[Early B.]

I recently added a tube preamp in my system. I immediately encountered a consistent, low level hissing sound from both speakers. I don't believe it's a grounding issue (it's a hiss, not a hum). It's more like a tube problem if I had to guess. My rationale is that when I power up the system, the hiss is loud, but after 15 - 20 seconds, the hissing sound slowly becomes softer as the tube warm up. Again, I'm guessing it would be one of the tubes on the power supply board because if it's one of the two 12au7 tubes on the preamp board, I'd probably only hear it on one side????

Anyone else ever have this problem? Is it a bad tube or something else?

Thanks.

[FullRangeMan]

This does not characterize a valve as defective, just noisy.
I would change this tube for other tube, other test is is to touch your finger on the suspected valve gently to see if anything changes, if the problem is the tube the noise will be changed, avoid hitting in the tube.
https://www.youtube.com/watch?v=13n1VCR6UXw

[borism]

It could be a noisy tube but I would also consider dirty or oxidized pins. To check for that you can just pull out and reset the tube in the socket a few times in a row. It can't hurt to try.

[Rusty Jefferson]

It could certainly be that both tubes are noisy,  or noisy in your system.  If you have efficient speakers (100db-ish) and your amp has say 25-28db of gain, and then you add a tube preamp with say 12-18db of gain (normal for 12au7), you could experience tube noise with all but the quietest tubes available.  I had this problem, and had to move to a lower gain amp and preamp, as I find any system noise unacceptable.

[Early B.]

OK, I'll let the preamp cool off for a couple of hours, then clean the tube pins.

I just noticed something very strange when I powered down the system -- it sounded like an aircraft engine coming to a stop -- first a little loud, then softening up. This was about 10 seconds. I've had tube gear before, but never heard this. 

[JackD]

That preamp has 23 db gain and it appears your speakers are also very efficient so some level of hiss when you get closer to the speakers may be a fact of life unless you want to use something like the Rothwell attenuators to knock down the gain on the preamp.

[Early B.]

It could certainly be that both tubes are noisy,  or noisy in your system.  If you have efficient speakers (100db-ish) and your amp has say 25-28db of gain, and then you add a tube preamp with say 12-18db of gain (normal for 12au7), you could experience tube noise with all but the quietest tubes available.  I had this problem, and had to move to a lower gain amp and preamp, as I find any system noise unacceptable.

Yeah, this could be a problem, as well. My speakers are 98dB, my amps have 22 dB of gain, and the preamp has 23 dB.   

[Early B.]

That preamp has 23 db gain and it appears your speakers are also very efficient so some level of hiss when you get closer to the speakers may be a fact of life unless you want to use something like the Rothwell attenuators to knock down the gain on the preamp.

Is there another way to reduce the gain in the preamp internally, either through lower gain, direct replacement tubes or resistor changes???

[JackD]

Not with the tubes as that is the low gain model in that line.  As to using resistors that would have to come from one of the resident tube designers like Steve or one of the others.  Looks like a fairly simple design on the inside.  Your amps listed in your system don't have high gain nor are they overly sensitive.

[FullRangeMan]

OK, I'll let the preamp cool off for a couple of hours, then clean the tube pins.

You need to clean the sockets also with a non greasy, no residue electric contact cleaner:
https://www.amazon.co.uk/Silverhook-SGEC1-Electrical-Contact-Cleaner/dp/B006ZP7S5E

[Early B.]

OK, I just removed the tubes to clean them and realized that the pair of 12au7 tubes appear to be generic USA tubes, not the premium Psvane tubes that was advertised. Not cool.

[JakeJ]

Sorry to hear that, Early.  Put it back on the seller to cough up the advertised tubes.  Is the ad still up?  If so get a copy of the description.

[Early B.]

Here is a pic of the 12au7 tubes. Are these generic or does anyone recognize the brand?

[Rusty Jefferson]

That looks like it could be an RCA Cleartop, or a knockoff.

It would be worth investing in a pair of NOS tubes from one of the well known dealers frequently mentioned here that you've specifically requested be very low noise (and microphonics if possible) because of the potential gain issues. 

[borism]

It does look like an RCA cleartop with side getter. I bought several from Brent Jessee - some labeled 'CONN' (organ manufacturer). They are highly regarded and were recommended by my preamp manufacturer. How is the sound after cleaning the pins/sockets?

[Early B.]

It does look like an RCA cleartop with side getter. I bought several from Brent Jessee - some labeled 'CONN' (organ manufacturer). They are highly regarded and were recommended by my preamp manufacturer. How is the sound after cleaning the pins/sockets?

I didn't bother to try them again. I'm waiting on a pair of new Genalex Gold Lions to arrive.

[thorman]

Just is case you want to try other 12au7's  I would highly recommend ( Upscale Audio ) Tungsram 12au7 tubes.....Reasonably priced with a neutral presence, excellent Bass and mid's . I have used /tried ,many and found the Tungsram's to be the most Honest and a " Best Buy " in my Opinion....You can pay more , but may not get better results.....

[Early B.]

UPDATE -- changed out the preamp tubes and the hiss is still there just as before. Perhaps I have too much gain in my system. I dunno.

[FullRangeMan]

It dont have a hum pot to adjust?

[RDavidson]

UPDATE -- changed out the preamp tubes and the hiss is still there just as before. Perhaps I have too much gain in my system. I dunno.

Very likely