>I guess the nice part of Monday morning is catching up with what's happened
>on the reflector over the weekend!
>
>Guess this is where I stick my neck out....................
>
>I believe that under the heading 'parasitics' we talk about 2 different
>things.
>
>1. 3-500's and the like in g-g with bent filaments etc
>2. devices with boiled gold on the grids.
>
>Case 2 I am quite prepared to accept as 'classical' VHF/UHF parasitics.
>
>Case 1 I am not so sure about. Just suppose that we have several amps of
>VHF. How does it bend the grid/filament? By definition, it's AC, so the net
>mechanical force just makes it vibrate.
Do grid current meters measure AC current or DC current?
- In the TL-922, SB-220 and L4-B, the DC grid current component travels
through an RF choke between grid and gnd. AC grid current travels
through the grid to gnd capacitors. Damaged grid RF chokes and bent
filaments often go hand-in-hand.
>TBut because of the inertia of the
>metalwork, it's unlikely (!) to vibrate at 100MHz. On this basis, the
>'flash-arc' or Rocky Point effect seems more likely.
>
Why don't I find arc-marks or more bad vacuums during testing and autopsy
of bent-filament shorted 3-500Zs?
>Resonances.
>
>Does it matter how many of these resonances there are? Provided:
>
>1. they shouldn't be at a hamonic of the operating frequency. If they are,
>then because they are almost certainly unloaded, you can get pretty massive
>voltages built up.
>
>2. just because there's a VHF resonance doesn't mean that there will be a
>parasitic oscillation. There has to be feedback and gain.
>
Agreed. . All amplifier tubes have output/input feeedback C. The
3-500Z is rated for max. input to 110MHz, which is approx. where the
anode circuit in the SB-220 resonates.
>Obviously, in a practical case, it's desirable to minimise the number of
>resonances, because of the possible problems, but even so, one resonance may
>not cause parasitics per se.
>
>I've never understood why calculations on parasitic suppressing resistors
>always seem to give lower dissipations from the fundamental current than
>practice does. Maybe it's me........
>
Are you referring to the method used to calculate suppressor R
dissipation in the March, 1989 *QST* article?
>...........snip
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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