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Old 9th April 2004, 07:52 PM   #21
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Default Yes Sy

Might call it a constant (although no tube is perfect). u=gm rp shows that, help our newbie friends. If we can keep u and gm constant, then rp will remain the same and the load lines in quadrant 1 would look great, quadrant 3 transfer characteristics would also be a straight line. Both would have great significance.

By contrast, typical tubes, and more revealing, variable mu or remote cutoff pentodes are variable in that all three parts of the formula are changing because of design considerations of the tube. Any good tube manual will explain the processes envolved if one wants to delve more deeply into it.

Anyway, back to work.

Good info to bring up Sy. Thanks and take care.

Steve
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Old 9th April 2004, 08:00 PM   #22
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Hi,

Quote:
Any good tube manual will explain the processes envolved if one wants to delve more deeply into it.
Varying the grid wire pitch is what triggers the variable mu.

Cheers,
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Old 9th April 2004, 08:15 PM   #23
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Quote:
Originally posted by EC8010


Hello, thoriated. But can you afford probes that maintain that 500MHz bandwidth to the probe tip? (I can't.)

As Frank says, grid-stopper values are heavily dependent on layout. I recently had to resort to 10k on a point to point layout (great for audio, less great for RF), whereas the PCB equivalent got away with a few hundred Ohms...
Hi EC,

So how do you select your grid-stopper values? By ear, scope, both?

Thanks for an interesting discussion.

Best,

George Ferguson
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Old 10th April 2004, 01:26 AM   #24
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Quote:
Originally posted by DrDeville
So how do you select your grid-stopper values? By ear, scope, both?
Good question. Both. My oscilloscope claims 300MHz bandwidth at its input BNCs, and the (original) probes claim to extend that bandwidth to the probe tip, but a few pF can make a huge difference to a circuit that's on the edge of oscillating at 100MHz. Fortunately, circuits that oscillate at more than 100MHz tend to be affected by anything that contacts them, and if you listen to their audio output whilst touching them with a probe (or even the blade of an insulated screwdriver) they tend to make more crackles and pops than you would expect, and that can be a pointer that something nasty is going on that your 'scope can't quite see.
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Old 10th April 2004, 07:48 PM   #25
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Hi, ec8010 -

Sorry for the delay in responding, but the scope probes I used came with the 500Mhz scope, so I presume they're good for the full bandwidth.
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