VTL IT-85 developing low hum within 1h

[Miracle1980]

Hello guys, as per title when I power on my integrated VTL-85 I can hear just the usual hiss from tweeters. Then, after 40/50 minutes I can hear an electric low hum from both channels.
The hum is not so loud but I can hear it with silence or low volume passages from my listening spot. It comes from the speakers and that doesn't respond to the volume knob.
My speakers are very sensitive... around 100db.
Is that normal or there is something wrong with my unit?

Power valves (4) are new, just replaced.
Preamp valves (2) and PI ones (4) are still the stock ones.

Thank you

[HotBluePlates]

... VTL-85 ... after 40/50 minutes I can hear an electric low hum from both channels.
The hum is not so loud but I can hear it with silence or low volume passages
...
Is that normal or there is something wrong with my unit? ...

Your VTL-85 probably needs service, with an eye towards the health of filter or bias-supply caps.

What I mean is the one time I've had the symptoms you describe, there was power supply hum (due to failing filter caps) that the amp-feedback mostly-squashed.  But every now & then some hum would pop up out of nowhere.  Opening the feedback loop on the amp exposed the hum that was there all the time; new filter caps fixed the issue.

There could be other reasons for your hum, but I'd start with opening the feedback loop from the speaker jack, and assessing the amp as-it-stands.  It should be reasonably hum-free even with an open loop, for any healthy amp.

[Miracle1980]

@HotBluePlates: do you suggest to have it checked ASAP or I can wait some time (1-2 months)? The hum is there but it's not so loud...

I first need to find a technician in my area (Frankfurt) as I don't know any... and, above all, I will have to prepare myself to be without music for some days.... ;)

Thanks

[Latole]

Miracle1980 did you listen the amp / hum with nothing plug to the amp input ?

For service, ask at any hi-fi store in your city. If they don't work on these amps they may know somebody.

https://www.yelp.com/search?cflt=hifi&find_loc=Frankfurt%2C+Hessen
 

[Miracle1980]

Miracle1980 did you listen the amp / hum with nothing plug to the amp input ?

Yes, I just did it again and the light hum is there also without inputs plugged.
I also opened the metal cage and I placed my ear very close to the amp... there is a small hum but I would say it's really normal... considering that it's a tube amplifier.

Can be related to the very high sensitivity of my speakers that, with 101db, are reproducing every noise?

[HotBluePlates]

@HotBluePlates: do you suggest to have it checked ASAP or I can wait some time (1-2 months)? The hum is there but it's not so loud...

If the hum is not too bad, and you do not see the gray plate material glowing red, you are probably fine to continue operating the amp until you get a chance to take it to a tech.

... I also opened the metal cage and I placed my ear very close to the amp... there is a small hum but I would say it's really normal... considering that it's a tube amplifier.

Audible hum like that from the amp chassis is due to a transformer or choke physically vibrating.  If it gets very loud (and trust me you'll say, "No way is that normal!!") that would indicate a problem.  However, I've had amps that hum acoustically because of imperfect potting of the transformer.