[Amps] High filament current on the 8877 amplifier tube

[MU 4CX250B]

Hi Tim,
I’ve never heard of that happening, though I’ve not investigated the
possibility with my own 8877s. I’m wondering if the date codes and
manufacturer for the odd tubes are different from your known good
tubes? Speaking from experience, a  small dent in the anode cooler
shouldn’t hurt anything, at least for EIMAC tubes, but one never knows
for sure.

If you can do it easily, you could try applying plate voltage with the
8877 filament turned off, to test for a cold short in the tube. If ok,
then, leaving the HV on,turn on the filament and cutoff bias to see
what happens as the tube warms up. You should see no cathode current
or grid current. If the tube passes this test, but hasn’t been used
for a couple of years, or if it’s a chinese or soviet tube, then just
let it cook overnight before applying drive power. Make sure you’ve
got a fuse in the plate lead to the tube, or else be sure to have a
fire extinguisher handy!
73,
Jim w8zr

Sent from my iPhone

> On Oct 8, 2023, at 12:48 PM, Tim Duffy <k3lr at k3lr.com> wrote:
>
> Most 8877 tubes that I test draw 11 amps of filament current at 5 volts.
>
> I have tested three recently that draw 15 amps of filament current. All of
> these tubes are used and have little "dings".
>
>
>
> Is 15 amps an indication of a problem tube? Or should I apply HV to the
> plate and see what happens?
>
>
>
> 73
>
> Tim K3LR
>
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