2. amps-preamps
  3. 1417714840

Tube Bias issue.

Hello all, I'm going to post my still unsolved problem again. I have a Rogue Zuess amp that you set the bias for each individual tube. The amp can be played in either 150 watts per channel in Triode mode or 225 watts per channel in Ultra-linear mode. I can set the bias in Triode mode at 40mv and it holds and works/sounds perfectly. If I switch the amp to Ultra-Linear mode both the left and right channel bias meter needles fluctuate wildly. The amp has been to Rogue and elsewhere and the result I'm told is that the amp is fine. I replaced the tubes from KT88's to KT120's which was a great upgrade but the Ultra-linear bias issue remains the same. I have Martin Logan Summits (2007) a Rogue Magnum 99 pre-amp, Sony XA9000ES SACD, Shunyata Hydra, all higher end Audioquest interconnects and speaker cables, mid level
Shunyata and Acoustic Zen powercords. I also put everything on 20amp dedicated circuits to no avail. I've tried switching cables, power cords etc.. unfortunately I haven't been able to switch out the speakers which seems like the only thing left. Could a crossover gone bad in one of the Summits or one of the speakers powered subs have an issue that could affect tube bias settings? The system sounds perfect in Triode mode but the speakers really bloom better with the extra power in Ultra-Linear mode that I miss. Any ideas at all would be so greatly appreciated. Thanks Robert.
rluthy57

Showing 11 responses by rluthy57

rluthy57 OP
35 posts
 
sorry at "Audiogon" thank you.
rluthy57 OP
35 posts
 
Thanks again for your help, the MLs are connected to the 4 ohm taps but I did try the 8 ohm taps to see if there was a difference and there was not. How would I know if the bias meters were reacting to sounds in the room through the speakers? The room is quiet while this is happening.
rluthy57 OP
35 posts
 
New tubes are in and I'm happy to report all is still fine. Best Christmas gift ever. Merry Christmas to all those at Audigon and those that gave their helpful advice and special super thanks again to A1marg and Atmasphere for actually solving my elusive compatability problem. Couldn't have done it with out you two. Merry Christmas : )
rluthy57 OP
35 posts
 
Thank you all for your most appreciated knowledge and replys. To answer some questions, the tubes don't red plate, they still look stable and normal while the meters are crazy in UL mode.
Yes I've tried the 4ohm and 8 ohm taps, same issue.
Yes the preamp is off and I've also disconnected all but the speakers, same problem.
I've also unplugged the speaker power cords just leaving the speaker cables on, same issue.
Yes I've tried every variation with outlets, dedicated, not dedicated, the Hydra, no Hydra etc...
I will try to get some BS speakers if you think that would tell me something.
Thank you all again for your thoughts and suggestions, to be continued. Happy Holidays.
rluthy57 OP
35 posts
 
Thanks, again. I tried the contact cleaner on the switch but that hadn't worked either. I am still working on a another set of test speakers.
rluthy57 OP
35 posts
 
Thanks man I appreciate it and will try that also.
rluthy57 OP
35 posts
 
Hello all, I disconnected the Martin Logans and hooked up a small pair of 6 ohm Phillips 2 way speakers into the amps 8 ohm taps and the ultra-linear bias fluctuation problem went away. Wouldn't this tell me that the amp is ok and the problem is with the Martin Logan Summit speakers? If it's the speakers I still don't understand why they sound so good when the amp is played in triode mode. What would the speaker affect in the amp to fluctuate the bias so wildly in ultra-linear mode? Still going crazy.
rluthy57 OP
35 posts
 
Gentlemen I don't know what to say, you have solved a problem that had tortured me for several years including thousands of dollars in upgrades of tubes, cables, dedicated circuits, shipping costs and upgrades by Rogue and Hollywood Sound, power cords and a Hydra. All made the system notably better at each stage but never solved the Ultra-linear issue. Luckily I had some spare 12AU7's so I was able to make the swap and the meters stayed stable at 40Mv in UL. I left the amp warming for 2 hours and kept checking the bias settings before I played any music still afraid that the sound would be affected. It sounds awesome most notably the subs with the increased UL power. I use Sonny Rollins Way Out West SACD as my reference disc and it was insane. Triode sounds great for normal listening but UL is a must with my set up for the volumes and type of music I listen to. I will upgrade all the 12AU7's as soon as I can. I appreciate everyones feedback and I thank you guys, you two geniuses A1marg and Atmasphere for solving this nightmare for me. Thank you both so much. Happy Holidays and all the best to you both and all that love this awesome thing of ours : ) Robert.
rluthy57 OP
35 posts
 
It's morning now and I'm pinching myself to make sure I'm awake and that this fix really happened last night. I never give up (super OCD/perfectionist/stubborn) but I could have never solved this out with out all your help and vast knowledge. I Can't describe in words how happy you all made me. Thank you all again.
rluthy57 OP
35 posts
 
Hi guys, I'm taking a shot and ordered 6 matched 12AU7 Genalex Gold Lions (Upscale Audio/Platinum) to replace the older regular Electro-Harmonix signal tubes that are in the amp right now. I'm hoping it improves what we've already done to this point, if not I'll have some great spares : )
rluthy57 OP
35 posts
 
I should have the new 12AU7s Gold Lions by Monday or Tuesday and hopefully I'll have some positive feedback to report.
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