[Amps] 8877 filament.

Randall, Randy Randy.Randall at UCHealth.com
Thu May 4 14:46:44 EDT 2017


That scheme was tried in TV's and consumer radios in the 60's and 70's and called an instant on feature.  The series string filaments were run through a diode when the set was "off" and the diode shorted when the set was turned "on".  It worked as you described, but tube life was reduced due to cathode poisoning.  I wouldn't try it on such an expensive tube.  If it was a workable solution someone would offer it on an amp today as a "feature".

Randy AB9GO

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of MU 4CX250B
Sent: Thursday, May 04, 2017 2:21 PM
To: Jim Thomson <jim.thom at telus.net>
Cc: amps at contesting.com
Subject: [Amps] 8877 filament.

>  A chinese 8877 is the ticket.   Only drawback is the 3 min fil warm up.

The 8877 is a fabulous tube, but there are times when the warmup delay seems like an eternity. I wonder how much the warmup delay would be shortened if the filament was run at, say, 3 V on a 24/7 basis, and then increased to 5V when the amplifier was powered up? The power expended during the "off" times would only be about 20 W (no ext cooling required) and my instinct is that the operating life expectancy of the tube would not be lowered, and might be extended.
73,
Jim w8zr
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