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