Feedback in Crack with 6AS7G?

[pkb]

High pitched feedback in crack??

I'm experiencing something that seems quite strange: what seems like very low-volume feedback through one channel in the crack.

It happens with both 6AS7G's I've tried.

The sound is sort of like my ears ringing and it will appear slowly. It got to the point where it was pretty annoying and I unplugged the headphone cable--and amazingly, my ear stopped ringing. Tt was gone. That's when I discovered that it wasn't my ears.

Also very strange is that when I unplug the headphone cable and then plug it back in, the sound isn't immediately there again. Sometimes it will stay quiet and sometimes it will creep back.

I'm wondering whether it could be feedback because, like feedback from a guitar or speaker, when the loop is broken and then re-established, the sound takes some time to re-establish the feedback loop.

Since there's no possibility that the drivers of these sealed headphones can be causing a microphonic reaction with the power tube, I'm wondering if it's possible that there is some electrical feedback look that might exist.

With the first NOS RCA 6AS7G, it would come and go, and occasionally be more persistent. When I sent that tube back as defective (thanks, TubeDepot) and got my replacement, the sound didn't immediately return. Now, that "feedback" might be creeping back to prominence.

I've also never noticed it from a 6080, interestingly. Could there be something special about the characteristics of a 6AS7G that would push the non-speedball'd circuit outside of its happy place?

Thanks all

[Laudanum]

  Some tubes are simply microphonic enough that it doesnt take an external stimuli, like the feedback from speakers, to "set them off".   What happens if you lightly tap on the tube?  Do you hear it?  It is strange that you have gotten two with the same issue in the same channel.  Im not so sure that the sound you are describing is actually a microphonic tube.  There could be something else going on maybe a solder joint?  If that could be the culprit the "chop stick test" could sniff it out but I'll let the gurus help out there, they are much smarter than me when it comes to this stuff and I could be off track.     

For some safe starters ... If it shows up when tapping the tube, maybe try the quiet 6080 and see if it does the same thing in the same channel when lightly tapped, and tapping on the top plate.  Those may be a couple of safe and easy things to try.  And I think that if it's a positive test with the 6080 as well, it would certianly point to something else.  Either way, nothing that the guys and gals here cant help you figure out.

The 6AS7 should be a drop in, so no electrical problems there, Speedball or not.  
Also, if I recall correctly, the 6AS7's can tend to be more microphonic, I think because of the larger glass envelope and internal design ... compared to the straight glass 6080's.   That doesnt mean that all 6AS7g's, with their ST bottles will be microphonic.  Just the tendency compared to 6080's.   For the record, I have several 6AS7's and they are all quiet on their own.  One or two are slightly microphonic if I tap on them or the top plate near them but no sonic issues otherwise.  

[pkb]

Tapping on the 6AS7 doesn't induce the ringing. Also, I may have mis-represented it a little, but the first 6as7 had the ringing in the right channel. The current one is showing up in the left (though so far, it's not as loud or as often--just noticeable when I'm listening to no-input Crack (a bad habit and a most extreme condition--certainly requiring an intervention from friends at the least (sorry for the nested parentheticals))).

Both the 6080 and 6AS7G http://i69.photobucket.com/albums/i45/rdh_skylab/DSC01621.jpg ping a little when tapped. I thought that was normal.

[Dr. Toobz]

I had an issue with a higher-pitched ringing that would come and go. Turns out that the USB DAC I was using with my Mac didn't do a great job of isolating the power supply from the audio, and some power-supply harmonic from the computer was taking a ride with the signal to end up in my headphones. It seemed like my 5998 was more prone to this than the 6AS7, but it's hard to say why.

Is the sound a ringing, or more like a medium-pitched rustling, like the sound of somebody shaking an incandescent light bulb? I ask this because I also sometimes hear "rustling" sounds when using an Electro-Harmonix 12BH7 as a driver, which come and go randomly. Another analogy would be the sound of water rushing through a distant pipe. It's happened on both specimens I have, but never on any other tube, even other brands of 12BH7's. My theory is that the EH tubes are constructed in such a way as to allow the wires that connect the pins to the innards of the tube to vibrate, and that resonance is making it to the triode assembly.

For what it's worth, switching to a better DAC and more robust driver tube (E80CC) has resulted in a dead-quiet background. The dodgy 5998 has been replaced by a computer-rated 7236, which also appears to be very quiet. The sounds come back if dropping in the aforementioned 12BH7's, however.

[pkb]

Another development: as the 6as7 heats up, the ringing becomes more prominent.

I've found one very strange solution: if I take a sock (lest I get burns) and squeeze the top of the bottle in line with the two heaters, the ringing stops. When I let go, it starts again. If I don't squeeze hard enough, it doesn't effect the ringing. I feel like I'm flexing the bottle a little to make it stop. The sound is now in both channels and has a sort of music unto itself. There are a couple soft harmonics that fold very slowly on top of one another--slow enough so that it's difficult to notice any change at all.

This problem does not show up in the 6080, which is very very quiet.

Does this give anyone the final clue? I'm still out to sea here with this problem showing up 2 for 2 with these RCA NOS black plate 6as7G's.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi315.photobucket.com%2Falbums%2Fll452%2Fpatrickbarter%2Fcrackamp2.jpg&hash=cd2782d3815a239efae793f826f785844f6ab15a)

[Laudanum]

 I dont think that chances of getting two noisy/microphonic 6AS7's would be great but I wouldnt bet against it either.  You say that the 6080 is quiet and if you use your hand to dampen the 6AS7 tubes they become quiet so logic says the tubes but something else could be going on.  Im not an expert at all here but noise in audio electronic can be "strange".   Dr. Toobz has a good example of similiar with his DAC's power supply and some tubes being more susceptible to the noise.   Have you tried a different source?   I would do so to see if there is more to it than just the tubes themselves.

[hasafraker]

Howdy kids!

I am here to tell ya, the 6as7/6080's and others can do this. I have the Sylvania Gold 6080 (and several others) that was included with my kit and it whistles and hums like an old crazy person. I have a slew of 6080's 6as7g's and one 7802 TungSol that are all dead silent.

My Sylvania, a couple "cheapo" 6080's and one of a set of Svetlana 6n13's all do this. Some of them didn't make any racket at first, one of my GE 5stars did it from the get go but after 80hrs burning in it stopped and doesn't do it at all now. Took me a while to find a quiet set of tubes because the included Electro Harmonix 12au7 would hum too, and that didn't start humming till after about 50hrs of burn in. I got a deal on a couple more, burned them in and after about 60hrs they started to hum too. drove me a little insane at first till I picked up a handful of  5814's in a cheap auction and they all sound good but... are perfectly quiet. Now I have managed to collect a small stash of Amperex 12au7's, 1 orange globe, a couple flashers, a very nice Dumont (which I'm currently using), and sadly a sweet looking Bugle Boy that works perfectly but has hum in the right channel.

The reality is that there is no guarantee that a new reproduction or NOS tube will not be microphonic. I have experienced noisy tubes in just about every piece of gear I have at one point or another. My Crack being the most sensitive because it's a headphone amp and you are must more prone to hearing hum or whistle through headphones so if it's not silent it's hard to miss. 

I personally have never purchased any of the silicone tube dampers on the market, BUT, for a tube that doesn't make a lot of noise it might help, generally speaking I would say tube dampers would be more beneficial if you don't have your amp isolated well from it's surroundings then it might help, if you're squeezing hard enough on the glass to stop the hum I doubt dampers will help. Keep looking for a quiet tube. I have had really good luck with the Russian tubes, anything JAN or Chatham marked, and I have several super nice Tung Sol's all silent. I also run my tubes in for about 80 hours when I get a new one before I give it a serious listen but I'm crazy like that

Good luck! Have patience, and don't give up, you won't have to spend a fortune to find a quiet tube or 5 lol. I watch ebay constantly just to keep my eye out for whatever looks like it might be fun or for anything I haven't tried yet that I want to. The 7802 that I have was a BIN ebay auction that I ninja'd at like 5am for $10 shipped and it's one of the nicest sounding tubes I have for this amp.