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Tube hiss - any idea what's causing it?

I own an Eastern Electric M520 integrated amp that I am very fond of. However, recently I have noticed that there is a great deal of rather disturbing hiss eminating from my Omega XRS speakers once I power the unit up. The hiss does not increase when the volume is turned up. It's always there and very noticable during quiet passages. It's definitely a hiss and not a hum.

I checked the tubes and noticed that when I touch or slightly jiggle the front EF86 tubes, the hiss gets louder/softer when they are moved. Could it be that these tubes are going already? I do not have many hours on them. Maybe 100 at best. Should I consider replacing them or perhaps checking the other tubes too - perhaps the 5 AR4 rectifier tubes? I doubt it's the power tubes.

Do you have any thoughts/recommendations for me at this point. I am not sure how to proceed. As always, thanks for your time.
jpstereo
slates
35 posts
 

You know come to think of this , I have the same issue in my ARC ref 1, I just bought 4 matched sets of EH 6922 Gold pin, and it seems like one or more has gone microphonic, with my ear up close to the speaker you could hear a hiss and a bit of a hum, the hum being the lesser. Also it's seems less in the right channel than the left. I will be honest, I do not have any dampers on the tubes.
How can I rectify which tube(s) it the culprit? The tubes have less than 10 Hrs on them.

thanks in advance guys.
bless
7 posts
 
Check out the tube sockets ,sometimes they don't hold the tube pins tight. Erwin.
ghosthouse's avatar
ghosthouse
4,296 posts
 
Try moving them around. I had some hissing and rotated all - Power and Pre-amp tubes, Left to Right. Checked bias and adjusted as needed. The hissing went away. Thought in my case it might have been an oxidized connection that changing out "fixed". In any case swapping all L to R seems to have fixed things - now for several months.
jpstereo OP
53 posts
 
I switched all the driver tubes around L-R and I still have the hiss. Getting some new EF86s and we'll see what happens. Not sure what my options are if that doesn't work.
herman
2,216 posts
 
The hiss is 99% for sure coming from the tubes. It is very unusual to get the results that Mr. Ghost did and have it go away. That is unless what he describes as hiss is actually something else. Corroded pins usually make more of an intermittent crackling sound than a steady hissing sound.

BTW hiss and microphonic are 2 different things. Hiss is just what it sounds like, a hissing sound that is steady. Microphonic means that external vibrations are picked up by the tube and you hear it through the speaker. If you yell at the tube and you hear your voice from the speaker then it is microphonic; meaning it acts like a microphone. You can also tap on the tube with something like the eraser end of a pencil and if you hear a ringing sound it is to some degree microphonic, but all tubes are if you tap them hard enough and tapping on hot tubes can damage them or cause them too fail so I don't recommend that.
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