[AMPS] Re:HV on tube w/o filament voltage

[Rich Measures]

Mr. Ehrhorn -
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>
>Don't you  think the HV filter will generally discharge much faster than 
>the cathode will  cool down & lose emissivity, Peter...?  Even if the HV 
>contactor/relay takes 50 ms to open and the tube(s) look like ~1k dc load 
>without bias, I think the HV is pretty well gone in another 50-100 ms. 

€  agreed.  However, in Class AB1 amplifiers using oxide-cathode tubes, 
to preclude the possibility of cathode-stripping, it might be a good idea 
to make sure that the bias supply bleeds down much slower than the anode 
supply.    This is why there is no bleeder R on the filter capacitor (C1) 
in the electronic switched Class AB1 bias supply shown in Figure 4 on my 
Web site.  

>Don't think much cathode cooling occurs in 100-150 ms. HOWEVER, if there's 
>any solid evidence to contrary it's something I'd sure like to know!
>
>73, Dick
>
>-----Original Message-----
>From:	Peter Chadwick [SMTP:Peter_Chadwick@mitel.com]
>Sent:	Tuesday, March 30, 1999 5:15 AM
>To:	'amps'
>Subject:	RE: [AMPS] Re:HV on tube w/o filament voltage
>
>
>The problem I could see is when switching off: if the bias goes down while
>the cathode ( rather than filament) is hot, it could lead to problems  with
>stripping. It might not do a thoriated filamenmt much good, because you
>start running out of emission, but generally, their thermal time constants
>are much less than cathodes.
>
>73
>
>Peter G3RZP



Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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