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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
9 responses Add your response
brf
1,715 posts
 
Try calling Synergy Audio and Visual, they are the Australian importers for ARC.
zd542
5,339 posts
 
I think you now have your amp set up right, like it is now. The 6550 is a much nicer sounding tube (in my opinion). I think the KT120's are a bit more powerful, but you don't need that with your speakers.
xti16
1,525 posts
 
When I got my Octave V110 (designed for the KT120's) I loved the sound for about the first 30 hrs (a very warm engaging sound). After that they sounded too forward in the mids and the highs were a hair rough. After 200 hrs they seemed to settle down. Bass is nice and punchy but the mids seemed a little too detailed but the highs smoothed out. Changing the drivers (12at7's) to Mullard's (very warm) helped a lot and made them sound acceptable to me.

That being I am somewhat spoiled. I prefer the Ei KT90 over the KT120. My next favorite is the vintage TS 6550 solid grey plate with my absolute favorite being the vintage TS 6550 sold black plate. Now I will admit the only tubes I can run at 110W power (620V plate voltage) is the Ei KT90 and the KT120. The others I can only run at 70W power (540V plate voltage). I do like the added 'control' of sound at 110W. I also have SED 6550 winged C's and SED KT88 winged C's along with the Gold Lion KT88 re-issue. The original Gec KT88 is very nice but a bit dry sounding like the SED 6550 and KT88's. Vintage Tesla KT88's are very nice but like the EAT KT88's they don't like the 540V plate voltage. Lost 3 of 6 EAT kt88's and 1 Tesla KT88 in less than 30 hrs.

Anyhow that's my opinion and the input and driver tubes can make the KT120's sound better but you have to try them for yourself to get the sound you like the most.
grannyring's avatar
grannyring
6,502 posts
 
I had the same experience with a different amp. The 120's sound more powerful and dynamic, but not as warm, lush and musical in the end.
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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