Driving 6V6 in to AB2

[FredB]

I was modeling driving a pair of 6V6s into AB2 operation with a pair of source followers. The 6V6 Leach tube model I am using seems to handle grid current badly.

So, my question is how well to 6V6s handle the grid current supplied by a source follower?

[redelephant]

Good question. The datasheet does include graphs with positive grid1, so obviously the tube can handle it. The grid current is pretty high though, almost 150mA at +15volts at the grid.

So unless u have a seriously hefty driver, those tubes wont behave well in AB2.

[FredB]

Good question. The datasheet does include graphs with positive grid1, so obviously the tube can handle it. The grid current is pretty high though, almost 150mA at +15volts at the grid.

So unless u have a seriously hefty driver, those tubes wont behave well in AB2.

The problem I am having with the Leach 6V6 model is that it fails to show any more grid current than about 10uA or so even with 25V positive grid voltage.

BTW Which data sheet did you see figures for grid current on? In the GE Data sheet for the 6V6/5V6 on page three the average plate characteristics graph appears to show about 45mA grid current at +15V grid voltage with a plate and screen voltage of 250V.

***

I've found the Duncan Monroe 6V6 model handles grid current. The stock Leach model seems to lack that capability. So, I guess that answers that.

[Steve Conner]

First thing to do is look for the control grid dissipation rating in the datasheet. If there isn't one, that means the maker never intended the tube for AB2. You can use it, but at your own risk. I heard that tubes with gold-plated grids aren't suitable for AB2, but that is just hearsay.

You could also come across a situation where the original tube was rated for AB2 but the Russian reissue isn't.

You're lucky that SPICE even models tubes at all, let alone accurately.

[FredB]

First thing to do is look for the control grid dissipation rating in the data sheet. If there isn't one, that means the maker never intended the tube for AB2. You can use it, but at your own risk. I heard that tubes with gold-plated grids aren't suitable for AB2, but that is just hearsay.

You could also come across a situation where the original tube was rated for AB2 but the Russian reissue isn't.

You're lucky that SPICE even models tubes at all, let alone accurately.

I wish my scope was more in alignment, and was a data logging version. That would make determining screen and grid dissipations possible.

I'm real glad that spice handles tubes at all. I have found that the tube models even with the simplified transformer models are sort of near the neighborhood. Good enough to be somewhat helpful.

I've also found at times it is necessary at times to break out the pencil and paper and draw up some load lines...

It's all good.

[Steve Conner]

Even with a DSO, measuring grid dissipation is no picnic.

A tube that was properly characterized for AB2 would have a "Maximum DC grid current" rating in the datasheet. That's the value that would be read by a grid current meter in your ham radio linear amp, or whatever. The tube makers knew how hard it was to measure dissipation, so they printed that figure as an easier-to-measure substitute.

So, you could measure the average ("DC") current drawn from the supply into the drain of your source follower, and compare it to that figure.

Grids of high-slope tubes can draw a LOT of current, because they have lots of fine-pitch wires right up close to the cathode. I've heard of people burning up grid stopper resistors with these AB2 driver arrangements.

[FredB]

Interesting point. I've read posts about burning up the resistors. What wattage resistors were they using?

As far as the model goes, and I know this is one of its more ify faucets, it looks like about 100mW of grid dissipation as far as I am willing to drive the tubes. I am presuming the current spikes on the screens have to be watched as well.

Just how far were they trying to drive the power tubes. My models show that I'd get maybe 25 watts out of an amp that was getting around 20 without the current buffers, although the distortion character appears to be much improved.

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Come to think of it, perhaps I could voltage limit the signal driving the power tubes by giving the phase inverter some careful attention...