[glyphmon]

While I am waiting for replacement tubes on the outside chance that it is a tube problem, and not having tested the current as yet, I would like to get an understanding of the path of the flow of electricity in this part of the radio.

My present understanding of this is that we supply the current from the R14/R15 junction (170V specified, tests at 175V).  This passes through the R4 resistor to supply the screen grids on the 6A8 and 6K7 tubes.  It is also connected via C15 (0.1 mfd) to chassis ground, and the 10 mfd C16 electrolytic.

From my research, in a nutshell, the screen grids regulate the output of the tubes.  Aside from a short which as previously mentioned is unlikely, what consumes the power coming into the screen grid?  I realize this is getting more into the physics of the operation of the tube than the radio and circuitry itself, but I'm just trying to wrap my head around it.

Based on the connections in this small subsection of the radio, and understanding that these tubes are expecting 100V on the screen grids, is anything else in the radio likely to be affected by the excessive voltage drop right here?  It seems to be reasonably isolated.  Does it harm the tubes to have lower voltage, or just as before, reduce the output of the tube?  If nothing else is likely to be affected, is there any potential harm with trying to change out the resistor for a smaller value that would get the screen grids closer to the 100V mark?

Also, I'm looking at the documents that MrFixr55 supplied about these tubes.  Unless I'm reading them wrong, the 6K7 screen current is listed as 1.7 mA with the plate voltage at 250V, and for the 6A8, the screen current is supposed to be 2.7 mA.  So, is the total current (hypothetical, according to data sheets) drawn by the two tubes of 1.7 mA and 2.7 mA, or 4.4 mA total, so unexpected?

I may be applying Ohm's law wrong here, but, is it not the case that, with both tubes installed, for example, the roughly 110V drop from one side of the R4 resistor to the other would imply:

current = voltage drop / resistance = 110V / 22K ohms = approx 5 mA

Or am I using the numbers and formula wrong?

Now, I have no idea what might cause the current to vary and by how much, but of course, the radio is supplying approximately 265V to the plate.  Does higher plate current necessarily or possibly scale other values in the tube's performance?  265 is 6% higher than 250, and 6% more than 4.4 mA is 4.98, not so far from 5 mA.  I'm just guessing here.

I know others have shared some calculations and unfortunately, those are hard for me to follow without seeing where the numbers are coming from.

I suppose at the most basic level, I'm trying to wrap my head around how these values are supposed to work out with the components involved, since everything appears to be correct as far as components are concerned, how do we make the math match what is specified?

Best wishes,
Roy