KT120s and Biasing in Premier 11A

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TubesnQuads
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KT120s and Biasing in Premier 11A

Post by TubesnQuads »

Hi, I'm new to the board. I live in Seattle in the US. For context, I have a CJ Premier 16 LS line stage, Premier 11A amplifier, Lector CDP-7 CD player (with NOS Mullard 12AU7s), Eastern Electric Minimax Phonostage, and Quad ESL-989 speakers fully modified by Electrostatic Solutions on Mye Sound stands. All wiring is Transparent Audio Super MM2. All set up in a dedicated listening room.

I have a problem I am hoping some of you know about. My Premier 11A has NOS Raytheon blackplate windmill getter 5751s and NOS Brimar 6FQ7s. I had no problem biasing the stock Sovtek 6550s with them and of course it sounded great. I just substituted new Tung Sol KT120s for the 6550s. When I powered up, all the biasing LEDs lit up. I waited 15 minutes for it to stabilize. Then when I adjusted the biasing screws, no matter how far down I turned them the LEDs remained on. I turned them all the way up and all the way down, but they remained on. Last night after several hours playing the system, I was able to turn one screw down enough to get the biasing LED to go off, but when I turned it back up to set the bias it lit up and remained on no matter what I did with the biasing screw. All other LEDs remained lit and did not respond to the biasing screws.

Has anyone else had this problem, and if so, what did you do? Is there something wrong with my biasing circuit? If it is going to be this way, what do I do about biasing the KT120s? Seems I could set the screws all the way down, all the way up, or in the middle. Is that a solution? Or I could try to replicate the one where I got the LED to go off, and set the other biasing screws to approximately the same point. Is that a solution? The tubes do throw off a lot of heat, but the upper shelf may be a little low for the taller KT120s, so I am not sure that that indicates anything. I really like the KT120s, but I am a little uncomfortable not having them biased correctly.

HELP?!
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Re: KT120s and Biasing in Premier 11A

Post by roberto »

Hola TubesnQuads,

Never turn the bias all the way to the right! Never do that. If you can not adjust the bias, it is because in some circuits you are able to change the KT-120s direct with the 6550s. If you can not adjust the bias, you should contact Jeff Fischel at Conrad Johnson, and tell him your problem. I think you have to do a little mod inside your amp, then this mod will allow you to use the KT-120s. You should not use them, if the LED are red or Yellow. The LEDs on each tube should be not lit when there is not signal applied.

Happy listening!
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Re: KT120s and Biasing in Premier 11A

Post by jahatl513 »

I think that is correct information and you do need to call CJ or email them
https://conradjohnson.com/contact/
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Re: KT120s and Biasing in Premier 11A

Post by ajf75 »

This doesn't match my experience as I'm currently running KT120's in my P11A and the bias adjustment procedure was uneventful. I recall finding that others also reported no issues (here or other audiophile forums) before I was willing to try these tubes myself.

Sorry that I can't speculate as to the cause of OP's trouble.
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Re: KT120s and Biasing in Premier 11A

Post by TubesnQuads »

It is curious. I wonder if the tubes will "settle in." I have an email out to CJ on this. Thanks for the warning Roberto. I am going to keep the bias setting on the low end until I figure this out.
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Re: KT120s and Biasing in Premier 11A

Post by joarfeve »

Hi,

I saw the same problem in some CJ power amplifier like LP125sa+, MV60SE, even ART Monos...it ended up being a batch of KT120... if you have the chance, try another quartet...

Greetings from Peru
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Re: KT120s and Biasing in Premier 11A

Post by roberto »

Hola Peru!...

On the other hand, we have separate bias for each tube. There is no need to be quads matched. That's what's bugs me. All tubes with the same problem? It looks to me that the bias resistors are not doing the job. They are out of the adjusting range, perhaps. If I were you, I will ask to Jeff Fischer at Conrad Johnson. He might has a better answer than me.
Happy listening!
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Re: KT120s and Biasing in Premier 11A

Post by Joe Appierto »

I had a system similar to what you describe (16LS2, Prem 11A,Quad 989) and never had the problem you describe when using KT120 tubes. I suspect it's something wrong with the batch of KT120s since you report no issue with another tube type. Also think it'd be a good idea not to use them until you identify the problem.
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Re: KT120s and Biasing in Premier 11A

Post by TDLM5 »

Hi, Tubesnquads...

The previous poster may be correct about the set of new tubes. Although new TungSols are what experts use and recommend these days. Do 6550’s work reliably in your amp?

Are you a member of the PNWAS? Their monthly meeting is this Saturday. Would be happy to talk about your issues at greater length if you are there.

My MV-125 has hit a rough patch, and I’m considering trying KT-120’s as well. I have inspected the underside of my amp’s PCB, and there are a few suspect solder joints. You might look at your amp’s underside.

You might try reseating - carefully- each tube. That seems like a long shot. Cleaning and retensioning the tube socket pins might be worth trying. That’s also a long shot.

Does Jeff at C-J “bless” the switch from 6550 to KT-120? Just curious.

Good luck. I hope you have it all sorted out by now.
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Re: KT120s and Biasing in Premier 11A

Post by TubesnQuads »

Thanks for all the replies. I reached out to my tube seller, Upscale Audio. They said that there is nothing wrong with my tubes, but that my tubes had tested in about the middle of the range for bias for KT120s, and that was apparently too high for my 6550-based Premier 11a amplifier. So they told me to return my tubes, and they would send me a replacement matched set that had a lower bias testing. I paid for return shipping, and they paid to ship the replacements, which I thought was very fair. I received the replacements last week, plugged them in, and they biased right up at about the same settings I had used for my 6550s. I came away very impressed with the customer service at Upscale Audio, and I would highly recommend them.
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Re: KT120s and Biasing in Premier 11A

Post by TubesnQuads »

TDLMS, I am not a member of PNWAS, though I have wanted to attend a meeting for some time. I did not get CJ's position on KT120s for Premier 11as directly, but others on this board report that they have spoken to Jeff and KT120s are the only approved substitution for the Premier 11a complement of tubes. I like my KT120s quite a bit better than the stock 6550s. I would not mind a NIB set of NOS Tung Sols, but those are pretty pricey and right now I have 6 6922s in my Premier 16 line stage that I want to replace with NOS Amperex or Mullard.
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Re: KT120s and Biasing in Premier 11A

Post by Wildcat »

Too late to really help here, but, my KT120s from Uncle Kevvy biased right up in my Premier 11. When you order tubes from Upscale now, they ask you for details on which amplifier you are buying tubes for. I would suppose that if they note we have a Premier 11, they would send the correct tubes which should bias properly. (I believe they added this because of an issue with a particular model of Audio Research power amp.)
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Re: KT120s and Biasing in Premier 11A

Post by roberto »

Hola. I am going to chime a little more. The Audio Research power amps, uses only one variable resistor for two tubes, not the case on C-J. The tubes for Audio Research amps must be matched, on C-J this is not necessary, because each output tube has its own variable resistor. Also with the aid of the LED, you do know when the bias is correct. Blinking LEDs are correct while playing the music. Not so when there is no signal applied.
Happy listening!.
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Re: KT120s and Biasing in Premier 11A

Post by Wildcat »

I think if I remember it correctly, the wrong tubes were blowing resistors inside the ARC power amp. It wasn't a case of biasing so much as it was actual damage to the circuitry. It was one of those cases where you were only to buy tubes from ARC with the proper specs. If I can find it, I'll link it here. (I'm thinking it was the VT100 or similar...?) I know that is what prompted Upscale to ask which amp you have, as they will not sell tubes if they know that they are going into that particular ARC amplifier.
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