>At 10:53 PM 2/11/03 -0800, Tom and Linda Hellem wrote:
>>Gentlemen:
>>
>>The information I have read concerning improvements to the SB220 contain a
>>recommendation to replace the grid chokes-to-ground with resistors and to
>>replace the stock 200 pf mica capacitors from the grid to ground with 1800
>>pf of capacitance per tube socket. My Digikey catalog lists some ceramic
>>disc 680 pf @ 1KV for .31 each and some silver micas, same value @ 500V for
>>1.86. Since I have two amps to modify, the cost is an issue and I am
>>wondering if the ceramic discs will do the job. Also, is the 1800 pf a
>>critical value, as I will end up with 2040 pf per tube using the 680's. Any
>>thoughts would be appreciated.
>
>Hmmm ... I have also read repeatedly on this reflector the recommendation
>to ground the grids directly, removing all of those components. In the
>absence of problems with my amplifier, I have not changed it from
>stock. The disparity in these recommendations tends to make me believe
>that this area is a good candidate for "if it ain't broke, don't fix it."
>
** A directly grounded grid (3-500Z) exhibits a grid resonance that is
c. 90MHz. A capacitor-resistor grounded grid resonates at c. 90MHz.
Grounded grid amplifiers are essentially neutralized below the resonant
frequency of their grid. Above that frequency, they are not
unconditionally stable.
- The advantage of a grid-fusing resistor is that it is likely to blow
before the tube sustains a grid/filament short. Shorted 3-500Zs are not
rare in amplifiers with RFC-grounded grids (L4-B/95MHz, TL-922/130MHz,
SB220/110MHz) nor are they rare in amplifiers with directly grounded
grids (SB-1000/155MHz, LK-500/100MHz, AL-80/155MHz).
- Note - In order to protect the grid fusing resistor from RF on the 10m
band due to the 5pF of coupling to the anode, the grid needs to be
RF-bypassed with c. 1500pF.
>
cheerz
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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