Zener on Rk to "fix" bias

[Steve Conner]

Yes, it's a scientific fact. We're betting on last week's lottery results here, not next week's.

The change in bias points between tubes will be worse by a factor of whatever cathode resistor you'd have used, divided by the dynamic resistance of the LED or zener or whatever you actually used in place of the resistor. So probably about 10.

[mooreamps]

Look, maybe I'm not understanding "exactly" what type of circuit we're all talking about here ; and maybe you're a fairly educated guy and all, but when you go with fixed bias, there just are no "dynamics" that change.


-g

[Dave H]

If we are talking about a fixed bias preamp stage here, it's going to behave just like a fixed bias output stage does. When the tube is changed for one with different characteristics the bias voltage has to be adjusted for it to have the same bias current as before but a diode biased preamp stage doesn’t have a bias pot so the bias point will change.

[Chuck H]

To expand a little on what Dave said...

If you fix the bias with -V on the grid or +V on the cathode and a respective 0V reference on the other end, the tube doesn't care. It's fixed bias. It is adjusted for the tube in the amp now for a chosen operating point. Plug in a tube with different operating characteristics and that operating point will certainly be different if no bias adjustment is made. That isn't to say it won't "work". Just that the operating point will be different from the original, intended design. As far as I'm concearned this IS a drawback considering the variability between preamp tubes. And not just different types. There is a lot of difference even from one brand of 12ax7 to the next.

FWIW the original post was in regard to a diode across the shared cathode resistor for a pair of EL84 tubes. The amp plays cathode biased, with soft attack and a little breath into the onset of distortion. As the crossover distortion starts to appear the amp "fixes" and tightens up a bit. This, for me, is helping to keep crossover distortion to a minimum and get a just overall bigger sound from a 2x EL84 amp. I'm also using the "Paul Ruby mod" with the zener values set 1 volt above the cathode zener voltage. The last bit of overshoot hash is eliminated with a conjuctive filter of 10k/1500pf which is barely audible except for the beneficial smoothing of the overshoot. I'm running the tubes at 355Vp and 330 screens. I am VERY pleased with the performance of the EL84's using this circuit. With the highish voltage I'm getting good bottom end but it seems to keep a little of the jangle. Though not nearly as sweet as running these tubes at a lower voltage. But that's not what I was going for. This sounds more like a little version of a big bottle tube in the way it plays under overdrive. But it keeps the cool cathode bias vibe from clean up into lightly distorted tones. There are no audible penalties due to the diodes.

Chuck

[mooreamps]

a diode biased preamp stage doesn’t have a bias pot so the bias point will change.

Fine, go build one and find out.

-g

[Dave H]

Fine, go build one and find out.

-g

I did build an EL84 power amp with a 15V zener in the cathodes for bias and it worked fine. I’ve never tried it for a preamp stage but I have noticed that for a resistor biased preamp the plate voltage can change quite a bit when the tube is changed so I’d expect the plate voltage of a diode biased stage to change even more without the negative feedback from the resistor. Anyway unless you always want it to be biased for max headroom the diode will most likely be close enough for rock and roll.

Dave H.