Thanks for the advice - I'm puttin an order in next week. I think I will give them a try.
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Thanks for your note of recommendation for Upscale Audio; we need to support the good guys. Be sure to put this info on US audiomart and also Sound Offer web sites and start doing business there. This site stinks now and the arrogant owners won't listen to the members. Let's vote with our dollars and move.
I bought a matched octet of 6550's for my ARC VT100 Mk II from Kevin a few years ago. I was also very satisfied with the experience, but subsequently Upscale announced they would no longer sell tubes for this particular amp. I am not sure where I will go the next time this amp needs re-tubing. I'm thinking of going with KT120s, but I dread paying $100 each from ARC. Any other VT100 MkII owners care to comment on this thread?
Mabonn, just a guess here, but I think it may be because the VT100 Mk II may not be user friendly when it comes to biasing the tubes. I've read somewhere that the bias pots are very close to some high voltage rails or contacts. If my surmise is correct, it's possible that Kevin may not sell you the KT-120s either for safety reasons. I know that he stocks both types of tubes. Btw, Kevin charges $90 for a pair of KT-120s, less than half the ARC price.
You should call him to see what the story is. Please report back. If Gary (Hifigeek) catches this post, perhaps he can weigh in about whether an electronics tech should install and bias the power tubes in a VT100 Mk II.
The reason ARC doesn't want those tubes in that series of amp is because that tube is mounted horizontally. Output tube sockets can become loose over time. It's less of an issue when the tube is mounted vertically. Because the KT-120 is taller, and heavier then it's 6550 brethren, it is far more prone to sagging in it's socket and losing contact with the socket. Thereby causing it to arc, damaging both the tube and surrounding circuit components.
Mabonn, just a guess here, but I think it may be because the VT100 Mk II may not be user friendly when it comes to biasing the tubes. I've read somewhere that the bias pots are very close to some high voltage rails or contacts.*
Good guess. I got a helluva shock when I was biasing the tubes on the VT100 Mk II I owned once, & I was being extremely careful. Just seemed very dangerous, all around.
Gary (Hifigeek) -- is Steveaudio's experience a rare occurrence with the VT100, or is my general recollection re installing and biasing power tubes in the VT100 on point. More to the point, as an ARC authorized tech, would you recomend that only someone who knows his/her way with electronics repairs handle a retube?
Bifwynne. Well obviously being someone who services ARC under warranty I'm a bit biased. Did I just say that? There are quite a few ARC amps that I don't recommend people re-tube and bias unless the bias voltage points are outside the amp away from high voltage traces. These amps run at around 400Vdc and pack quite a wallop as Steve found out! Imagine someone with a weak heart or an unsteady hand, or improper test probes attempting to set bias. All it takes is one slip, and a trace can blow right off the circuit board and leave a crater in the board just for starters. Is paying for a few hours of labor worth that kind of dangerous exposure? I leave it to each person to answer that.
If I recall, Keven said he would not sell tubes for VT100 Mk II amps because of the design. He did not specify electrical design or physical design. I have re-biased this amp and I understand the dangers. I trust myself to do it without shocking myself. Perhaps I need to talk to Kevin in more detail to find out what his actual concerns are.
Gary (Hifigeek) is right on as usual. I own an ARC VS-115 amp which has bias test jacks OUTSIDE the amp. The ARC supplied plastic screw-driver thingy is a very safe tool to use to access the bias pots. I bought the metal screen tube cage, but will NOT pull the top panel that covers the works in order to screw the cage on. I rest the cage on top without screwing it on just to protect the tubes from an accident. It's just too damn dangerous to expose the electronics!
I just re-tubed my Ref 5 and VS-115 using Kevin's tubes. The old tubes from the Ref 5 and VS-115 had over 3000 hours on them. Major, major improvement!!
Regarding using the KT120 tubes in VT 100 amp. As some of the responders of the OP will remember I have just changed over from 6550 to the KT120s. (check older posts from about a month ago). Got them from tube depot here in Canada, but they also ship to USA. They are indeed a heavier tube and overall more robust construction, but fit well in the amp with enough clearances all around and very easy to bias. After a few days burn in, they sound very nice and I'm pleased. The horizontal mounting issue may be valid but I don't think so really. The socket pins on this amp are TIGHT. Also, they seem to run a little cooler but that might just be my imagination....
In addition, the front end tubes (6922) should be replaced every other time the output tubes are replaced. Set up after new tubes are installed is a royal pain and the tubes MUST come from ARC in order to set the front end up properly for lowest distortion. I don't set up the front end of Mk1's and MK2's unless I get the tubes from ARC. Mk3's use a different circuit topology.
Has anyone re-tubed a CAT JL3 Signature (KT120) from Upscale Audio?
I live in Sydney, Australia and I ordered a CAT SL1 Ren 5 months ago and haven't received it as yet. I don't want to think how long it will take to get 32 matched KT120 tubes. I would very much appreciate it if anyone could provide any feedback with regards to re-tubing CAT JL3 Sig from sources other than Ken Stevens's Convergent Audio.