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In Reply to: SV KT-88 Question posted by HiFiBlues on April 01, 2001 at 11:36:30:
I had SVKT88's in my ARC VT100Mk2 in January, that also would not come up to bias. By the time my saga was over I had the Mk3 upgrade from ARC. Whichever KT88's you got, they are in short supply, and to obtain a quad or two matched to the close standards that ARC amps need to function properly is virtually impossible. I agree with Kevin that you may have a damaged tube in the circuit. When it ran, it sounded great, but after a very short time, the tubes, which I returned, looked like they had 20 times the wear that they had been put through. I found out about mine after a few days of being in, after maybe 20 hours of play, when I went back in to check bias.By the way, changing the input/driver tubes in that amp is playing with fire too. Better have brand new, matched tubes, and then adjust the rail voltages for each side as well. Talk to Leonard, or to Chris Ossanna, at ARC, They are very helpful. Power down and check the 1 and 100 ohm resistors next to each tube to see if any have failed. They act as fuses, and the 1 ohm ones go first. Unless you have a catastrophe, where they blow like firecrackers. Don't rum it with those tubes until you check everything out, unless you've got the upgrade itch.
Bob
Follow Ups:
Bob, thanks for the info. Sounds like you've been where I am. Could you please explain your saga a bit more. I understand that you've upgraded, but I'd like to know why. Was it because the tubes didn't work?? Too much trouble??
Thanks..
"Don't want no MTV!!!"
Dear Hifiblues,My saga is in the archives from January. First I put orange globe Amperex 6DJ8's into the 6922 positions. These came out of my preamp, which I had upgraded to some low noise tubes. I neglected to adjust the voltage rails, which I only found about later.
Next I bought two "matched" quads of Svetlana KT88's from a vendor who was supposed to be reputable, mentioned well by the vendor I bought the other tubes from. When I put them in, there was an immediate increase in dynamics, bass, and midbass, compared to the ARC supplied Svetlana 6550C's which were only about 6 months old at that point. I had retubed in the summer.
The trade was a decrease in clarity, especially in the upper midrange. It was certainly different, but not everything I was after. When I opened the amp after a number of days to check bias, it only came up to 124 on one channel. So I called Leonard at ARC. He said that I needed to adjust the rail voltages, something usually to be done by an experienced technician, but I was game with my handy voltmeter, and went at it. Well, something was wrong with the 6DJ8's, because the voltages were all wacky and would not come into line. The voltages varied a whole lot as I switched tube positions, which is what you need to do to help bring it into line. At one point, I shorted across a resistor next to one of the KT88's, but everything seemed to be okay after replacing the fuse. When I switched power tubes around to see if that had an effect on bias, two resistors went off like a firecracker next to a tube, probably the one I took out with the short.
So in this state of discouragement, in over my head, I sent the amp to ARC. I did not send the Amperexes because I had determined that there was something wrong with some of them. For a preamp they may have been fine, but the loading in the power amp required a tube that could take more abuse.
ARC tested the KT88's I sent them. The match numbers were 306-121, 310-118, 310-119, 318-116, 321-119, 324-116, 326-110, 335-115. Look at your old ARC tubeset. The match numbers on the tubes will read within one digit for each number within a quad. ARC will only use tubes if they are this closely matched.
Since my amp was in the shop, costing a fortune for Fedex, I decided to go for the Mk3 upgrade. One reason was that they highly recommended it. Another was that it got rid of the 6922's in favor of a Sovtek 6H30, which is where ARC is taking all their new designs. Look at the Ref2Mk2. The Sovtek 6922's were grainy and restrained on the highs, whereas these new tubes are not. Also, it would not allow me to tube roll, Sovtek making the only one.
Now that it has broken in a month, I can say that it is truly worth it. It has the dynamics and bass punch that the KT88's gave the Mk2, but also has a clarity and purity across the board that it did not have before.
The tube dealer took back the KT88's, minus a charge, minus the shorted tube, which was fair. He thought the limitations that ARC placed on power tube matching to be unreasonable. I can only say that if I had had a well matched set of very good quality 6DJ8's, and the same for the power tubes, it may have worked fine. But I'm not taking chances anymore with my amp. This escapade cost me a lot of money.
Talk to the people at ARC. They are the people who know their equipment best, no matter what anyone else might say. They have been down the road with many different tubes and many different designs, and also know most of the tube vendors you will deal with.
Good luck.
Bob
That was a saga! Unfortunately it did'nt sound like you were ever able to hear what the KT-88's might really do in that amp. I feel your pain right now, although I'm hoping that I can reach a less dramatic conclusion with a little help from folks like you and the rest of this bunch. I appreciate you sharing this info. I think this is a great way to let other people know they are not alone in their woes.Good luck with your MkIII...Let's hope it stays home where it belongs!!
Also bought a tube tester as part of the saga, to have a better idea of what I was putting into my equipment. You may want to test the KT88's at this point to make sure they are in good shape.Take care.
Bob
> > > Whichever KT88's you got, they are in short supply, and to obtain a quad or two matched to the close standards that ARC amps need to function properly is virtually impossible
---------------------------------------------------------------The only thing you need is KT88 that are matched, and are not on the "cool side". Tubes being on the "cool side" can occur with any power tube. It just means the tube will not come up to idle in that circuit. So use a hotter tube.
As to matched Svetlana KT88...don't listen to dealers that say they can't be had. We have imported directly and have 1000 pieces in stock. And re-tube an ARC VT100 every week or so.
What ticks me off is one dealer from "up north" saying "the Svetlana KT88 isn't as good now" since the American importer had hiccups.
Such a silly comment. The ones we have are stunning and consistent with the best Svetlana we ever received.Truth is he can't get them for some reason...so tells the guys the Tesla KT88 is better. Using that tube is an almost guaranteed problem at some point because it truly is flawed. It's either a stupid or dishonest comment. And the end user suffers when his amp goes poof. You can't play games with that amp.
What you say about the front end tubes is true. You can't just plug in new 6922 without adjustments. Doing the power tubes is pretty easy.
Dear Kevin,I have read that you are a reputable dealer, and am interested in your opinion on this. I bought two "matched" quads of SVKT88's from a dealer in California. When I had the tubes tested at ARC during the ultimate rebuild of my VT100, the match numbers ranged from 306-110 to 335-119. When ARC matches their SV6550C's, the match numbers are the same for the foirst number over a quad, and vary by only one for the second number within a quad. In other words, they seem matched to within 1%. These others that I got were only within 10%, and ARC would not use them.
How closely do you guarantee your matched quads and pairs. With my Mk3 now I only need to match pairs, but the new tubeset was still within 1 digit all around. I theorize that in a push pull circuit this gives less distortion.
I am happy with the SV6550C's now, and the 6H30 drivers, but am curious.
I think our friend here has some bad tubes. I don't want him to ruin his amp. Do you guartantee your tubes against damage to the amp they are installed into?
Bob
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