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

Showing 5 responses by atmasphere

atmasphere's avatar
atmasphere
11,566 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.
atmasphere's avatar
atmasphere
11,566 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.
atmasphere's avatar
atmasphere
11,566 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...
atmasphere's avatar
atmasphere
11,566 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.
atmasphere's avatar
atmasphere
11,566 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.

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