Metal Oxide Screen Stoppers

[anatech]

Hi tubelab,

The voltage rating also applies to the dielectric strength of the coating.

Well, yes. That too. However, I'll bet that the dielectric strength of the coating to external objects is a fair bit higher than the rating. Add to that, it's common practice to do as you have, mount those resistors off the PCB. Mounting them over holes is even better for dissipation.

My only comment was that the failure mode is normally flashing over between spirals if your applied voltage does not create excessive dissipation right away. I wouldn't be surprised if a normal overheat also causes flash over as the element goes open. As you've discovered, lot's of fun!

In short, in most sane applications I find that metal oxide resistors are more than satisfactory. They are not as quiet as metal film types, but they are much less noisy than carbon composition types are. Those ceramic compositions look interesting too.

-Chris

[Ty_Bower]

Originally posted by tubelab.com
I have run a bunch of different tubes through the Simple SE under many different conditions. The screen resistor, and everything else remained cool until I put in some JJ EL34's, and plugged my guitar preamp into the amp. Things were still OK, until I dialed up my favorite Jimi preset. After about a minute of this the first resistor started sparking out, some cool distortion ensued, and then silence.

I repeated the experiment. First, my old blow proof Chinese 6L6GC, no problem, dive bombing feedback, toggling the pickups on and off on a Les Paul, nothing bad happened. Ditto the KT88's. Enter the EL34. As soon as I held the guitar in front of the speaker and yanked on the pickup switch, the screen resistor acted like a micro Chernobyl. The JJ EL34's are not at fault, they are still hapilly alive 2 years later.

From my short experiments, beam tubes appear to draw about one third the screen current of an otherwise equivalent pentode (comparable plate voltage and total idle current). This is to be expected - beam tubes are supposed to have the screen grid aligned in the shadow of the control grid, which substantially reduces the number of electrons that make it to the screen. I understand there may be some pentodes out there that do have aligned screens, but I haven't seen any yet myself.

[anatech]

Hi Ty,
Yes, that was what I was thinking when George pretty much proved this as fact. It wouldn't surprise me to see even more of a difference in current draw between pentode and beam tube screen currents.

-Chris

[Tweeker]

I think the GU50 has aligned screens.

[Wavebourn]

Originally posted by Tweeker
I think the GU50 has aligned screens.

Yes. But anyway I use metal oxide resistors for grid stoppers that are coated by non-flammable material. Who knows what may happen...

[Johan Potgieter]

There is also a larger variation in G2 current with beam tubes, again because of the grid alignment. Looking up roughly similar operation between pp EL34 and 6L6GC (output about 50W), the EL34 variation (0 output - full output) in Ig2 is 20 - 40mA, and that for 6L6GC: 5.6 - 25mA.

[Wavebourn]

Originally posted by Johan Potgieter
There is also a larger variation in G2 current with beam tubes, again because of the grid alignment. Looking up roughly similar operation between pp EL34 and 6L6GC (output about 50W), the EL34 variation (0 output - full output) in Ig2 is 20 - 40mA, and that for 6L6GC: 5.6 - 25mA.

When NFB is applied ig2 rises up sharply on clipping, it is a nice feature: I use it to control anti-clipping optical compressor.
According to my musical tastes slight compression is the lesser evil than clipping, even though tube clipping sounds less harsh than SS clipping.

[anatech]

Hi Anatoliy,

According to my musical tastes slight compression is the lesser evil than clipping, even though tube clipping sounds less harsh than SS clipping.

Me too!

I use it to control anti-clipping optical compressor.

A nice application for an otherwise wasted current. Pretty smart if you ask me.

Hi Johan,
I think that's one reason I prefer the 6CA7 to an EL34.

-Chris

[FRE]

I don't know whether I'm qualified to have an opinion on this, but would it be reasonable to use zener diodes instead of screen stopper resistors? The typical applications in some tube manuals specify a specific screen voltage, and it seems to be that that could be conveniently obtained by using zener diodes of appropriate rating. That should also work well with an output transformer having screen taps.

[Wavebourn]

Originally posted by FRE
I don't know whether I'm qualified to have an opinion on this, but would it be reasonable to use zener diodes instead of screen stopper resistors?

"Stopper" is a jargon term that means stopping of parasitic oscillations by adding of energy losses to an oscillating system, such as a resistor in series with a capacitor or in parallel with an inductor. Zeners are devices that have such combination of properties like relatively high voltage drop, but relatively low dynamic resistance. Also, they have a capacitance that is much higher than typical resistors have.

The typical applications in some tube manuals specify a specific screen voltage, and it seems to be that that could be conveniently obtained by using zener diodes of appropriate rating. That should also work well with an output transformer having screen taps.

Zeners that regulate voltage in respect to cathode of the pentode may be used to decrease distortions.
Zener from B+ source to screen grid reduces constant part of B+ voltage on a screen grid, while almost all variations of B+ voltage will be applied to it. Is it what do you need, or not?