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Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Herein my ignorance of how tubes work will probably be implied, but oh well...
I see a few VTTC schematics where the filament supply is switched separately to allow the filament to be warmed up prior to the application of HV to the plate. I also see plenty of schematics where no such switch is used, and (presumably) the filament and plate supplies are switched simultaneously. Which is "right"? Is there a general rule? Why does it matter if the cathode is cold when the plate voltage comes up? Doesn't a cold cathod just mean the tube is "off"? "off" seems like a safe thing to me, but obviously I'm missing something
In case anybody wonders to what these questions tend, I just bought a 3-500ZG at a swap meet and want to build a VTTC around it. Yeah, I know... too many coil projects going in parallel... I'll never get anything done!!
Registered Member #494
Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31
J. Aaron Holmes wrote ...
Herein my ignorance of how tubes work will probably be implied, but oh well...
I see a few VTTC schematics where the filament supply is switched separately to allow the filament to be warmed up prior to the application of HV to the plate. I also see plenty of schematics where no such switch is used, and (presumably) the filament and plate supplies are switched simultaneously. Which is "right"? Is there a general rule? Why does it matter if the cathode is cold when the plate voltage comes up? Doesn't a cold cathod just mean the tube is "off"? "off" seems like a safe thing to me, but obviously I'm missing something
In case anybody wonders to what these questions tend, I just bought a 3-500ZG at a swap meet and want to build a VTTC around it. Yeah, I know... too many coil projects going in parallel... I'll never get anything done!!
Regards, Aaron, N7OE
put the filament allways on, on my GU-81m VTTC is powered off a separate supply. your tube looks like to be pretty good, I would put also a soft start on it, maybe with a 1-2 Ohms power resistor in series with the filament that is shorted after 3 seconds by a relay, it would make your filament much more happy and you will not hear the "thunnnng" from the filament transformer at every tube start.
when I work for much time with the VTTC, I just leave the filament plugged in, and then when its needed put on the anodic voltage, this makes much longer the life of the tube.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Thanks guys! Ok, so the idea is that warming up the tubes first keeps the filaments healthy. That's probably enough to go on! Time to find a couple of those mythical "inexpensive" variacs, I think
Christopher: I found your site a few days ago while searching for VTTC info (just after buying this tube). Your coils look awfully spiffy!!! I'll challenge myself to produce something nice-looking, but I think I'll probably stick with wood and PVC. Every time I try to work with acrylic or polycarbonate, I end up wishing I hadn't bothered. Practice makes perfect, I know, but "practice" with plastics is just too darn expensive!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
When I'm not playing with HV, I like to play with tube audio amps. The tubes in these are a lot smaller than the ones you guys use in your VTTCs, but the same principle applies. You're always supposed to turn the filament power on first, and let the filaments reach operating temperature, before applying the plate voltage.
The reason you usually hear is because of a thing called "cathode stripping". The amount of electrons that a filament can give off depends on its temperature. If it's not hot enough to supply the full quantity of electrons needed for operation of the tube, it gets attacked by ion bombardment as the electric field tries to just rip the electrons it wants off anyway.
This is mainly a problem with oxide-coated filaments and heater cathodes. The thoriated tungsten filaments are tougher and will take some abuse. You know you have one of these if it runs with the filament shining like a light bulb, instead of the dull red glow of most tubes.
I actually like how I have to turn the heaters on, then go and wait 2 minutes before firing up the HT. It gives me a nice feeling of retro-ness.
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VTTC query: Filament startup?
