Plate Current VS Plate Voltage Graphs

[Captn Dave]

I have a 6L6 setup where the cathode current (therefore bias) is set by a bad and unmarked resistor. There is some evidence to suggest that 100ma is the correct cathode current. I'm trying to find the operating point for verification. Which Chart is relavent to the task of determining the correct cathode current?

I suppose what I'm asking is that given the relatively low current at the supressor grid and control grid, do designers assume the plate current and cathode current to be equal?

Thanks

[ray_moth]

I guess you mean "the relatively relatively low current at the screen grid and control grid". I would say it is quite a rough approximation to consider cathode cureent and plate current to be the same. The screen current, even in a beam power tube, is a significant proportion of the cathode current and varies with screen voltage.

[Captn Dave]

Yes I did mean to say screen grid, and I should have approached the question differenty. Allow me to try again.

I'm sure we've all seen plenty of designs that use a pair of 12 volt filaments in series to set the cathode bias (Is there a term for this design?). It seems that most of these designs in my collection of schematics specify a 20 or 22 volt bias.

The vintage guitar amp I have on the bench now uses a 39R resistor in series with the parallel combination of a single 12 v filament and a (failed) power resistor of unmarked value. As you would expect, with the open power resistor) the 12AX7 is seeing way too much voltage and the operating point of the 6L6 power tube is way down there. The power resistor must be replaced but with what value?

I set it up on the bench and noodled around with different resistors. 100R yields 12.7 for the tube filament and 22.9 for the bias. Not too bad for the tube filament but still pretty high for the bias. A 44R resistor drops the bias to about 20.6 volts but the filament only sees 8.8 volt. I'll probably set it up for a 22 volt bias which should yeild an 11 volt filament and call it a day. OR- a 21 volt bias and 10 volt filament is another possibility, humm.

But, I'm a geek and I so would like to know, without a schematic, how would one go about reverse engineering the design operating point and determining the correct bias?

And so that's why I was asking about cathode current.

Thanks