Crack low frequency hiss [Solved]

[x181]

I've completed building my Crack and most measurements check out OK.

I've done a resistance and voltage check twice.  Resistance seems to check out OK but I noticed that the voltage on terminal 1 and the connected terminal on row 6-10 (can't recall which terminal exactly as I don't have the Crack in front of me right now) is about 110V to 114V which may be problematic.  Also, it seems like it takes about 10 seconds for the voltage to stabilize on terminal 1.  It could be my probe but it's as if it fluctuates around 130V or 140V then drops to 110V, although it should drop to around 90V.

What I hear is a low frequency hiss which sounds like something you would get if you amplified a microphone's input; it sounds like an over-amplified over-sensitive microphone picking up ambient air noise.  It's not "microphony" in the sense that it has feedback, it's just like a quiet hiss which doesn't seem to increase when the volume is turned up.  I do start to hear a buzz, which I am assuming is the 120Hz hum after the volume is set to about 75%+.  Is this normal?

I also do hear a high frequency buzz but only when my RCA cables are connected to my computer.  The frequency changes when I perform different tasks on my computer so I am assuming this is just the interference picked up by my sound card, amplified.

My main concern though is the low frequency hiss.  Could the capacitor/resistor terminals 12/14 be a possible source of this?  I have a feeling their connections may not be very good.  What are some other possibilities?

I can post the resistance and voltage values when I get home.  I was hoping this was a common problem with a straightforward solution.  Thanks in advance.

Update:
I purchased a few Winged C tubes and the hiss was still apparent.  However, now the hiss is gone so I'm guessing the smaller tube was the culprit and just needed a good burn-in.  I also re-soldered the capacitor joints just to be safe.

It's been a while since this issue corrected itself, but as you can imagine, I've been enjoying my amp too much to remember to come back here and give an update.  I thought one is due since this is an excellent product.

P.S. Thank you for the chocolates.

[Doc B.]

The tube are usually responsible for hiss. Give the tubes some time to form their cathodes and the hiss may go away. Re: 120Hz at 75% volume setting - this is not quite enough info, as we don't know what your normal listening level setting is. If it's 75% then you may need to check cabling, etc. If 75% blows your head off like it does with a lot of setups, the 120Hz is probably irrelevant at actual listening levels.

[x181]

Thanks for the quick response.

I noticed the opposite to be true with the tubes.  The amp starts out quiet then the hiss slowly fades in after about 10 or 20 seconds and stays consistent at any volume level.  Even after an hour of use, I can hear the hiss.

My typical listening volume is around 25%.  As I was checking to see if the hiss increased in volume as I turned up the pot, I noticed the buzz (sounds like a guitar amp) around 75% which I am assuming is the 120Hz hum.

The 6080 tube does have what looks like a crack in one of the grids.  Could that have something to do with it?

[Grainger49]

Since this comes up so often I have made this FAQ #4.

[Paul Joppa]

It takes around 20 seconds for the 6080 to warm up, depending on the tube - those cathodes are large!

It takes 50-200 hours for a cathode to form, depending on the tube. Most of this is done at the factory, the tube barely works at all when first sealed off. But it is not uncommon for it to need another 50 hours to settle in. Some factories are better about this than others, so kind of "luck of the draw".

The 120Hz is likely RF radiation from somewhere - could be diode reverse recovery, could be flourescent lights, could be a computer power supply. It peaks at 3/4 volume because that's the midpoint of the volume control (it's logarithmic, not linear) so the impedance is highest at that point. If it becomes a problem, a tiny capacitor from grid to cathode of thedriver tube might kill it. I've never tried it (never had to) but every environment is different.

[x181]

Thanks for the replies.  It seems the best course of action right now is to use this amp for about 50 to 100 hours and see if the hiss goes away.  It seems everything else is normal.

[STURMJ]

You should try another source too.  It sounds like you are using a computer, there have been others hearing a buzz with computer as a source.   I doubt that this will make the hiss go away, but it wouldn't hurt to try.

[x181]

Thanks, I haven't tried another source yet but I did find that the high frequency buzz varies in frequency based on my CPU load which leads me to believe my soundcard, although it's supposedly shielded, is picking up interference from the CPU or PCI bus.  I've set some Daft Punk - Discovery tracks on loop.  That should "exercise" the headphones and amp...while I am at home at least.

[Natural Sound]

Thanks, I haven't tried another source yet but I did find that the high frequency buzz varies in frequency based on my CPU load which leads me to believe my soundcard, although it's supposedly shielded, is picking up interference from the CPU or PCI bus.  I've set some Daft Punk - Discovery tracks on loop.  That should "exercise" the headphones and amp...while I am at home at least.

I had a 120hz "buzz" in my amp, right channel only. But it would come and go. When it buzzed I found that if I gave the driver tube a very light tap the buzz would stop. I replaced the tube and never had a problem again. Have you tried any other tubes?

[Laudanum]

Was just going to go through my standard "always have a couple spare tubes on hand".   Easiest thing to try and eventually, you will need a spare to get you back up and listening to tunes.

[x181]

I just bought the kit so I haven't had a chance to buy or try tubes.  I notice that the large 6080 rattles when I rotate it in my hands.  Best way to describe it is it sounds like a combination lock setting into its proper place.  I noticed this when I first received the kit and figured it was supposed to be that way (which may be the case).  I tried tapping on the 6080 while the amp is live and I hear a ringing sound only though the headphones which according to the manual hints at a bad tube, or rarely a capacitor.  Is there any way I can isolate the tube as the source of the problem without having to buy a new tube?

Forgive me if I sound ignorant, as this is the first time I've used a tube amp, but I also notice a blue electrical wave radiate across the surface of the grids inside the tube after about 10 seconds.  I am assuming this is normal as well.

[Grainger49]

If I understand your first problem correctly, I have heard of there being something inside tubes that rattles.  If it isn't metal, shorting the elements there is a problem.  But if not it should have no effect on the sound or life span of the tube.

Most tubes will pass a ringing sound through the audio path if you tap on them.  The worst offenders, truly microphonic tubes, will ping or ring when the selector switch is moved or the top plate tapped.

As for the blue light show, you are just lucky.  I've been looking for the blue tubes for years and can't find them.  Seriously, that is just from a trace gas inside the tube.  It harms nothing and gives a show in the dark.

Come to think of it, my stock Sovtek 2A3 tubes in my Paramours had a blue glow at the top of the tube.  It could be seen only in complete dark.  It was a deep blue.

[Laudanum]

There really is no way to troubleshoot a suspected tube problem other than to try another tube.  These guys can walk you through all sorts of troubleshooting but the amp can work fine and measure fine and still have some noise that is due to the tube itself.   Fortunately, 6080 and 12AU7 tubes can be had inexpensively.  There are expensive new old stock (NOS) brands and variants of each type but they also are both available inexpensively in the more common, non "boutique" brands or in current production tubes.  If the conclusion is that it may very well be a noisy tube that doesnt sort itself, Im sure that Bottlehead will send you a replacement.  But with tube gear, you should have atleast a spare of each tube, even if you arent interested in tube rolling.  They eventually get tired and things happen where a tube can just go south for whatever reason and accidents happen during dusting or whatever else.  Both the 12AU7 and the 6080 / 6AS7 are available as a new production tubes in the $10 range if you dont want to buy old stock vintage tubes.

http://tubedepot.com/6as7.html
http://tubedepot.com/12au7.html

[Grainger49]

To add to what Desmond said above, I have recently seen a few products that didn't measure right and it was a bad tube.  I don't remember if they were worn out or new bad tubes.

And I second the suggestion that everyone should have a set of tubes for a "backup."

[x181]

It seems that the hissing has mostly isolated itself to the right channel and is barely audible in the left channel.  Any ideas what is going on?