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Subject: [Amps] ALPHA 77 SX UPDATE
From: (2)
Date: Wed, 21 Aug 2002 05:43:44 -0700
>I finally really checked out the tubes on the ALPHA 77 SX.  Initial ohm 
>reading of filaments showed a very low resistance ... less than 2 ohms but 
>something didn't seem right.  Applied 5 vac filament voltage directly to 
>tube and measured filament current and voltage.  Filament current was zero.  
>Immediately measured resistance ... it was less than 2 ohms, but a with 
>gentle knock on the tubes, resistance went to zero.  There must have been a 
>few filament threads hanging on that gave the previous continuity reading.  
>The first time I checked out the amplifier I got output, but it was probably 
>just one working tube and the filament was fragile and lasted for one test.  
>Looks like I need two 8877 tubes.  Anyone know where I can get any 8877 
>for a good price?
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. 

Another consideration is that 2, 8877s have 0.2pF of feedback-C.  Since 
the 8877 is capable of oscillating) well into the uhf range, this amount 
of feedback-C is nothing to sneeze at.  [intermittent vhf/uhf parasites 
have a reputation for causing intermittent hv-to-gnd arcs, so lower vhf-Q 
suppressors might not be a bad idea] 

cheerz, Bob

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

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