The AM Forum

What would be a high filament voltage for 572b filaments. My 500 is supplying about 6.4 volts to the modulators. Is that high enough to shorten the life of the toobs???

Hi Bill,  Spec sheet says 6.0 to 6.6, you're cool.

Remember the 572B has the same heater rating as an 811A so I would think emission is not great for even a new tube. Consider the 813 with a 50 watt heater compared to a 572B with a 25 watt heater. All you get with the 572B is higher plate dissipation.

As I recall, the voltage on a thoriated tungsten filament should be +/-  5%.  Low filament voltage will shorten tube life just as much as high filament voltage.  You should never run the voltage low enough to reduce the tube's peak emission within the manufacturer's rating.

<fumbles for coffee cup> Glug glug glug... <...cackles fiendishly...>

...why I crank the sucker up until it GLOWS BLUE WHITE[/color][/b] and then it flashes like a flashbulb!    :o 

 Right before that point is when I keydown, that's the pernt of maximumm outzputz rite dere! 

yeah, that's the ticket!  ;)

           _-_-

...wait a second! I don't drink coffee - nevermind, just ferget it!   ;D

I've had a Fluke 77 for 20 years.  It's never gave me an ounce of trouble.  It's calibration has never been checked though since I took ownership.  Probably a good idea to have a cal check done on a meter especially for measuring critical voltage like filament voltages when dealing with tolerances at tenths and hundredths of a volt level. 

I had a Fluke at work years ago, what a great meter. re mark,
I'm with you, I measure the fils right at the pins and set them to spec.
my dx100 (deenex100 as tim calls it) I put a 3 amp variac in the
front panel to be able to set the voltage on the 813's in it exactly.

Good info here!

I have a soft-start circuit and variac on the filament supplies for my 833A modulators and the 845 audio driver tubes, as these tubes are getting scarce and expensive, and like all of us, I want to get as many operating hours from these tubes as possible. I pre-heat the filaments and limit the inrush current with 50% of normal filament voltage (i.e. 5 volts) for 15 seconds, and then the full filament voltage is applied (10 VAC). The variac allows me to keep the filament voltage as close as possible to the nominal rated operating voltage for the tubes.

Question; As I indicated, I soft-start at initial turn-on at 50% of the normal filament voltage. Is there a safer (read lower) inital turn-on voltage that will result in less stress to the filament structure on turn-on?? The issue as I see it, is that a lower intial turn-on voltage will stress the filament less, but the ramp-up to full voltage will be proportionally greater, and this sudden non-graceful ramp-up could stress the filament as well.

Any thoughts?

Thanks & 73,

Bruce

Mack,

That is not my question. I completely understand the requirements for correct filament voltage for operation of the tube. My question pertains only to start-up voltage on the filament, and what is the recommended voltage for initial start-up of the tube when using a soft-start turn-on. As far as I know, the transmitting tube manufacturers do not specify this information, but perhaps a member of the forum can provide some insight based upon actual experience.

As I indicated, in my rig I apply 50% of the normal operating voltage for 15 seconds before the full filament operating voltage is applied to the 833As and the 845s. To maximize the filament life during this turn-on sequence, should the initial voltage be 20%, 40%, 50%, 75%, etc. of the normal operating voltage for the tube??? If the initial turn-on voltage is too low, perhaps the filament structure can be overstressed during the sudden ramp-up to full voltage.

Thanks & 73,

Bruce