>Hello
>
>I am new to this reflector. I have an amp with 4 813's and need to know how
>to calculate the value of the resistors in the plate parasitic circuit.
>
>any help would be appreciated.
There is an article in the March, 1989 issue of *QST* magazine.
>It currently has 4 unknown values in parallel inside a 1 inch long coil
>about 1 inch round.
>
Briefly, MOF resistors are a better choice for the suppressor resistor
(Rs) than carbon-composition resistors. The reasons: MOF resistors can
dissipate more power without changing value, and they have less intrinsic
L (increases VHF stability). However, the conductive film of a MOF
resistor is just below the surface, so wire should not be wound on or
around one. The suppressor inductor (Ls) needs to be adjacent to Rs and
separated by an air gap of around 2 mm to prevent arcing between Rs and
Ls. Typical values of Rs are 50 to 100 ohms.
- More Ls results in better VHF suppression. However, too much Ls will
burn out Rs, so a compromise is in order. Thus, Ls should be whatever it
takes to make Rs get quite hot at the highest operating frequency---but
NOT hot enough to burn out Rs. I'm guessing that about 4 to 5 turns on a
3/8" winding form would be a ballpark figure for Ls for an 813, at 15m
max., at 2800v.
- Materials that tend to yield a high VHF Q increase the VHF Rp
(parallel equiv. R) of the suppressor assembly. More VHF Rp means more
VHF amplification. Thus, 'heavy-duty' copper and silver conductors
should be avoided. All things being equal, compared to copper, a
resistance-wire Ls typically lowers VHF Rp by about 40%. Stainless-steel
safety-wire is a good substitute for resistance-wire. A smaller diameter
conductor for Ls is better for reducing Rp--- Provided it can safely
carry the RF circulating current from the tank at the highest operating
freq. For a graphical comparison of a copper versus a resistance-wire
Ls, see Figure 12 at:
http://www.vcnet.com/measures
Rich---
R. L. Measures, 805-386-3734, AG6K
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