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Subject: [Amps] ALPHA 77 SX UPDATE
From: (2)
Date: Wed, 21 Aug 2002 20:33:33 -0700
>Glad to see the recommendation for the clamping diode to ground on the 
>choke. Without this, the path to ground is through the filament winding on 
>the transformer. In the Alpha amps this may cost more than a new 8877. Why 
>not leave it as is and add a gaseous clamping device from cathode to ground 
>rated at a value below the safe cathode to filament voltage for the tube? 

What is the source, availability, rating and cost of a suitable gaseous 
clamping device.?  What is the failure mode ?  Would such a device not 
fire fire from the RF potential at the cathode ?

>They have only a few pf capacity and would be much simpler to install. As 
>Rich says--mo simpler is mo better. Eimac says that it is not necessary to 
>float the filament for RF below 30 mHz.

>Sam, W5LU
>>I have seen the open heater phenomenon in kaput 8877s that were used
>>in amplifiers that ran one side of the heater at gnd potential.  If you
>>don't have a free supply of 8877s, it might be a good idea to eliminate
>>the possibility of a cathode-heater arc burning out the heater during an
>>inadvertent hv arc to gnd.  To do this:
>>1.  float both sides of the heater xfmr winding.
>>2.  add a bifilar heater choke of 10 - 15 uH.  Bypass the cold end of the
>>3.  connect the heater to the cathode - as would normally be done above
>>4.  add a 10 - 100 V transient suppressor diode from the cold end of the
>>DC-return choke to chassis-gnd.
>Join the worldís largest e-mail service with MSN Hotmail. 

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,  

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