[Amps] sb 220 1mh rf choke help

R.Measures r at somis.org
Sun Jun 20 06:08:33 EDT 2004

=F0  Hello, Pat -- The 1mH choke was not defective.  It smoked for a=20
reason -- i. e., too much grid current.  However, the surge in grid=20
current was probably not caused by operator error.  All tubes have a=20
small amount of anode-to-input feedback-C.  This is insignificant at=20
HF, but not at VHF.  It might be a good idea to check the tube whose=20
choke smoked for a filament-to-grid short since high grid current=20
exerts lateral stress between the grid and the filament.  I would=20
replace the smoked choke with a 0.25w to 0.5w carbon resistor that has=20=

a similar # of DC ohms.  20 - 30 ohms is typical.  However, to protect=20=

the resistor from 10m RF, add 1000 or so pF to the existing grid bypass=20=

C.  In the event of an oscillation condition, the small resistor will=20
open and hopefully protect the tube from failure.  cheerz

On Jun 19, 2004, at 11:04 AM, N7uvh at aol.com wrote:

> does anyone know of a good online retailer of the 1mh rf choke used in=20=

> the
> sb-200 at the bottem of the z500 tube.  I have replaced my recifiyer=20=

> board with
> harbuch and looks like I have smoked a choke.   I found one place but=20=

> was 22
> buck each.
> tnx
> Pat
> n7uvh at aol.com
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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