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
atmasphere's avatar
atmasphere
11,901 posts
 
If the bias is OK in triode mode and without a preamp attached, then we can assume that it is not noise coming into the amp from the preamp.

We can also assume that its likely that its not noisy driver tubes.

This leaves the switching means itself. My theory is that the switch that is used to go from Triode to UL is dirty. This might be an easy fix if this is the case. Get a spray can of contact cleaner from Radio Shack, or a spray can of Deoxit D (you never ever want to use anything more powerful than these products on a tube device BTW!).

Locate the switch, and with the amp off, find a way to get a shot of the spray into the switch using the pipette that is supplied with the contact cleaner. Then operate the switch vigorously about 15-20 times. Then try it out.

If that solves it- great! If not, at this point unless you have a strong technical background (and from the posts here I am assuming you do not) then it must be repaired by a competent technician.

The $5.00 can of contact cleaner is worth a shot, if you can pardon the expression.

Please report back.
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.
atmasphere's avatar
atmasphere
11,901 posts
 
If the bias is fluctuating, it is likely due to noise in the circuit, if OK in triode mode.

Its not likely to be influenced by speakers.

If there is a bias control, it might be worth cleaning it with the contact cleaner as well, then working it back and forth. If there is a balance control, the same for that.

If neither has an effect, the unit has to be serviced by a qualified technician.
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.
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