02-01-2017, 09:06 AM
(02-01-2017, 12:56 AM)Nowhere-Man Wrote: Yes, interesting point. I should clarify I'm very keen on seeing how helpful basic theory is so I like to apply it to something on the bench. Here is an exam question from 1959 that reflects the standard voltage drop estimate:
"It is found from tube tables that the tube to be used should have a negative grid bias of 8 volts and at this bias the plate current will be 12 milliamps. What is the required cathode resistance."
I alway prefer to say divide current "into" volts rather than divide "A" "by" "B". So, 8 volts div 0.012 = 666. An unlucky number at that.
So, the "tables" have shown currrent at a set bias. Maybe I should find the bias of the 13 milliamp valve anode because that value was the downfall. In this case too current will rise and drop as signal pushes the DC bias more into positive (less negative).
Don't get me wrong the theory just adds a bit of interest but the applied engineering side is already done for me. 33 K is the standard value and that tests just fine.
If you are referring to the ECH81 triode section then the bias voltage for an Ia of 13mA is already given in the data sheets, it's 0 (zero) volts, which more or less tallies up with the IaVg curve in those data sheets:
http://www.r-type.org/pdfs/ech81.pdf
Lawrence.
