Hi,
This is not a well-known amplifier, so here is a review I wrote years ago about it:
http://www.musicgearreview.com/reviews.php?man=Montarbo&cat=Guitar_Amplifiers&mod=165%20NR
And Schematics I had directly from the company:

Quote from: J M Fahey on January 01, 2010, 01:37:08 PM
Hi Matb5
Great amps, specially their PA equipment, tape echo and the like.
I think they even had their own microphones.
Your amp *may* work on 120V, but will do so poorly, underbiased, etc.
You have a grounding problem.
You have 2 ways to plug your amp into the transformer (I hope that's what your "converter" is) and probably two ways to plug your transformer into the  outlet.
That makes 4 combinations; try them all.
One of them should be the less noisy, and to boot, anyone which lights a test-screwdriver when touching the chassis, is *dangerous*, so mark your power plugs (and always use that screwdriver) to play silent and safe.
Ah !, post your schematics.
Be sure they are readable.
Thanks.
PS: I guess you're Italian.

Thanks for this fast and informative answer.
The amp is a Montarbo 165NR. One of the best purchases (and surprises) I've made. There is not much info on this amp on the internet, so I had written a review years ago, for those who are interested: http://www.musicgearreview.com/reviews.php?man=Montarbo&cat=Guitar_Amplifiers&mod=165%20NR
I don't know Montarbo's products other than this amp, but the quality of this one and the reviews I've read here and there about their sound equipment make me agree with your comments.
I DO have a grounding problem, you're right. The amp isn't grounded (simple 2-prong plug). There is a ground point on the back of the cabinet, but I don't really know how to use it. I attached the schematics I got directly from Montarbo a while ago.
The transformer is grounded, so I can only plug it one way. I will try plugging the amp differently.

PS: I am French, that was close

Greetings (and happy new year),
I'm new to this forum, this is my first post.
I have a Montarbo guitar amp that I really like. It was made in Italy in the 70s, and is solid state.
Here is my problem / question:
I now live in the US, and, as the amp is original, it is designed for the 230V/50Hz electric system. I've used it with success through a converter (which by the way converts voltage but doesn't change the 60Hz frequency). Problem: this is not very convenient, and it generates quite an amount of noise.
Sorry if the question sounds dumb (I'm in the newbie section, right?), but is there any risk in using the amp directly on the US 120V system (with the proper plug adapter)? My poor knowledge of electronics makes me think it might loose power, which is not really an issue, but would it explode in my face? be damaged? sound dull? I wouldn't ask the question if I was interested in doing things the other way (plugging a US amp on European system) as I assume some electronic components might not appreciate the voltage difference.
Thanks!

PS: I think I have some schematics somewhere of this amp, if that can help