Driving Grid Positive into A2

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Paul Barker
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#1 Driving Grid Positive into A2

Post by Paul Barker »

It’s said.

1/ You can’t autobias the output valve because cathode resistor won’t free up grid current. you cant cap couple due to blocking distortion.

2/ You can IT couple

3/ You can direct couple with a cathode follower operating at greater current than max grid current. Obviously this valve is dc shifted to bias point of output valve as part of levelling mechanism within a +/- power supply.

1/ I accept isn’t any good.

2/ on surface is good negative power supply biasing output valve . BUT Interstsge transformer has a dc resistance, so the -ev bias will alter as grid current flows. So positive peaks will flatten proportional to dc resistance of IT coil due to changing bias.

That only leaves option 3/

Right or wrong?
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Nick
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#2 Re: Driving Grid Positive into A2

Post by Nick »

Well, all driving methods wiil have a source resistance, so you will get some current rated distortion. But yes, that’s the restriction with IT coupling.

I don't understand why the CF current has to be equal to the max grid current though, I would expect grid current to increase the current through the cathode follower not reduce it.
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#3 Re: Driving Grid Positive into A2

Post by Max N »

It’s worth trying a mosfet source follower as an alternative to the cathode follower. I think I actually prefer it subjectively.
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#4 Re: Driving Grid Positive into A2

Post by Paul Barker »

Thanks both
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#5 Re: Driving Grid Positive into A2

Post by Max N »

Paul, I hope you don’t mind me asking this here, I think it is relevant.
I think it’s relatively easy to get the source resistance from the follower so low that a grid stopper on the output valve becomes significant.
So the next question in my mind is ‘what steps can we take to reduce (or eliminate) the grid stopper?’
Is it just a case of keeping the wire from the follower to the output stage grid as short as possible?
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#6 Re: Driving Grid Positive into A2

Post by Paul Barker »

If you go to 801a thread I’ve put up first attempt at a driver. The cathode of driver has autobias resistor I could have bypassed. when driving long cable runs with capacitance, not bypassing this resistor stops capacitance interacting with CF. The resistor Not bypassed prevents oscillation getting starting. I’ve chosen that method to work for the 801a input capacitance.

When circuit is built and not just a figment of my crowded mind I can experiment. But I think it’s a safety against oscillation worth harnessing @ the cost of higher output impedance.

There will be other similar situations , when whole amp built, listening tests will decide with/without cap.
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#7 Re: Driving Grid Positive into A2

Post by Nick »

Max N wrote: Wed Jul 07, 2021 2:30 pm Paul, I hope you don’t mind me asking this here, I think it is relevant.
I think it’s relatively easy to get the source resistance from the follower so low that a grid stopper on the output valve becomes significant.
So the next question in my mind is ‘what steps can we take to reduce (or eliminate) the grid stopper?’
Is it just a case of keeping the wire from the follower to the output stage grid as short as possible?
I would turn that question around a bit Max, I would ask if the source impedance is low enough do we need a grid stopper? The stopper works with the grid capacitor (and miller if relevant) to create a low pass filter to stop oscillation. If its working in this way, it implies to me that the feedback point has to be before the grid stopper otherwise it would have no effect. So if the impedance of the source (at RF) is low enough, would that not also act to prevent oscillation. Just a case of getting the source driver as close as possible to the grid.

If not the above, than maybe a ferrite instead of a grid stopper would be worth trying.

You could also try a diode across the resistor to allow grid current to pass in that direction.

Not tried any f the above. Personally I think its a bit pointless to worry about the effect of a small resistor, it seems to be a bit of a fiction that without the resistor and an 0 ohm source driver the valve will be perfectly linear, so its maybe chasing a ghost.
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#8 Re: Driving Grid Positive into A2

Post by Max N »

I was thinking about the A2 case, but it may be that I don’t fully understand how the grid load appears to the driver. I seem to remember estimating ages ago that when a 211 enters A2 it changes from an easy/high impedance load to something like 2Kohms. Or rather for part of the waveform it is high, and for part of the waveform, when the grid is positive, it is 2K. And to minimise the distortion due to the changing load, you would need your driving impedance to be small compared to 2Kohm. So then the grid stopper becomes significant.

I think you are right, a low impedance driver very close to the grid probably won’t oscillate. I don’t have easy access to the bench at the moment, but will do some experiments when I can. And if I do hit problems I will try the ferrites. Thanks Nick

Paul, I think I get what you are saying (took me a while to get my head around it) but I don’t think it applies with a fet follower because the biasing is different.
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#9 Re: Driving Grid Positive into A2

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Max N wrote: Wed Jul 07, 2021 4:59 pm

Paul, I think I get what you are saying (took me a while to get my head around it) but I don’t think it applies with a fet follower because the biasing is different.
My heads in bits for personal reasons so I cant get my own head round what its thinking. Which is actually why Im taking so long to even start drawing the amp on paper.

I get youre fears of something horrible occuring but me and Nick have really driven an 833a into A2 and no bad sound. Dave t,t Bass would say “Shoddy”.

Ive had to fight myself into submission not to just build the ach2 circuit Nick tried and shared with me. But I wanna do it my way! if it works!
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#10 Re: Driving Grid Positive into A2

Post by Nick »

I also drove the 100th in a similar fashion.
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