[R-390] C227? recap note

[John KA1XC]

That's a good description from Roy.

I also found that this cap IS needed for stability as well, but for reasons
not particular to the way that stage works.

The front end in my EAC 390A liked to take off on the higher bands and the
oscillations varied with the ANT TRIMMER control. It turned out that the
C227 .047 uF bypass was open, and since the cathode is connected to the RF
GAIN pot via an unshielded lead in the wiring harness it was pumping RF
somewhere where it shouldn't of been and things got unstable.

This cap is often overlooked in recaps; I replace it now as a standard
practice if the RF deck is being recapped. The new bypass cap can be located
adjacent to the tube socket in the *same* compartment as its tube pin
connection and grounded locally as well, rather than connecting it through a
relatively long (inductive) lead to the adjacent compartment where the
original cap is located. The old can cap stays there for looks but is
disconnected.

See link for pic, the old cap is of course is barely visible and the new cap
is the yellow axial leaded one.

http://img226.exs.cx/img226/2469/c227recap5yo.jpg

73,
John


> 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
>
>
> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Cell 301-928-7794
> Work: Voice: 301-975-3254,  Fax: 301-948-6213
> roy.morgan at nist.gov --