The AM Forum

Ok, see attached.  If I have 2200 volts on the plate of a 4-250 abd -150v on the grid, this thing will be pulling over an amp? 1.4? and 50 mils grid?

It's late and I'm tired.

Thats at +50V bias

Carl
KM1H

Be careful, ruining a 4-250 like that could melt down to your cellar.

Ok then. No Chernobyl tubes please.

So how does one read this ^&*#%* chart anyway?

For your example its easy. Where the -150 Eg and 2200 Ep intersect continue the line to the right. It is BELOW the 0 Ip point which means its well into cutoff.

For actual operating parameters its easier if you toss all the unrealistic curves and expand just the section of interest. You can also start with a clean graph and just copy over what you want.

Carl
KM1H

Ed,
Your vertical red line assumes the plate voltage does not change but in the case of an amplifier say a modulator There will be voltage across the mod transformer primary when there is a change in plate current. So as you modulate the voltage across the primary will swing between the saturation voltage (few hundred volts) of the tube to about 2X power supply  when the tube is off (Push pull) As this happens the voltage on the plate is swinging around based on the load resistance seen by the plate. Your line will no longer be vertical but slanted. This is called the load line.

Thanks for the replys!

So, at -150 volts the tube is cutoff (which is what I thought). So in simplest terms to run it in class C it would take a 100v (more or less) sine wave to increase the grid voltage above the cutoff line for a portion of the cycle.

For this example, class A would have a grid bias at say  -75 v - tube is on (conducting) all the time.
class ab the tube would be biased into cut off partially during the input swing.
class C is just biased further into cutoff (for the same input). 
Attached is a diagram showing what I think I got from the reading I did (the only electronics I know comes from reading about Amateur Radio and a couple of classes I took on the basics years ago,

IIRC - which is a big if - a good rule of thumb is to have a fixed bias on the tube for protection and then add in yer operating bias via gridleak. You could just have a total fixed supply for it all. The HK's I'm working on want -200. you could also even some cathode bias in some cases just to be a superfreak. There's no 1 right way.

IMO the RCA datasheets are by far the most helpful. They often give you the values of screen rx's and grid rx's for various values of plate voltages.

You pretty much have it OK Ed. The bias needs to be set in the linear modes for the specified idle current, the graphs are approximations.  In any mode for transmitting its good to have around 2X the cutoff available when receiving. Even a very slight conduction can create diode noise on receive.

Carl
KM1H

Ok guys thanks for the answers. Is there a book reference I can get that would go into a step by step method of working this out?  Most of the books I have don't really go into details like the Load line.  That must have been something that got covered in the classroom!

http://www.ax84.com/media/ax84_m225.pdf

There is nothing all under one cover.

Some of the better reads are Radiotron Designers Handbook 4th Edition, Eimac Notes plus Care and Feeding, Full data sheets, The Editors and Engineers (W6SAI era) Handbooks from the 60's and later. W6SAI also was a prolific writer for the ham magazines.

Plus this, Ian is a very sharp cookie who Ive known around 20 years.

http://www.ifwtech.co.uk/g3sek/

Carl
KM1H