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