when you tighten the screw. Sometimes that is difficult to get at. But if you can grab it you can usually keep the t-strip from twisting as much.
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