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
 
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
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 : )
atmasphere's avatar
atmasphere
11,901 posts
 
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?

It might be worth a shot- of course an easier tube to find is the 12AT7, which has the same mu: 70 instead of 100 of the 12AX7.

I prefer both tubes over the 12AX7 as its a lot easier to get bandwidth. I would think that you want as much gain as possible without pushing the amp into instability. The problem here seems to be that the feedback loop does not have proper cutoff frequencies set up, so with a heavily capacitive load (and in UL operation, which will have a little more voltage output due to the increased power and reduced grid capacitance) it can become unstable. Sorry for the technical mumbo-jumbo.

Apparently the condition is not able to damage the amp over short duration, so I think its worthwhile to try the various tubes (12AU7, 12AT7 and 5751) to see which offers the best sound along with stability. The only trick is that depending on the tube brand, the sound might vary enough that the brand may supersede the type. That is good if that's the case, then its all just about the best sonic performance.

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.
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 : )
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