Ultralinear with zener on the screen

[kstagger]

This may have been covered before, but my search was useless.

Last night I was discussing Ultralinear output stages with a friend of mine and he mentioned using zener diodes to drop the voltage on the screen. This could be important if driving a vintage 6550 or perhaps a sweep tube. This reminded me of an old article in Sound Practices which I looked up:

from "The Classic Williamson 1993 Style" by Bill Kleronomos (Sound Practices #4)

"An unusual method of applying active screen voltages to the tubes is used. Instead of the usual dropping resistors, I used a set of 5 Watt Zener Diodes for special benefits : screen voltage is always held at or below manufacturer's recommended value regardless of screen current. Distortion products are materially lower because of the stiffer screen supply provided by Zener regulation - this is especially true for the EL-34 pentode with its wider ranging screen current than the usual beam power tubes. And less screen voltage requires less bias voltage and therefore less peak signal voltage from the driver at max. output. Bench tests with EL-34s and 6550s in the output sockets confirmed an approx. 20% reduction in THD when the original dropping resistors were replaced by Zeners."


Thoughts? I was going to give this a try on my current amplifier I've been playing with. I haven't seen anyone else use zeners in the UL taps like this.

[Wavebourn]

Better screen regulation in respect to anode, but not to cathode, right? What's the purpose?

[ray_moth]

Distortion products are materially lower because of the stiffer screen supply provided by Zener regulation

That is incorrect. The screen will not be regulated except, as Wavebourn says, wrt the plate (and B+). That is not a stiiff screen supply unless B+ itself is regulated.

[Tom Bavis]

Stiffer than a resistor dropping the same voltage, certainly.

[jnb]

Yes.

Perhaps the O/P can visualise it as the transformer tap leading the screen up and down at the end of a long pole, where the zener is the pole.

[wa2ise]

Should one use a bypass cap across the zener diode? Would there ever be a condition where the screen grid stops drawing current, and the cap would maintain the desired voltage waveform on the screen grid?

[kstagger]

in my haste at work, I forgot to add that that the author of the article suggested using 75V Zeners for the 525 power supply.

My main concern would be the zener noise.

I have some 100V and 25V 5W zeners in my junk box - maybe I will give them a try.

[SpreadSpectrum]

I am intrigued by the idea.

Personally, I would think that the zener noise voltage would be such a small magnitude compared to the power supply voltage that I would not expect it to be a big problem. However, you could always build one bypassed and one not and then use your ears or test equipment tell you if there is a difference.

I'll eventually try it, but my project is coming along too slowly to promise anything in the near future.

[tubelab.com]

I did some experiments along these lines when I was torturing some sweep tubes at plate voltages that would melt the screen grid. The results were promising but there is only so much to be gained. If you use too big of a zener the G2 voltage can approach zero on low going plate voltage excursions. For those who are sandophobic a gas regulator tube can be used. The photo below shows a 6CD6 with an 0B2 between the UL tap and the screen grid. Do not copy my unique breadboarding method. I think that an 0A2 migh have been more suited because when I really cranked it the 0B2 would extinguish on peaks causing some really rude distortion.

[kstagger]

last night I soldered in a 25V/5W zener and gave it a quick listen... everything sounded 'bout the same. Didn't have time to do any measuring with the rest of the family down with the flu.

This is on a complete test rig single mono amp using a single Shuguang EL156 running on outboard regulators.