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[Amps] FW: High filament current on the 8877 amplifier tube

To: <>
Subject: [Amps] FW: High filament current on the 8877 amplifier tube
From: <>
Date: Mon, 9 Oct 2023 15:20:45 -0500
List-post: <>
I have nothing to offer about the excessive current draw, but I have read a lot 
of warnings NOT to apply plate voltage to an 8877 until the cathode comes up to 
full temperature to avoid damaging the oxide cathode.   "Do not apply HV or 
draw cathode current unless the heater has been within factory specified range 
for 3 full minutes.   If the heater is started very slowly, the tube warm-up 
time should be extended.   Never run a (8877) tube when the heater voltage is 
low.   Doing so will poison the cathode"  -


-----Original Message-----
From: Amps <> On Behalf Of MU 4CX250B
Sent: Sunday, October 8, 2023 4:12 PM
To: Tim Duffy <>
Subject: Re: [Amps] High filament current on the 8877 amplifier tube

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!
Jim w8zr

Sent from my iPhone

> On Oct 8, 2023, at 12:48 PM, Tim Duffy <> 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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