[R-390] C227? I can't find it

[Roy Morgan]

At 10:23 PM 3/28/2005, Barry wrote:
>...Amelco RF deck. ... the modification that adds a 47pF cap across the 
>0.047uF cap (from cathode to ground) on the first RF amp (V201).
>...
>According to TM 11-5820-358-35, C227 is supposed to "prevent cathode circuit
>degeneration."   I'm not sure exactly what that means.  Was this cap
>intended to prevent the tube from oscillating?

Barry, and others,

I didn't see anyone talk about what "cathode degeneration" is, so here goes:

The first RF Amp tube, a 6DC6 had R202, 220 ohms in it's cathode line, and 
C227, 0.047uF bypassing that to ground.  The grid is connected to the AGC 
line through the RF input transformers.  The cathode resistor creates DC 
self-bias because of the tube current. If that resistor were not bypassed 
for RF, it would create "bias" at those frequencies in opposition to the 
signal applied to the grid.  This is "cathode circuit degeneration."  The 
result would be less gain than desired.

The fix is to bypass the cathode to ground for RF voltages, thus the 0.047 
uF cap.  We could assume that the cap is pretty much a short circuit to RF, 
and even more so at higher frequencies. However, caps are made of real 
stuff, including coils of metal foil or film on paper, and so practical 
capacitors can be somewhat inductive at some frequecies.  This effect would 
depend on how the cap was made, and one batch might work well and another 
batch, or caps from anther manufacturer might not.

It appears that in some RF decks, the caps used caused reduced gain in the 
first RF stage due to inductance (and maybe series resistance) in the cap. 
So the filed change added the small 47pF cap to make it really bypass at 
the higher frequencies.

Roy


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