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

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
I don't think that the tubes will go bad just sitting on the shelf for 5
years, they should work the same as the last time they were used.
Having said that, it is always a good idea to wait until the cathode
reaches full emission before applying plate voltage and rf drive.

I would think that these tubes reach that point within a minute of
applying filament power. You could apply filament power to the tube on
the work bench and monitor the tube temperature. With the tube out of
the socket and heater power applied via use of heavy clip leads you can
feel the base of the tube with your hand and see how long it takes to be
able to feel the cathode heat reach the outside of the bottle. (It
won't be hot, just luke warm).

Thanks.

I wasn't concerned about them going bad per se, just tiny bits of gas
released from the inside not getting 'getted' and causing a flashover
when plate voltage is first applied after long-term storage.

It seems that any time I fire up an Eimac ceramic triode after it's been
sitting a long time it flashes over once just to spite me and stress
the supply. Maybe it's just bad luck or other operating practice on my part.

Jim, N7CXI


ken scharf wrote:
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
I don't think that the tubes will go bad just sitting on the shelf for 5
years, they should work the same as the last time they were used.
Having said that, it is always a good idea to wait until the cathode
reaches full emission before applying plate voltage and rf drive.

I would think that these tubes reach that point within a minute of
applying filament power. You could apply filament power to the tube on
the work bench and monitor the tube temperature. With the tube out of
the socket and heater power applied via use of heavy clip leads you can
feel the base of the tube with your hand and see how long it takes to be
able to feel the cathode heat reach the outside of the bottle. (It
won't be hot, just luke warm).

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

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!

ken scharf wrote:

As for the flash over in those ceramic metal tubes, maybe some trace
radioactive elements in the ceramic has something to do with this?

No, it's just the result of very slow outgassing from the metal and
ceramics.

To handle this problem, all vacuum tubes are constructed with some kind
of 'getter', a chemically activated surface that combines with stray gas
molecules and keeps the vacuum clean. In small glass tubes, the getter
is the silvery metal film (usually barium) deposited on the glass, but
such metals are too volatile for use in transmitting tubes.

The getter used in transmitting tubes is a metal such as zirconium or
tantalum, which needs to be hot in order to function. In large glass
tubes the main getter is typically the dull grey coating on the outside
of the anode, which is intended to run very hot. In addition, there are
often secondary getters in cooler locations such as the grid and
heater/filament structures. However, the anode of a metal/ceramic tube
is always quite cool, so the main getter has to be in the hottest place
available, typically on top of the cathode post.

All this means that transmitting rubes need to be *used* in order to
keep the vacuum clean. Tubes that haven't been used for a while may need
to be fired up with heater/filament only, to give the getter a chance to
function before high voltage is applied. Cooling is also required, of
course. How long this takes will depend on the individual tube, but 24
hours is usually enough to reduce the risk of high-voltage flashover.


Just to be clear, a leaky seal is a completely different problem.
Outgassing is typically worst with brand-new tubes, but will eventually
settle down. In contrast, a leaky seal is continuous. Although the
getter may be able to handle a very tiny leak for a while, the leak will
eventually win.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Thanks, Ian. That's a very clear and concise description of the issue.

This may be overkill, but since it costs nothing I'm thinking of four
six-hour cycles, allowing the tube to cool to room temp or so between
cycles.

Since you're here, Ian, I'll slip in an unrelated question. What do you
think about using regulated DC for the heater on an 8877 ? I ask because
I'm thinking of using a couple of spare controller I/O pins and a
home-made R2R network to ramp up a series-pass regulator as part of the
overall controller startup.

On the other hand, I suppose a relay and appropriate power resistors
would be simpler... ;-)

Thanks,
Jim, N7CXI

Ian White GM3SEK wrote:
ken scharf wrote:

As for the flash over in those ceramic metal tubes, maybe some trace
radioactive elements in the ceramic has something to do with this?

No, it's just the result of very slow outgassing from the metal and
ceramics.

To handle this problem, all vacuum tubes are constructed with some kind
of 'getter', a chemically activated surface that combines with stray gas
molecules and keeps the vacuum clean. In small glass tubes, the getter
is the silvery metal film (usually barium) deposited on the glass, but
such metals are too volatile for use in transmitting tubes.

The getter used in transmitting tubes is a metal such as zirconium or
tantalum, which needs to be hot in order to function. In large glass
tubes the main getter is typically the dull grey coating on the outside
of the anode, which is intended to run very hot. In addition, there are
often secondary getters in cooler locations such as the grid and
heater/filament structures. However, the anode of a metal/ceramic tube
is always quite cool, so the main getter has to be in the hottest place
available, typically on top of the cathode post.

All this means that transmitting rubes need to be *used* in order to
keep the vacuum clean. Tubes that haven't been used for a while may need
to be fired up with heater/filament only, to give the getter a chance to
function before high voltage is applied. Cooling is also required, of
course. How long this takes will depend on the individual tube, but 24
hours is usually enough to reduce the risk of high-voltage flashover.


Just to be clear, a leaky seal is a completely different problem.
Outgassing is typically worst with brand-new tubes, but will eventually
settle down. In contrast, a leaky seal is continuous. Although the
getter may be able to handle a very tiny leak for a while, the leak will
eventually win.

Jim Barber wrote:
Thanks, Ian. That's a very clear and concise description of the issue.

This may be overkill, but since it costs nothing I'm thinking of four
six-hour cycles, allowing the tube to cool to room temp or so between
cycles.

What would be the reason for dividing the 24 hours into four parts
(bearing in mind that the 24h is already little more than a guess)? Also
note that the tube must be cooled anyway, to avoid overheating of the
base seal. 5V at 10.5A is a lot of heat input, and all of it must be
removed somehow.


Since you're here, Ian, I'll slip in an unrelated question. What do you
think about using regulated DC for the heater on an 8877 ? I ask
because I'm thinking of using a couple of spare controller I/O pins and
a home-made R2R network to ramp up a series-pass regulator as part of
the overall controller startup.

On the other hand, I suppose a relay and appropriate power resistors
would be simpler... ;-)

There are two separate issues the the need for a regulated heater
supply; and the need for a slow ramped startup.

The need for a regulated heater supply depends on the regulation of your
mains voltage. Eimac specify +/- 5%, so if your mains regulation is
worse than that, a separately regulated supply could be worthwhile.
Mostly you need to prevent the heater voltage dropping too low, because
electron emission from a chilled cathode may not be able to deal with
the peak anode current.

If you ever plan to use the amplifier from a Field Day generator, then
the answer changes to a definite "yes". Otherwise you can get a nasty
scenario where you switch to TX, the increased demand pulls the
generator voltage down, and the heater voltage along with it... and then
you're trying to pull peak anode current from a chilled cathode. With
two or three stations sharing the same generator, there will frequently
be a "double low" or "triple low" when stations are transmitting at the
same time. You can't do much about the effects on anode voltages in this
FD situation, but you can prevent unnecessary stress on the cathodes by
regulating the heater voltages.

As it happens, the 8877 has a 5.0V heater, and low-cost 5V switch-mode
supplies are very readily available. A PC power supply would do, but is
much larger than needed for an 8877 (10.5A). Nominal 5V power supplies
can generally be modified for other voltages in the 5-6V region by
jimmying the master voltage divider. Switching noise should not be an
issue if you choose a supply that is shielded, and filtered at both
input and output.

But this raises an issue related to your second question. It is not
normally necessary to avoid prevent inrush current surges with
*indirectly* heated tubes such as the 8877... BUT... a voltage-regulated
heater supply MUST be modified to prevent it from driving a very large
current into the low resistance of a cold heater at startup. I haven't
tried this personally, but some people have created a ramped startup by
simply adding a large electrolytic capacitor across the right half of
the master voltage divider (you can check this using a 6V light bulb).


By the way, I learned a lot of this stuff by reading the mail on the
AMPS list over the years
(http://lists.contesting.com/mailman/listinfo/Amps). There are also
searchable archives.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:
ken scharf wrote:

As for the flash over in those ceramic metal tubes, maybe some trace
radioactive elements in the ceramic has something to do with this?

No, it's just the result of very slow outgassing from the metal and
ceramics.

To handle this problem, all vacuum tubes are constructed with some kind
of 'getter', a chemically activated surface that combines with stray gas
molecules and keeps the vacuum clean. In small glass tubes, the getter
is the silvery metal film (usually barium) deposited on the glass, but
such metals are too volatile for use in transmitting tubes.

The getter used in transmitting tubes is a metal such as zirconium or
tantalum, which needs to be hot in order to function. In large glass
tubes the main getter is typically the dull grey coating on the outside
of the anode, which is intended to run very hot. In addition, there are
often secondary getters in cooler locations such as the grid and
heater/filament structures. However, the anode of a metal/ceramic tube
is always quite cool, so the main getter has to be in the hottest place
available, typically on top of the cathode post.

All this means that transmitting rubes need to be *used* in order to
keep the vacuum clean. Tubes that haven't been used for a while may need
to be fired up with heater/filament only, to give the getter a chance to
function before high voltage is applied. Cooling is also required, of
course. How long this takes will depend on the individual tube, but 24
hours is usually enough to reduce the risk of high-voltage flashover.


Just to be clear, a leaky seal is a completely different problem.
Outgassing is typically worst with brand-new tubes, but will eventually
settle down. In contrast, a leaky seal is continuous. Although the
getter may be able to handle a very tiny leak for a while, the leak will
eventually win.


So if you use your linear often you only need to heat treat the tubes
once, on installation.