>
>On Wed, 08 Aug 2001 14:37:01 -0400, Pete Smith wrote:
>
>>Well, I don't know, but when the T/R relay on my SB-220 began to stick in
>>the "on" position, it turned out to be the bias contacts that were the
>>worst burnt. Maybe a sign that they were trying to open too soon?
>
>_________________________________________________________
>
>The contacts must be opening more than just bias, most likely the actual
>cathode current.
// The Ic is normally 0.15A - which should not crispy-critter 10A
ontacts.
> Since this is DC, you should be able to suppress the
>arc by placing a cap and resistor in series across the contacts. The
>best value has to be determined empirically. I'd try a .1 uf in series
>with 100 ohms and then tweak the values for minimum visible spark at the
>relay contacts. You should be able to get the spark down to barely
>visible.
// Good idea for arc quenching. However, 0.15A should not be a problem.
My guess is that the problem is related to the use of V-cutoff bias
during RX. I would try converting to resistor-cutoff bias. This done by
removing the +110v PS from the bias contacts, and moving one end of the
100k-ohm [R27] resistor from the NC contact to the NO contact. This
converts the bias to resistor-cutoff. Also, with the SB-220's original
V-cutoff bias ckt, when a 3-500Z fil,/grid shorts, the (unfused) filament
transformer will melt down unless the 220 is quickly switched off.
However, with R-cutoff bias, nothing bad results when a tube shorts.
cheers
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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