Denny wrote:
Jim, I would suggest a bit more than a minute or two.. You may want to
pop that question over on rec.radio.amateur.amps and get the group
opinion... I would suggest a 24 hour filament preheat after long
storage... There is no more effort in letting it cook 24 hours than
for 3 minutes...
I have no direct experience with the 8877 but I do speak 4cx800... The
majority of the amp builders getter new tubes with filament heat only
for up to 24 hours before applying HV.... One of the failings of hams
is that we seem to think that turning on the filament is wearing out
the tube... Whereas the broadcast industry turns the filaments on and
leaves them on because they know what wears out the tubes is cycling
the filament from cold to hot over and over...

I just finished bringing a 3CX1200D7 back to full output with a
controlled filament over voltage for 6.5 minutes with no HV applied...

cheers ... denny / k8do


Jim Barber wrote:
I have several Eimac 8877's I bought as tested new-equipment pulls
around 5 years ago. I tested them again myself at the time, then packed
them away.

Here it is 5 years or so later. The question for the panel is how long
should I "cook" the filaments before putting plate voltage on?

Thanks,

Jim, N7CXI

It's also important to bring heaters/filaments of large tubes up to
power SLOWLY!. In the old days, they'd use a variac (with a large
knob!) and slowly bring the voltage up so the heater/filament wouldn't
be shocked by the sudden inrush of current. In a dual 4-400A amp I
built years ago I just put a large resistor in series with the primary
of the filament transformer and shorted it out with a relay after 15
seconds delay. (Used a transistor and an RC circuit to control the
relay). After the the filament was on full power another circuit on the
relay removed an interlock on the relay controlling the HV power supply.
There was also a resistor in series with the plate transformer that was
shorted out by a relay (the coil of that relay in parallel with the
primary of the plate transformer). This limited the inrush current to
the filter capacitors to protect them and the rectifier diodes.
There were TWO power switches, on for filament and one for plate, plate
power could NOT be applied until the filaments had full power.

4-400A's being directly heated, come up to power rather quickly.

As for the flash over in those ceramic metal tubes, maybe some trace
radioactive elements in the ceramic has something to do with this?
If you don't believe it, bring a Geiger counter near the tube!