More buffers

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Max N
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#1 More buffers

Post by Max N »

I want to have a play with some buffers, for driving output stage grids (6b4g, 300B, 211), possibly into grid current. I'll try pentodes, but would also like to try some FETs. But which ones? I think I'm looking (in valve terms) for high transconductance and low Cag. Which translates into (in transistor terms) high Gfs and low Crss.
MJ used FQP1N50, which are obsolete.
Ale Moglia (Bartola) tested STP3NK60ZFP, DN2540, IXTP08N100D2, and preferred the STP3NK60ZFP which has also protection diodes.
Should I just pick one and have a go, or is there a firm favourite. It's not quite as easy as using the data sheet number, because the transconductance in particular seems to fall off at low current. I should probably do a spreadsheet, unless someone has been there before me and can recommend something?

If anyone is wondering how this fits into the PP 6b4G project, it is part of that. Or at least that that would be what I would use for listening to the effect of the buffers in circuit, in the first instance. I also want to see if driving ITs with such a low impedance brings benefits.
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Nick
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#2 Re: More buffers

Post by Nick »

I have not tried this in this context but a cascade may be a nice thing to try. Tracking the upper device with a zener between the two gates and a large resistor to b+ should in theory remove the variable gate capacitance.
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Max N
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#3 Re: More buffers

Post by Max N »

Hi Nick. For some reason I’ve always found it harder to figure out solid state circuits than tube circuits. I’m making some progress, but I have to mentally convert the ss circuit to an equivalent valve one. I think it’s just my age makes it hard to learn new stuff.
So I have wrapped my head around the typical circuit where one device acting as a follower is loaded by another acting as a constant current sink.
So just let me check my understanding of what you are recommending. We would be using two devices to create an improved follower. The second device is bootstrapping the capacitance of the first device. The resistor and zener are biasing the two devices.
Am I on the right lines?
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Nick
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#4 Re: More buffers

Post by Nick »

Yes, but its the lower device being loaded by the voltage set by the upper device. It was just a idea, may not work. Its like the super cathode follower that allen wright used.
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Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
Max N
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Posts: 1280
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#5 Re: More buffers

Post by Max N »

Ok, I’ll try out the super buffer along with the others.
I had another thought. If I was using the MJ statistical regulator power supply, could I tap off at appropriate places along the zener string for bias voltages? Also for things like raised heater supplies, screen grids?
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Nick
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#6 Re: More buffers

Post by Nick »

Yes you could but I would be wary of pulling too much current or injecting noise into the string. Adding a MOSFET follower would work well on the taps.
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
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