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
7,068 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.
atmasphere's avatar
atmasphere
7,068 posts
 
What tap on the amplifier were you using with the MLs?

MLs generally have a very low impedance and are rarely suited to work with tube amplifiers! It might simply be that the impedance of the speaker is playing a role.

Here's a theory:
Like any other loudspeaker, they can act as microphones and pick up sounds in the environment. Normally this can't affect the amp, but if the impedance of the speaker is very low (and it is) and the amplifier impedance is higher (maybe?) you might be able to see the effects of sounds in the room on the bias meters. Easy enough to find out...
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.
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