An introduction and a question

[W2PFY]

AM (Double Sideband w/carrier) is a no-no for Technicians on 10 meters.

yer right, maybe I need to call mind busters to get some pills so I don't forget this stuff!

[WD5JKO]

I've always wanted to add some NFB to my single stage 8877. I have at least 6db of extra drive available for NFB.

   Tom,

  Here is what I do with my 8877. That air wound choke going to the cathode of the 8877 has 4 ohms DC resistance. The choke goes to a ~5v Zener bias diode. This resistor adds 4v of bias at 1 ampere cathode current. This is true cathode degeneration, a form of NFB. The little 6 hole ferrite bead cured a parasitic that I was seeing.

http://pages.prodigy.net/jcandela/MLA_2500_8877/socket_view.JPG

   I have no point of comparison with/without the choke. I can say though, that with low B+, the idle current is low (about 30ma), yet on AM at > 250 watts carrier output, the Trapezoid pattern is as good as the exciter. Switching to my GSB201 with 4 X 572B's, and 100 ma idle current, I see Trapezoid curvature (slight 'S' pattern) at > 150W AM. Adding a tuned input to the 572B amp did not help. The exciter within it's capability has a perfect Trapezoid.

Jim
WD5JKO

   

[KM1H]

Tom, adding about 10 Ohms in series with the drive to a filament tube creates NFB and improved IMD. Thats a common mod to the SB-220 in order to reduce gain, improve stability and the NFB is a bonus. I also do it to many SB-200's as 100W drive is overkill to a pair of 572B's, especially with customers who should either get an SS amp or take up another hobby that doesnt require so much mental stress

Ive never tried it with a GG cathode tube as all the ones Ive worked with already have IMD in the -40 region. I see no reason why it shouldnt work.  It does work with grid driven tubes as already mentioned and that can go right on up to the 4CX250 family and similar.

[K1JJ]

Thanks for the info Jim and Carl -

That seems like an easy mod to do.  

The question that keeps bothering me is the varying DC bias caused by the resistor. I can see how the cathode resistor would be FB if the tube was in a steady plate current state like class A. The DC bias generated by the resistor would be constant. But in AB operation, the plate currrent is varying all over the place, thus the resistor bias is too. I would think this would cause distortion like any sagging DC bias would add. Would the benefit of the degenerative RF NFB be canceled out by the varying DC bias, no?  

In contrast, when using a zener, the DC voltage across it is constant no matter what the current.

What am I missing?

T

[k4kyv]

AM (Double Sideband w/carrier) is a no-no for Technicians on 10 meters.   Their only voice privileges on HF are SSB between 28300 and 28500 kc.

Another one of those bogus anti-AM clauses that got shoved through under the radar.  The alleged reason was to keep CBers from getting a Tech licence, modifying their good buddy rigs and turning 28.3-28.5 into an extension of 11m. A bit of paranoia or stretch of the imagination, to say the least, and given the current drop-off in interest in CB as it exists to-day, no longer serves any useful purpose even if it ever did.  The SSB-only clause only serves to point newcomers who might otherwise modify an old CB rig to 10m, or homebrew a simple crystal controlled low power 10m AM transmitter, firmly in the direction of appliance operation from the outset, kind of like that entry-level "Foundation" licence in Canada and the one in UK that require the use of commercially built equipment and prohibit homebrew.

If you want to have some fun playing with some heads, try operating AM in this segment.  A lot of Hammy Hambone types have the mistaken belief that AM is illegal in this segment for everyone, not just for Techs, and will try to play radio cop and order you off the frequency because AM is "illegal" there.  Just tell them you you are pretty sure you'll get away with it anyway, and if they don't like it they'd better report you to the FCC.  

Ashtabula Bill used to get a big kick out of doing that.