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ARC Vsi60 with KT120 valves - issues

Hi guys,
I'm a newbie here and also new to the world of tube amps. I bought my Vsi60 back in July and it came supplied with the KT120 valves. I did what I considered an extensive demo with my speakers (ProAc 1sc) and my CD player (Cyrus CD6SE) at the shop and was blown away at first. However after receiving my amp and setting it up at home, I felt that the sound was way too bright. Violins sounded thin and edgy, and the overall sound was one of brightness and artificiality (violins in some recordings sounded like synthesizers to my ears!). At first I thought it was a burning in process but after about 150 hours or so, I decided I had to do something about the amp or sell it! So I decided to get a quartet of Winged C 6550 and changed the preamp tubes to the EH 6H30 (with the gold pins) and the sound was exactly as I hoped an amp would sound. It was rich, lush - instruments and voices sounded very lifelike with just the right amount of edge to the tone to make it sound clear without being harsh or artificial. I love the sound and my amp now!

However, I am still curious about the KT120 valves since many people on this forum have expressed their preference for this valve. My query to the more experienced guys out here is - what is the correct bias for the KT120 in this particular amp? The booklet that came with it only states the correct setting for the 6550 valves (I kept the same setting for the KT120s) and the shop basically told me to stick to the same settings for both tubes.

Since ARC does not have a customer email address and I live in Australia, I would appreciate some feedback regarding making this KT120 sound at its potential best particularly with regards to bias settings etc. As it is my first valve amp, I am afraid of blowing it up or taxing it if I play around with the bias settings without understanding it more.

Thanks in advance.
tritonus
stu223
3 posts
 
Couldn't agree more the kt 120 is too bright , sound stage well there is none . Midrange I'm trying to find it still !! Will try the 6550 or sell it
hifigeek1
770 posts
 
Like tone controls, the KT-120 will sound different depending on the amp and system, so there is no right or wrong answer. Bias specs will vary depending on the amp it's put in. ARC runs that tube at 65mV.
hifigeek1
770 posts
 
Note: There are a few integrated amps that ARC does throttle back the bias on. I would suggest you contact ARC and ask.
salectric
755 posts
 
For what it's worth, in my Emotive Audio Vitas, I liked the KT-120 best at 65ma. This is with a B+ of 500v so it's not running the KT-120's anywhere close to full power. That said, in the same amps I prefer the sound of the KT-77 Genalex reissue.
wolf_garcia
6,708 posts
 
Although I understand that mentioning tube rolling results in another brand of amp might be meaningless, I offer the following: I've used the stock Tung Sol 6550s, Sovtek KT88s, and KT120s in my Jolida 502p. All sounded great, but I prefer the 120s (driven at the amp's recommended 500mv bias for no particular reason except maybe the stock specs, and the sound) as they seem to have a really good grip on things without the harshness or lack of soundstage noted by others. Also noted by others is an overwhelming acceptance of 120s generally, with criticism being surprisingly rare (thus welcome as I get bored with too many positive reviews of anything). I read a review of a newish ARC 75 watt integrated recently and it was found to have harsh highs by that reviewer, which is surprising and simply TAWDRY for a ARC amp costing around 10 grand. I've had some success in making a good thing better with driver tube swapping...currently using Psvane 12AX7-T IIs and Mullard CV4024s (12AT7) that seem to give the 120 tubes (and my ears) room to thrive.
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