KT120s and Biasing in Premier 11A

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TubesnQuads
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KT120s and Biasing in Premier 11A

Post by TubesnQuads » Sun Mar 18, 2018 2:32 pm

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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roberto
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Re: KT120s and Biasing in Premier 11A

Post by roberto » Sun Mar 18, 2018 8:44 pm

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 » Mon Mar 19, 2018 9:05 am

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 » Mon Mar 19, 2018 3:13 pm

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 » Mon Mar 19, 2018 3:29 pm

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 » Mon Mar 19, 2018 8:37 pm

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 » Mon Mar 19, 2018 11:02 pm

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!
ML CLX speakers, BF-210 sub, Classic One Twenty SE amp. ET-7 C-J pre. Exasound E-32DAC, Mac Mini i7, Anthem MCA-3 power amp, Anthem AVM-60, Oppo BDP-103. ML Stage X Center, Motion 4, BPTSignature 3.5 Plus condiciones.

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Re: KT120s and Biasing in Premier 11A

Post by Joe Appierto » Wed Mar 21, 2018 10:45 pm

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 » Wed Apr 04, 2018 2:22 am

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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