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In Reply to: Re: Kt88's took out my friend's VT 130 SE posted by kentaja@yahoo.com on September 17, 2004 at 13:00:53:
I am telling people what WILL happen. The fact that you doubt it is based on what? Nothing from what I can tell. Before you dispute what I say call and talk to Chris at ARC. Untill then you might like to retract your statement.Better yet buy one and stick some KT 88's in it. Then be "suprised".
Follow Ups:
Can you point out to me where I discounted your experience? I don't believe I did.I questioned, and that was it, questioned, the assertion that the amp has no cathode follower. I was assuming you were referring to the typical driver stage in ARC amps executed as a cathode follower. And again, I did not claim the VT130 had a driver stage executed as a cathode follower. I said I did not know since I had never seen a circuit diagram of the VT130. My error here was in assuming. I should have asked you to clarify this point since ARC input stages typically employ cathode followers in several locations, with the driver stage generally always a cathode follower.
What I did say was that ARC amps, from the beginning, share a very common theme when it comes to circuit topology. A look at their circuits over the years will confirm this point. UL output stages are virtually identical since day one. Some changes in operating points, or number of tubes, but the same UL circuit. Input stages have gone through many revisions, but still sport similiar circuit topologies, even with the newer generation of hybrids. Changes in tubes have caused them to either scale up or scale down the circuit complexity based on gain, interstage coupling, actual tubes used, etc.
I did not say anywhere that running KT88 in VT130 was an OK thing to do. I have visited with ARC on this topic in the past, and in general, ARC frowns on KT88, KT90 or just about any other tube in the circuit, unless the circuit was designed around that particular tube.
Whether or not the failure of the VT130 was due to using KT88 in the output stage and the circuit having no driver/cathode follower stage, I do not know. How could I? But my guess here is the KT88s where probably of questionable quality and bit the dust, taking out some of the amp. Again, just my guess based on a sketchy description of actually what happened and having a great deal of experience with ARC amps and bad tubes in ARC amps.
Actually, my real failure was in responding at all. Just trying to pass on a little information regarding ARC tube circuits, in general.
I just am trying to give a heads up to people that beleive that it is common knowledge that KT 88's are "always" interchangable with 6550's. My info comes to me from the direct experience of a an old friend who has been using ARC gear since the D50. This is what ARC told him. I have no reason to dispute it.It just seemd to me that you were trying to contradict what I was saying. Weren't you? If not I appologize for the kurt tone.
We just had a misunderstanding as to what each of us was trying to say.I agree that the 6550 and KT88 are not always drop-in replacements. Much like 6DJ8 versus 6922. Many times that can be direct replacments.
It is always a good idea to contact the maker of the gear before substituting tubes. Generally everything will work just fine. But their are times, i.e. VT130/KT88 where making the switch can have some rather bad results.
There are times when it would be nice if ARC would give users a bit more information about replacing tubes rather than their assumption that you just need to buy a set from them. The input driver tube adjustments on the VT 100,100 mkII,50 are agood example of this. No mention of it in the manual at all. I'm not sure really how critical it is but it would be nice to know about anyway.I agree with you on tube life. For my use, tube life is very important.I would rather spend my time listening to music when I want to and not changing tubes. I do like an amp that won't stress them so badly that they die prematurely. That's just one of the many things I love about my ARC amp.
And as an ARC customer that has visited with factory, you know to person up there they are more than willing to help their customers. A simple phone call can save a lot of headaches! Can't even begin to say how much time folks like Chris, Leonard, etc. have spent on the phone with me over the years helping with questions, working through issues on gear that is in for service, etc. That kind of customer support keeps me coming back for more!
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