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Subject: [Amps] ALPHA 77 SX UPDATE
From: (S. J. Blackwell)
Date: Wed, 21 Aug 2002 13:46:23 +0000
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? 
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
>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.

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