[Amps] Three questions about the 8877 in G-G

[Radio WC6W]

Three questions about the 8877 in G-G

  Recently, I've been contemplating building a low band, 160-10 meters, grounded grid 8877 amp as I've got a socket, a bandswitch and a suitable cabinet -- That's a start!

  And not having built anything in the G-G format since my pair of 813's, 40 years ago, I started by reviewing some of the ancient literature, plus some of the knowledge of the internet which has left unresolved the following queries:


  1. What is the proper treatment of the heater circuit?

  Eimac recommends an ungrounded supply.  Some designs interconnect the heater & cathode, some keep them isolated, some run trifilar chokes, some bifilar and a separate one for the cathode, and some sans one at all in the heater leads.  

  I'm planning on using a switching supply, with an appropriate soft start, to power the heater but, it would add a bit of complication if this voltage need be isolated from the main circuit ground.


  2. What is the proper treatment of the cathode circuit?

  Why are the cutoff bias resistors typically rated at such high wattage?  I've seen 10-25 watts in the literature.  It seems that a small fraction of that would be more than sufficient.

  Why do so many designs include fuses in this circuit?  Is this necessary with plate and grid current fault cutoffs?


  3. Is there any easy way to measure the plate phase angle, for an autotune sense, in a G-G design?

  The grid driven schemes rely on having an independent input signal which is not the case in G-G -- the plate phase will pretty much track the cathode signal.

  I've seen a couple allusions to a method employed in commercial transmitters but, no circuitry on the 'net.  And speaking of netsearches, I've noticed, starting perhaps months past, the search engines exhibit an increasing dearth of cogent results related to our hobby.      

73 & Good morning,
   Marv WC6W

http://wc6w.50webs.com/




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