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Bottlehead Kits => Crack => Topic started by: dstarr3 on January 22, 2023, 04:05:51 PM

Title: Is mine working as expected?
Post by: dstarr3 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!
Title: Re: Is mine working as expected?
Post by: Deluk 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.
Title: Re: Is mine working as expected?
Post by: Paul Joppa 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.)
Title: Re: Is mine working as expected?
Post by: Paul Birkeland 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.
Title: Re: Is mine working as expected?
Post by: dstarr3 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.
Title: Re: Is mine working as expected?
Post by: Paul Joppa 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.
Title: Re: Is mine working as expected?
Post by: Doc B. 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.
Title: Re: Is mine working as expected?
Post by: dstarr3 on April 02, 2023, 11:05:41 AM
An update on this situation. Definitely not a ground loop, no difference in noise floor with RCA inputs connected or disconnected.

Tightening the transformer bolts definitely helped a lot. I just went to town on it until I started stripping the head of one of 'em, lol. Overkill surely, but, well, it helped.

I went through the process of testing every single combination of every single tube I have, because I didn't think it was possible that the three difference RCA black plates I have all have the same ringing issue, but it turns out, that's got to be it. With my HD6XX and the tubes BH provided, dead silent noise floor, not a whisper of hum or buzz, perfect. So we can definitely confirm mine is assembled properly, at least. Tested both of my RCA clear-tops with the BH-provided power tube, still dead silent noise floor. Tested all of my RCA black plates with my now-known-to-be-good clear tops and the BH-provided preamp tube, and every single one of them had ringing to some degree.

I really, really love the sound of the RCA black plate (noise floor aside), but 0/3 in terms of defects is pretty poor. Is this bad luck or is this just what RCA black plates do?

Again, for perspective, the ringing is at least subtle. I can only hear it on my HD6XX, not any of my 600-ohm cans, and I can only notice it during moments of super-quiet music or silence between tracks. But still, when I do notice it, it's pretty annoying. And I'd love to know if this is just what life is like with RCA black plates or if I have just had bad luck
Title: Re: Is mine working as expected?
Post by: Paul Birkeland on April 02, 2023, 11:26:49 AM
You are mentioning ringing and buzz in these posts.  Is the buzz completely gone with all combinations of tubes?  What does the ringing sound like exactly?
Title: Re: Is mine working as expected?
Post by: dstarr3 on April 02, 2023, 12:54:55 PM
Oh, yes, this has gotten confusing, sorry, lol

Yes, buzzing is all gone regardless of tube combination. Tightening the transformer bolts more solved that. The ringing is very quiet, only audible on my HD6XX during very quiet music or silence between tracks, and only when using any of my RCA black plate power tubes.
Title: Re: Is mine working as expected?
Post by: Paul Birkeland on April 02, 2023, 02:29:00 PM
Can you describe what the ringing sounds like?  Is it a constant noise? Is it periodic in nature or random?
Title: Re: Is mine working as expected?
Post by: Mucker on April 02, 2023, 07:27:10 PM
I have a 2399 tube that I use with my Crack (same shape as a 6AS7) and it has a high frequency vibration that makes the top plate of the amp ring. If I move the tube around slightly or tap on the plate I can alter the ringing, sometimes it disappears but eventually it will come back. What nearly eliminates it is using a tube damper ring (42-44mm for 6AS7 shape). The 2399 is a tube I really like so this solution works well.

I use RCA 6AS7 tubes in my Woo Audio WA2 (as well as Crack) and they all seem to vibrate/ring. The tube dampers help with those as well. I have never experienced ringing with 6080 or 7236 tubes. Anyway, I attached a pic of my 2399 with damper and a link to some dampers (44mm will work best).

https://www.ebay.com/itm/224291554306?chn=ps&mkevt=1&mkcid=28&var=523080473869&srsltid=AfAwrE76GBA05L98l5j-2yIkHQ1N614Wu11Ux55QOGatvxTm2gJdeoRp078