Zener in series between UL and screens: any drawback?

[Merlinb]

Is there any drawback in adding a zener diode in series with the UL connection, adding a resistor to ground in order to always ensure at least 2 mA through it

I don't see a problem with this. You probably don't even need the resistor, since screen current is never zero. Zener noise probably isn't a problem either, since the screen grid is so insensitive.

 

[zintolo]

Thank you @Merlinb , easier to try without.

 

[lcsaszar]

AC attenuation of g2 voltage due to UL ratio (referred to peak negative anode voltage) + DC shift due to zener voltage > cathode voltage.
The zener will restrict the g2 negative peak (must not go below the instantenous cathode voltage), hence it will restrict the available output power.

 

[waltube]

Thread 'Ultralinear with zener on the screen'

2008-01-29 7:35 pm

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...

• kstagger
• Replies: 26
• Forum: Tubes / Valves

it seems that no progress since 2021 on this strange way


Walter

 

[zintolo]

AC attenuation of g2 voltage due to UL ratio (referred to peak negative anode voltage) + DC shift due to zener voltage > cathode voltage.
The zener will restrict the g2 negative peak (must not go below the instantenous cathode voltage), hence it will restrict the available output power.

Hi, as I wrote many times, here I'm using low UL percentages and fixed bias, plus flattish loadlines, so (almost) no power reduction.

 

[zintolo]

@waltube
thanks Walter, I totally forgot that thread!

 

[baudouin0]

I'am guessing its a gu50. Another option is a mosfet to the screen. That way you can combine 0V, HT and plate in any ratio you wish without a UL tap.

 

[zintolo]

6P6S running at 320V 33 mA on a 8 kOhm Raa.

@baudouin0