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In Reply to: RE: Well that was quick....... posted by Jim McShane on October 11, 2014 at 14:04:34
Was I reading the plate curves right though Jim? Do they actually predict about -40 vdc for the bias with plate current of 62mA and plate volts of 430 vdc, or am I doing that wrong?
Edits: 10/11/14Follow Ups:
Let this go Vinnie, you're just confusing yourself. There is often a LARGE variation in plate and/or cathode current between tubes. So the curves may show -40 but that's only true for a few tubes of the type. Due to internal construction variances & variances of heater voltage the -40 might need to be -32 on the next tube or -51 on the next after that and so on.
The curves only apply to a "bogey" tube under very specific circumstances. The typical tube in a typical amp often shows significant variances even with identical circuit characteristics - because the tubes themselves often perform far differently than the bogey tube does.
You misunderstand Jim. I am just trying to figure out if I am doing it correctly. I am quite happy to go with your advice on not trying to mess with the grid volts, but I do want to know if I am reading the predicted value right for future use.
Besides, if I am it means the readings that I am getting for bias matches the predicted and the bias listed in the operating posts is much different for some reason. I would like to know that if it is true.
Edits: 10/12/14
Vinnie,
The exact grid voltage is essentially meaningless. Only in the most general and loosely defined terms does it have any meaning at all. The tolerances on grid voltage are quite loose so the curves only approximate what you'll find in real world situations. That is because the curves are only correct if the tube behaves exactly as designed.
You can look on a plate curve diagram and if you know the current and the voltages then the diagram will tell you what the grid voltage should be - as long as the tube is built EXACTLY to the spec - which they almost never are.
Can I suggest a little research on how tubes work is likely to make this much more clear? Your posts have changed the verbiage some, but essentially you are asking the same question over and over.
Should the grid voltage on the curve diagram match the tube in your project? In a perfect world maybe - but in the real world the tube's tolerances render an exact number meaningless. The next tube in the EXACT SAME circuit will behave differently.
JimThe link below is the answer I was looking for; you just kept misreading the question.
I was reading the plate curves incorrectly, that's what Tre' showed me in his post and that was what I was asking you.
Edits: 10/13/14 10/13/14
Vinnie,
What I'm trying to get across is that the curves Tre' posted are only accurate for a very few tubes. The VAST majority of 845s will not end up with -51 on the grid even with 430 plate volts and a current of 62 ma.
Just keep that in mind!
We have probably beat this to death, so "Roger on that". : )
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