protecting output tubes- screen voltage vs screen resistors

[imaradiostar]

I've been experimenting with different output sections and trying to get maximum power from different tubes. It's not tough to get a lot of power- it would seem that it's tough to do it without damaging the output tubes!

Assuming that the screen voltage sets the amount of current a tube can draw at a given load and voltage I can adjust screen voltage to hit a certain power level. Clearly this varies with the tube- 6L6's don't make much power till the screen grid voltage approaches the plate voltage. I found that EL34's and KT88's can get by with much lower screen voltage and still draw lots of current. Of course they'll also light the screens up like a light bulb filament if you overdrive them when set up for maximum clean power.

How do I determine the point where I can meet my power goals but still have high enough screen resistor values to avoid destroying the screens?

jamie

[Old Tele man]

...there were some very "high-end" tube stereo systems that used a microprocessor with built-in A/D converter that actually "monitored" the idle current of each tube and "tweeked" their bias voltage (as necessary) in realtime.

...something similar could be done with plate current and plate voltage to calculate the instantaneous plate dissipation in realtime and then output a control signal to an adjustable high-voltage regulator "chip" that controlled the screen grid voltage to both tubes.

...not totally "simple" but obviously "do-able."

[Wilder Amplification]

How do I determine the point where I can meet my power goals but still have high enough screen resistor values to avoid destroying the screens?

jamie

Go dual rail.

In all seriousness, if you look at the characteristic curve charts for KT88s and EL34s, you'll find one that shows screen current at different points on the load line. In every one of them you'll notice that once plate drop hits a certain point, screen current will go through the roof. It's a characteristic of the tube when it hits maximum power, not the surrounding circuitry. At some point in the plate drop, screen voltage is now above plate voltage which will cause the screens to "steal" more current. Once screen current increases to the point at which they over dissipate, you get into screen glow land.

It is this screen glow phenomenon that was one of the big driving factors that got me to start researching and prototyping dual rail designs.

[imaradiostar]

I have the screens 200 volts below the plates- it's still not enough. Some designs simply can't make the power without a certain amount of screen voltage anyway.

I know that an amp I build isn't likely to abuse the output tubes like I can with a signal generator but I'd still like to find a little more reliable solution.

jamie

[Wilder Amplification]

I have the screens 200 volts below the plates- it's still not enough. Some designs simply can't make the power without a certain amount of screen voltage anyway.

Correct...because the screen voltage has a direct effect on the transconductance of the tube.

I know that an amp I build isn't likely to abuse the output tubes like I can with a signal generator but I'd still like to find a little more reliable solution.

jamie

I honestly don't think you'll find one. Look on the charts in the datasheets...you'll see that every tube will start seeing a screen current increase when plate voltage drops below a certain point no matter what you set the screen voltage to.

Max power transfer occurs when the load line crosses the zero volt grid curve. Most guitar amps that operate "sliding screen" have the screen voltage and hence the curves well above the load line. As the screens draw more current, the screen resistor causes the screen voltage to drop which causes the grid curves to "slide down" the graph (hence the term "sliding screen" operation).

If the screen resistor is sized correctly, it should pull them down right to the point where the Vg1=0 line crosses the load line when the grid drive signal drives the control grid to 0 volts.

[imaradiostar]

Shouldn't we be able to do a little simple math and calculate a resistor value then? I know it won't work for all conditions but at least for the conditions we're likely to create.

There are millions of amps out there that don't blow up- what can we do to still make big power but be just a little more certain that an amp won't blow up? Just up the screen grid resistance so that any substantially large overdrive condition causes the screen resistors to limit the current to the screens? In my experience this just makes for a change in feel as you approach an overdriven condition- sometimes desirable in a dirty little guitar amp but not so much in a bigger cleaner bass or preamp gain driven guitar amp.

Yes, I'm planning a high-gain amp build but I want it to have a worthwhile power section too!

jamie

[pdf64]

The only way I can think of to get max clean power, but limit screen dissipation when overdriven, is to have a current limited supply to the screen.
In every amp I've checked in the past year (since I became aware of this issue), the screen dissipation increases to at least twice the data sheet max level when overdriven. That includes EL84, EL34, 6L6, 6V6 types, but not KTXX. With a higher than optimal load (ie 16ohm load on to 8ohm output) it goes higher still.
Is anyone aware of a simple, robust high voltage current limiting circuit available in the public domain? A pair of 6L6GC only need 20mA screen current for max clean power, but can draw well over 100mA when overdriven.
The supply would have to drop maybe 350V when limiting the screens to 20mA under heavy overdrive, assuming a B+ of 450-500V.
I've tried PTC thermistors in the screen supply, but due to the temperature variation inside an amp chassis, and the lag effect for the thermistors to cool following heating, none that I was able to use gave a satisfactory result.

[Steve Conner]

To protect output tubes, keep the amp in your living room and play quietly.

Even dual rail designs can overdissipate your screens. In a way, they can be worse, because they have smaller screen resistors, so the screen supply is stiffer and can deliver more screen-BBQing current.

One idea I once had was a light bulb in series with the screen supply. Same idea as the bulbs they use to protect tweeter horns in PA cabinets. When it kicked in, the bulb would light to inform you of a problem, and you could keep playing, it would just pump like a compressor. If things got really bad, hopefully the bulb would blow before any of the screens did.

It would work better over here, where the light bulbs are 240 volts. You now get little 240V halogen capsules that look like they'd work well. Maybe in Americaland you can use two or three 120V ones in series.

My Ninja Toaster power supply had foldback current limiting on both plates and screens, but it wasn't designed to kick in in normal operation. It originally had a screen current sensor that shut off the B+ if it thought the tubes were about to fry, but now and again it would pull the plug on you in the middle of a guitar solo! I ripped it out and threw it away years ago.

In the amps I've built so far, the screens don't obviously glow even when they're cranked into square wave clipping. They're also so loud that I rarely need to crank them that far. So I don't think it's a major issue.

[Enzo]

And the new CFLs will look even cooler in that application.

[Steve Conner]

Gah! I forgot about the lightbulb ban.

[Enzo]

I just bought a box of 100w this evening... and a few CFLs too. No shortage of incandescents here.

Light bulb limiters aside, I also work on light dimmers, and my collection of porcelain bulb sockets with 100waters in them are my loads.

[imaradiostar]

I guess this thread has gotten out of hand and drifted a bit from my original intent. I appreciate all the input.

More than anything I'm debating how to set screen resistor values. I guess there is no hard and fast rule- copying existing output sections is probably a good starting point.

jamie

[Wilder Amplification]

I wonder if running a screen bypass cap would help. If I'm thinking correctly here, it would keep the grid curves from sliding down very far if at all since it would hold the screen voltage itself constant, yet the resistor would still be limiting current to the screens.

[Merlinb]

I wonder if running a screen bypass cap would help. If I'm thinking correctly here, it would keep the grid curves from sliding down very far if at all since it would hold the screen voltage itself constant, yet the resistor would still be limiting current to the screens.

Unfortunately no, if you did that you'd be back where you started. The screen current simply gets drawn from the bypass cap instead of through the resistor! In other words, if you're preventing the grid curves from sliding down, then you're not limiting screen current!

[Wilder Amplification]

Unfortunately no, if you did that you'd be back where you started. The screen current simply gets drawn from the bypass cap instead of through the resistor! In other words, if you're preventing the grid curves from sliding down, then you're not limiting screen current!

Yeah I see what you're saying there...didn't think about the cap becoming the new current supply.

The problem here is that under "normal" designed operation tube amps aren't supposed to clip. When you push them into clipping (which a lot of us do with guitar amps), you're exceeding the design limits.

So when designing output sections, one must decide what their main objective is..max clean power transfer or being able to overdrive it without sending the screens into meltdown. It's just like designing car engines...do you want it to be a high RPM engine while being able to live to tell about it? Or is it just going to be a "daily driver" motor where max power is not of concern? Unfortunately most guitar players all wanna be able to have their cake and eat it too, but in most cases you just simply cannot have that. Nothing's free and there's always tradeoffs.