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
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.
czarivey
5,946 posts
 
Fluctuating bias is huge problem that can destroy tubes very quick. At least the fluctuation should stop after short period of time less than a minute before you can play the music through(which is acceptible but not perfect).
There's definitely something bad when you connect to UL power supply(second grid). My wild guess would be going through applicable electrolytic caps that are part of UL supply circuit. I'd usually ID second grid pin# and trace all the way back to rectifier examining visually and then testing with DMM.
If the amp is still on warranty, use your old tubes in UL mode with music till they start blowing and destroying amp before you send it to the rebuild service back to Rogue(this way they will definitely know that the amp is NOT OK). You can use dummy load resistor to save your speakers while 'cooking' your amp. Repeat procedure while the amp is perfect or demand another unit.
atmasphere's avatar
atmasphere
11,861 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
11,861 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.
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