[AMPS] AL1500

[2]

>
>By comparison, my PA70-V uses the same darlington EBS circuit to the 
>cathode, but Q2 does not exist as this was a feature
>"enhancement" in the PA-77 which later replaced the PA70 series.  Instead, 
>a 1/4-amp grid fuse is placed in series with the 8.2 V
>zener diode to the tube's cathode.
>
>Now, to Rich's point (I believe)...the fuse may in fact blow faster in 
>this arrangement than in the case of the PA-77 where a
>sampled switching transistor operates a relay to force a standby 
>condition.  The relay uses no special acceleration circuit.  So, it
>would be an interesting test to measure the "break" time of the fuse 
>versus a transistor/relay combination.
>
A conventional relay adds c. 15mS delay.  My assumption - based on 8877 
autopsies - is that this delay may be problematic because gold-sputtering 
occasionally happens.  In my experience, fast fuses serve well in 
protecting the screen grid against excessive dissipation.  Thus, I am not 
surprised that the 70-V's 1/4A grid-fuse performed its job 
satisfactorily.  
--  Sometimes mo' complicated is not mo' betta.  

> ... ... ...
cheers, Paul

-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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