Biased

[richidoo]

How many of you have had an easy time biasing the power tubes of your push pull amps?  In my limited experience, they always use one turn pots with a hair trigger sensitivity, and the screw in the pot must be filled with tar because it is hard to turn and sticks. There is never an easy and definite approach and land exactly on the setting, it is always turn back a lot, then turn it forward again, let it snap from the sticking, and see where it lands, repeat. Granted, we are adjusting a $5 part to .001VDC, but it could be better.  It seems to me a 10 turn pot would deliver same range of adjustment, but with more sensitivity.

Then god forbid you check it again a week later or at a diferent time of day. Ideally the tubes will have moved bias readings together and both will show same error so you could leave it alone. But I hate it when they change in relation to each other for no apparent reason, other than the obvious - welcome to tubes sucker.

I guess this is most prevalent during tube break in, they will change a lot, then finally settle down and remain kinda stable. New Sensor tubes hold tight bias, IME. 

I have read that PP zero crossing distortion will increase when the PP pair is not equally biased. Can anyone confirm that's true? 

On some amps, the bias is critical, it's easy to hear when they are a slight bit off. With others it doesn't seem to be noticable.  What's been your experiences with biasing tube amps?

[Black Sand Cable]

The only tubes I'm running at the moment are on some Primaluna stuff.....which sets the bias all on its own!  I know that when I had a set of EE mono blocks here, I was setting the bias every week as I could tell when it had gone out.

Other then that, I cant offer any opinion as I'm a tube newbie!

[mgalusha]

I hate that, especially when a decent 10 or 15 turn trimmer is only a few bucks.

If the current isn't the same through both tubes in a PP amp it will put DC on the output transformer, which will cause the transformer to saturate much easier and no doubt increase distortion. I don't know if the zero crossing distortion would increase. I could see it resulting in a slightly asymmetrical waveform but that would be something to measure. Unlike transistors tubes never really shut off, so crossover distortion should be less of a problem, at least in my mind, which is kinda sparse on some days. 

[richidoo]

Autobias is a great idea. BAT and VTL offer it too, at higher price points.

Mike, makes sense that DC offset in the transformer primary is more of a concern than the crossing.  Is there a typical range of offset that will become audible in your experience? I can hear 10mA difference on 300Bs and Snapper alike. And 20mA difference sounds kinda like like a wad of lint on the needle with the 300Bs, growly lower mids and harsh highs.

[mgalusha]

Rich,

I don't know what a specific value might be to become audible but 10 - 20mA of difference is quite a bit IMO. A typical EL34 amp is biased around 40mA per tube, so 10mA is quite a bit. With say 450V on the rail, that is 4.5 watts of power. I would think that more than a couple of mA of offset in a push pull would be detrimental.

mike

[richidoo]

Yes, you're right. EL34s biased to 300mV which meant 30mA. So 10mV adjustment means .3mA bias difference. Seems very small, but I'm pretty sure I could hear that.

On the 300B bias is 500mV in class A. B+ fuse is 400mA, so it must be a 50mA bias per tube. A 10mV difference on the adjustment means .5mA current to the transformer. Interesting! Thanks for giving me some perspective.   I must be imagining it. But it feels so good to believe you can hear like Steve Austin.

[mgalusha]

The mV of bias may not translate directly to mA. It depends on the resistor you are measuring across. If it's 10 ohm resistor (common) then it does translate nicely. 300mV across 10R is 30mA. But I have seen other and somewhat odd values. 1 ohm is also fairly common. Of course that translates nicely as well. It's the weird ones that are confusing.

Some transformers will be more sensitive to DC than others. If your 300B amps will run in SE mode, then the output transformers are going to be pretty tolerant of DC as the full current to the tube goes through them in SE mode unless they are parafeed (capacitor coupled).

[richidoo]

Yup, 10ohm resistors.  Thanks Mike
Rich

[rollo]

Biasing the Audio research DR250Servo Mk2 [cap coupled] can be a drag with 16 6550s. Set at .65mv as per the spec sounds fine . When I played with a 20 % adj. in either direction you could discern a difference. The higher the setting the more top end info comes through.
   With the Cyber 211 SET monoblocks [ transformer coupled] the setting is 4.80mv. When I use the GE211 tubes a setting of 5.20 brings more detail. When using the RCA 211 the standard setting sounds better. By adjusting the Bias manually one has the benifit of changing the sound. Brighter or more laid back. Auto biasing is fine but IMO eliminates the setting choices.
   Now that changing Bias thingie. Depending on house voltage it can drop a few MV during the day hours and settle back at night. For the most part both the RCA and GE tubes hold very well. 


charles

[richidoo]

I typically see about 5% fluctuation in bias settings due to varying AC night to day.  But the bias settings per pair remain OK.

[rollo]

   Grant Fidelity reccomends biasing ones tube 20% down from rated bias level. Says saves tube life. I have found biasing can act like a tone control . I now bias my 211s 20 % lower [ 4.8 now 3.7]. to my surprise I perfer the sound. Go figure.


charles

[Deton Nation]

Separate biasing for the 4 power tubes for each mono amp on the Manley Mahis. They dont stick but are pretty sensitive. And if you turn a tube up on the left you have to check the right because they will go up so you have to balance individually and across tubes. Not that difficult but a tiny pita.
M

[richidoo]

Good tip! In any amp bias setting can make a big difference, even solid state.  Different tube brands can benefit from tweaking bias also to find the tube's best bias point in your amp's circuit, not just fast swap like a signal tube. You just have to make sure the circuit can handle it if you increase the bias current. +10% / -25% should be safe range for most amps, but always a good idea to contact your amp maker for advice on safe bias range.   If you go too hot, the B+ fuse should blow, but not if the designer sized it to protect something else, assuming you will follow his bias setting in the manual.

In what way does the sound change with the bias reduction? Softer, warmer?

[rollo]

 A 10% or 20% boost will shorten tube life. For me lowering the bias just made it a bit smoother. No lose of detail or highs at 20% at 10% could not tell a difference. At Say 5.2 in lieu of 4.8 the top was more evident and livlier. That is what is recommended by Rochlin for my amp. However I find that setting bright. I'm at 4.0 now and liking it. It makes a difference. Time to play around a bit more.


charles

[richidoo]

Thanks.   Sometimes more brightness is needed, on amps with cheap slow OPT, for instance.