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
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almarg
9,670 posts
 
Ralph, note in an earlier post that the OP indicated he tried both of the taps the amp provides (4 and 8 ohms). Also, he indicated that the problem occurred even if the AC power plugs of the speakers were disconnected. That would seem to rule out the powered woofer section of the speaker as having anything to do with the problem. And I'm not sure that an unpowered electrostatic element can be microphonic.
12-15-14: Rluthy57
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.
My speculation, as stated in my initial post, was:
... if the bias readings do not fluctuate with those dummy loads in place, per my comments above it would not necessarily be indicative of a speaker defect. My suspicion in that case would be that some kind of interaction between the amp and the speakers is resulting in an oscillation.
If an oscillation is occurring, at some inaudible frequency, the amp's feedback loop would presumably be involved, and therefore pretty much the entire signal path of the amp would presumably be involved. I see that the small signal tubes used in the amp are two 12AX7's and four 12AU7's. What type and make of these tubes are you using, and are they the tubes that were originally supplied with the amp? Just a guess, but perhaps changing the 12AX7's to lower gain equivalents might resolve the problem.

Regards,
-- Al
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atmasphere
11,841 posts
 
Thanks for pointing that out Al!

Some amplifiers are unstable with capacitive loads. I generally associate that with transistor amps (with poorly designed feedback elements), and a tube amp can have troubles too for the same reason. Changing the driver tubes to ones with less gain (12AU7) will reduce the gain of the amp and its feedback. With feedback you either want a little or a lot. I'm sure it won't hurt anything to try- at worst it will just affect the sound.

Actually this seems the simpler explanation so I am going with a basic incompatibility (amplifier instability when used UL) with this speaker. 12AU7s in place of the 12AX7s may well solve it.
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.
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almarg
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Awesome! Glad to have helped solve this extremely frustrating problem. And thanks for the nice words :-)

Ralph, in the interest of optimizing sonics do you think it would be worthwhile for Robert to try a pair of 5751's in the 12AX7 locations? As you no doubt are aware, 5751's are commonly used as lower gain substitutes for 12AX7's, having a "u" (amplification factor) of about 70% of that of the 12AX7. That is much higher than the u of the 12AU7, of course, so there is no guarantee that the problem wouldn't return with them. But other parameters such as Rp and Gm are much closer to those of the 12AX7 than the corresponding parameters of the 12AU7, so I'm thinking that they may therefore be more optimal sonically.

Looking at an older GE tube manual I have, though, I note that the 5751 has a rated maximum plate dissipation of 0.7 watts, while the corresponding figures for the 12AX7 and 12AU7 are 1.2 and 2.75 watts respectively (for both the "A" and no suffix versions). I suspect that is unlikely to be an issue, but having no knowledge of the specific design I'm not totally certain.

Best regards,
-- Al
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