Output Pentode Screen at half Plate voltage

[Andrewbee]

Greetings all.

I have a question. I have heard / read of an old standard of running the screen voltage at one half of the Plate voltage.
I have seen several driver stages using this but output stages I think only two (Music Reference is the only one I can remember).
I can understand a guitar amp and wanting to keep the screens cooler by running a lower voltage while the plates are very high.

In my situation, I am working through an EL84 PP amp and have a pair of original Dynaco Z565's but want to run Pentode for now.
The tube charts give characteristics at plate and screen voltages as 250 volts or 300 volts. I took a quick look at a few other tubes and they all are similar in that none I saw had specs for a screen at half the plate voltage.

My thoughts are to run the plates at 300 volts and regulate the screens at 150 with a VR150.

So, where does this half b+ for the screens come from ?

Thanks for the help.

[SY]

Originally Posted by Andrewbee

So, where does this half b+ for the screens come from ?

Legend.

There's really no reason that one particular screen voltage is the universally correct one. The optimum depends on the tube, the load, the operating point, and the required output power. For your setup, the "book" values would be my starting point, not some audiophile myth.

[Yvesm]

Originally Posted by Andrewbee

So, where does this half b+ for the screens come from ?

Pearhaps from the behaviour of old Pure Tetrodes where plate voltage was not allowed to go below the screen one unless you want to build an oscillator or blow the screen.

Then came Beam Power Tetrodes and Pentodes . . .

Yves.

[smoking-amp]

"So, where does this half b+ for the screens come from ?"

The TV Sweep tubes sometimes have the "book" values of screen voltage at roughly 1/2 the idle plate voltage. Like 250 V and 150 V (6JN6, 6GE5). The later Sweeps were almost always designed for lower screen voltages than the earlier audio types. The very early pentodes would have lacked the aligned grids to hold screen current down, so would have required reduced screen voltages to avoid screen meltdown. Looking at the "modern" Sweeps on a curve tracer, one will often find that a reduced screen voltage helps alleviate kinks in the curves near the "knees". This "screen current distortion" effect (which sets in when plate V drops below screen V) is usually only relevant to the signal at near clipping levels (for the Sweeps, but can be more problematic with the Audio types since plate V can more easily drop below screen V, but is usually a smaller kink then ). If one does use a lower than normal (book) screen voltage, the g1 negative bias will have to be less too, so less output AC current will be available from the tube. That will require a higher B+ and a higher Zpri OT to get rated power then.

[Troncones]

Understanding how to use pentodes is such a challange for me. SY said start with "book" values. What book is this?

The data sheet (ref 6BQ5) has plate and screen at the same voltage is this "Book" value. Where I loose it is I haven't seen a schematic that shows Va and Vg2 at the same level. Vg2 is usually 10% to 20% less than Va. I understand this is to help reduce distortion near the clip points. I believe that Guitar amps play with the separation of Va and Vg2 as a means of creating controlled distortion.

The successful use of pentodes seems to be determined by Vg2. Any help or names of books is appreciated.


T

[SY]

Originally Posted by Troncones

Understanding how to use pentodes is such a challange for me. SY said start with "book" values. What book is this?

I'd use the Mullard datasheets- they are very complete and comprehensive. RCA tube manual also gives much of this information. Generally, the people who made these tubes knew pretty well how they should be run.

[smoking-amp]

Here are some curve tracer plots for a 38HE7 Sweep pentode, which has noticeable screen current kinking.
Screen voltages at 175 V, 125 V, 75 V, 50 V, and 25 V. The vertical plate current axis scaling gets progressively reduced as well as the grid voltage step size.

[smoking-amp]

The previous 38HE7 was a typical low screen voltage horizontal Sweep tube.

Here is a high screen V 6JC5 vertical sweep at screen V of 230 V, 135 v, 75 V, and 35 V. Vertical axis plate current scaling progressively reduced as well as g1 V step sizes as before.

edit: and one more at 350 V screen (same screen V overating as the 175 V for the 38HE7)

[Wieslaw Lipowsk]

Originally Posted by Andrewbee

So, where does this half b+ for the screens come from ?
Thanks for the help.

It can come from the middle tap of the power supply transformer, as per the schematic:

[Miles Prower]

Originally Posted by Andrewbee

So, where does this half b+ for the screens come from ?

I have no idea as I've never heard of this, at least not concerning audio. May be an RF practice?

You have a wide variety of different screen voltages and characteristics, depending on the design and likely usage. A lot of audio finals (6V6, 6AQ5, 50C5, 50L6) were designed to operate the plate and screen at the same voltage to either make ultralinear operation possible, and/or to avoid the need for voltage dropping and regulation.

Other types (807-oids, most TVHD finals) were designed to maximize efficiency, and so run the screen at a very low voltage to minimize power loss in the screen circuit where it contributes nothing to power output. These types typically put the screen very close to the cathode, and can't stand high voltage at the screen anyway.

As for screen voltages, try the spec sheets. Even if you don't find any recommended audio bias points, and loads, you still might be able to find a decent audio loadline.