Fixed biasing el84s

[jordan86]

I purchased a lot of working unmatched 6n14n (EL84) tubes on eBay. I’m trying to do some crude tube matching with a friend’s Victory Sherriff 22. It’s basically a fixed bias, el84, plexi with bias test points. So I pop a pair in, see what current the want to pull on a middle of the road bias setting and write that down.

FWIW…I’ve only ever used el84s in cathode biases amps.

What I’m noticing is that some tubes bias way colder than others. At like a “middle of the road” setting on the bias resistor some tubes bias at like 10ma while a few others bias at 35ma. I understand the setting of the bias resistor is irrelevant in the context. I can get all tubes to the proper bias range (Victory recommends 22ma) by adjusting the bias resistor but my question is why do some tubes bias so hot/cold and does that suggest that some tubes are good or bad? Does a weak tube that needs more bias voltage suggest it’s bad? Or is this all normal behavior and should be expected?

In comparison to a new stock set of JJ’s almost all the NOS reflektor 6n14n bias very cold.  Like 10ma less.

[shooter]

normal behavior and should be expected?

pretty much

In comparison to a new stock set of JJ’s almost all the NOS reflektor 6n14n bias very cold.

from the 'ol guys in Audiophile, Tubes need about 20 hours "burn in" to settle into their "normal behavior"

just for "fun";
dial-in any 2 tubes
2 meters are best but connect at least one to monitor bias
then plug in and jam while watching the meter
you'll find some interesting "swings"
I do this for about 5-10 minutes with the meters collecting Min/Max and AVG
THEN I set my "final bias"  typically falls below the "static" ~~60%

[jordan86]

I’ve done just that, Shooter. Paired up two that measured closely. 16ma on my initial readings. Biased up to 20ma at idle. The swings are HUGE when playing. In fact, with the master set to full the tube light up considerably. Maybe that’s bad?

Do you think it’s better set bias based on those max swing readings? Keeping those under 100% max dissipation? I guess that would make more sense.

[acheld]

Swings will be huge when playing.  That is expected.

Biasing is always going to have trade-offs.   Bias high, and your tubes might sound better (to you), but you'll get lower service life.  Bias cooler, and you'll get better service life.  Bias too cool, and your tone will suffer, but your tubes will last your lifetime.

I personally aim for 60-65% max plate dissipation on my fixed bias amps.  Some go with the traditional 70% number, others higher.  My ears can not tell the difference between an EL-84 biased at 60% (or even 50%) and one biased at 70%.  That's me.  Your ears are probably different.

For many years Blues Juniors were factory (fixed) biased at 80+%.   In mine, a set of EL-84s might last 6 months or less.   That was tiresome.   After I modded the amp to 70%, the tubes last years, and sound the same (to me).   

Interestingly, BJ IV's are more conservatively cathode biased and seem to hold up well.

[shooter]

I bias at AVG, +/- guitarist play style


the point mainly is to point out "matched" is more a theory or selling point, not a reality.

[jordan86]

I bias at AVG, +/- guitarist play style


the point mainly is to point out "matched" is more a theory or selling point, not a reality.

Thanks. Sounds good! Yeah, I’m well aware that tight matching isn’t all that critical. I know some guys welcome the desirable mojo of mismatched tubes :) 

[pdf64]

With signal, cathode current increases above its idle level.
Guidelines such as 'bias at 60%' or whatever apply at idle.

[HotBluePlates]

I purchased a lot of working unmatched 6n14n (EL84) tubes on eBay. I’m trying to do some crude tube matching with a friend’s Victory Sherriff 22. It’s basically a fixed bias, el84, plexi with bias test points. So I pop a pair in, see what current the want to pull on a middle of the road bias setting and write that down.
...
What I’m noticing is that some tubes bias way colder than others. At like a “middle of the road” setting on the bias resistor some tubes bias at like 10ma while a few others bias at 35ma. ...

Apply voltages to a tube's electrodes, then supply a suitable negative voltage to the control grid (G1).  In this fixed-bias setting, every tube may land on a different plate current.  All you know from this one data-point is how close/far this tube is from the plate current predicted by data-sheet-curves.  Rinse & repeat for every other mix of electrode voltage & bias, and you could create your own set of curves for that tube and compare them the "Average Tube" represented by the sheet's data.

The next bit of useful data is Transconductance (Gm), or how much plate current changes with a G1-voltage-change.  The catch is all tubes exhibit higher Gm when their plate current is higher, so if you're comparing Gm to a data-sheet-figure, you will first need to adjust the tube-bias to get the same idle plate current as what the data sheet figure had.  Now when you test Gm you can make a meaningful comparison between your tube and the "Average Tube" in the data sheet.

When you have used a tube testing setup that can make measurements using either fixed-bias or cathode-bias, you will find a wider variation of results with fixed-bias.  Cathode-bias is self-adjusting to some degree, and different idle plate currents result in different bias voltages, which tend to increase-current when it's low and lower-current when it's high.  The net result being that a group of tubes exhibit less idle plate current variation when they're cathode-biased, simply because the bias method pulls the tubes towards an average.  Likewise, Gm figures tend to be less-different, but that's because the idle-plate-currents tend to be less-different.


I tell you all that for 2 reasons:
   1.  There's no feedback-mechanism with the fixed-bias you're using to observe the idle plate current of these tubes, so wider variation is Normal.

   2.  If your test conditions don't match your usage conditions, the test results are less-relevant to your use case.  If you're checking these tubes in fixed-bias, but your amp will use them in cathode-bias, then they will exhibit a smaller range of variation in your amp than you observed in your friend's amp.

[Merlin]

At like a “middle of the road” setting on the bias resistor some tubes bias at like 10ma while a few others bias at 35ma.
In comparison to a new stock set of JJ’s almost all the NOS reflektor 6n14n bias very cold.  Like 10ma less.

So it sounds like your bias voltage is in the low-current region where the grid curves get really bunched up. We can expect sample variation in this region to be much greater than if you were 'targeting' a higher current.