[Boatanchors] Carbon Comp Resistors: The Darkness Gathers?

[Jacques Fortin]

Hi David,

Your statement: "The metal film spiral trimmed resistors will definitely kill the drive on the Collins 30L-1.
These are on the grids of the PA tubes."
Raised some doubts because I am just restoring a 30L-1 actually.
So I have the following comments:

1_ I measured three variants of 47 ohms, 1W resistors on my HP 4192A bridge: Allen-Bradley Carbon Composition, Corning Glass Works tin-oxide and Yaego metal-film. For the AB and CGW, the inductance part of the measured impedance is 0.01µH at 13MHz and depends on the length of the connecting wires, leaving me believe that the resistive elements within those resistors are not more inductive than an equivalent piece of wire. The Yaego measures 0.022µH with equal connecting wire lengths, so OK for this one, it have to be the "spiralling" effect contributing, BUT that equals to 3.9 ohms of inductive reactance at 28MHz, so less than 10% of the resistive value. Not so a big problem.

2_ About "killing the drive": have you remarked that the 30L-1 is not driven by the grids of the 811As ?
This is a grounded-grid amplifier: the 47 ohms resistors on the grid circuits are just there to bring the negative DC bias to the grids themselves, and accessorily serve as fuses in case of a power tube hard failure.
The grids are AC-grounded by the 220pF caps (in fact that value was chosen to create a broad series-resonant circuit with the 811As grid inductance).
The drive is applied to the cathodes of the 811As thru pi-network matching circuits, so whatever the distributed inductance of the currently available 1W size 47 ohms "grid" resistors, I cannot figure how this can, in any way, "KILL THE DRIVE"...
I agree, however, that using wirewound resistors there will create problems (some tried this before)...

About the CC resistors: the ones available now from Ohmite and Stackpole (wherever they are made), are "coated" with a kind of lacquer.
The result of lessons learned about the moisture penetration and resulting degradation process ??
 
73, Jacques, VE2JFE