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A question about VTTC

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hyparh

Sun Mar 17 2019, 05:25PM

Registered Member #1054 Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29

Thanks for the information MRMILSTAR! Today I tested my PC PSUs and it seems that only one of them is suitable for the job. The rest of them give voltages under 12V or a bit higher (up to 12,2V), but not enough.
I measured 12,62V from the last one - right on spot :)
Do I have to soft-start the filament every time or it must be done only the first time I power up the tube?

MRMILSTAR

Wed Mar 20 2019, 03:53PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

The soft-start procedure should be done each time. You are trying to avoid the high in-rush current to the filament while it is cold. As the temperature of the tungsten filament increases, its resistance increases. After the filament heats up, the current reduces to its normal spec value. Using the 833A vacuum tube as an example, the normal operating specification for the filament is 10 amps at 10 volts, but at start-up, the in-rush current can be as high as 40 amps if the full 10 volts is applied.

An analogy is the incandescent light bulb. Have you ever noticed that they always fail when you switch them on? This is because of the high in-rush current applied to the filament at turn-on. If the voltage were applied gradually, the bulb would last much longer.

Proud Mary

Thu Mar 21 2019, 02:13AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Where a device is only to be used for a few minutes at a time, I find it often convenient to use lead acid or NiCad cells as a floating power supply for thermionic valve or X-ray tube heaters and filaments.

hyparh

Thu Mar 21 2019, 01:44PM

Registered Member #1054 Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29

MRMILSTAR wrote ...

An analogy is the incandescent light bulb. Have you ever noticed that they always fail when you switch them on? This is because of the high in-rush current applied to the filament at turn-on. If the voltage were applied gradually, the bulb would last much longer.

This is indeed the case, I didn't think of it this way till now... Thanks!

Proud Mary wrote ...

Where a device is only to be used for a few minutes at a time, I find it often convenient to use lead acid or NiCad cells as a floating power supply for thermionic valve or X-ray tube heaters and filaments.

That's an interesting idea to think about :)

hyparh

Thu Mar 21 2019, 09:49PM

Registered Member #1054 Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29

I have one more question about the cooling of the tube. Is it enough to use two PC fans (I have a lot of them) - one for the radiator and one for the tube itself? I also have a microwave oven fan, but do I need it for that purpose?

MRMILSTAR

Fri Mar 22 2019, 03:25AM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

I'm not familiar with that particular tube. For my 833A tube I use a 5" muffin fan mounted high enough so that it blows across the anode caps on top of the tube. Your spec sheet may specify what type of cooling is needed.

hyparh

Fri Mar 22 2019, 03:31PM

Registered Member #1054 Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29

Here are some values from the spec sheet:

Filament voltage: 11,9-13,3V

The next ones are "no more thanâ€¦"

Anode voltage: 3000V
Transitory anode voltage: 6000V
Anode power dissipation using air cooling: 1500W
Anode power dissipation using water cooling: 2000W
Anode temperature: 200 degrees (Celsius)
Grid connection temperature: 120 degrees
Cathode connection temperature: 120 degrees

I hope the translation from russian is good enough :)

This site also provides useful information about the tube:

From the first table below: "Minimum required Air flow: 2500 l/min (83.3 cfm)"
I donâ€™t know if a PC fan can achieve air flow of 2500l/minâ€¦

MRMILSTAR

Fri Mar 22 2019, 06:04PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

That will require a stout fan, a lot more than a PC fan. That's one of the problems with high power tubes. You have to find a way to cool them. Cheap solutions might be an old vacuum cleaner motor or hair dryer with the heating element disabled. The fan will be noisy though which is something you have to put up with.

hyparh

Sat Mar 23 2019, 09:36AM

Registered Member #1054 Joined: Wed Oct 10 2007, 10:51PM
Location: Bulgaria
Posts: 29

I suppose I'll go with the hair dryer :D Thank you!
Looking at the specs for anode voltage (3000V)... Is it a problem to use a level shifter after the MOT? As far as I know a tube in a VTTC will perform better with level shifter (although in this case it exceeds the nominal anode voltage).

MRMILSTAR

Sat Mar 23 2019, 02:06PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

My 833A tube has 2 specifications for maximum plate (anode) voltage. The spec for convection cooling is 3000 volts while the spec for forced air cooling is 4000 volts. This indicates to me that the maximum anode voltage is mostly a question of heat dissipation. I use 4000 volts for my 833A and sometimes up to 4500 volts with no problem. Your spec sheet mentions a "Transitory Anode Voltage" of 6000 volts. This sounds like your tube can safely handle a 6000 volt anode voltage for short periods of time. My guess would be that your tube could safely take 4000 volts on the anode with forced-air cooling especially since you won't be operating it for long periods of time. A MOT with a voltage doubler will put out about 4000 volts, possibly a little less. If you have access to a variac, you could vary the voltage into the MOT to get any anode voltage that you desire. That is what most VTTCs use. I have a variac on mine.

I saw one hair dryer with a 1900 watt rating that puts out 41 cfm. That power rating includes the heater. Having seen this, maybe a hair dryer isn't enough since your spec sheet says 83 cfm. Two hair dryers? Another cheap solution for cooling is a small shop vac operating as a sucker through the tube cooling fins. The ingested hot air might damage the shop vac motor though since it relies on the intake air flow to cool the motor. I bought a very small shop vac for $10 on sale. Very noisy though. The salvaged vacuum cleaner motor is also a good idea. I see old vacuum cleaners being discarded a lot. The motors almost always work. Of coarse you can still use the single hair dryer if the run-time is short enough to avoid over-heating. I can't say what length of time is "short enough". It sounds like the best cheap option that satisfies your 82 cfm spec is to use a salvaged vacuum cleaner motor to blow air into the tube cooling fins unless you like the idea of using two hair dryers.

I should tell you that I am not an expert on VTTCs in general. I am only familiar with the one that I have built based on a 833A vacuum tube. If there are others on this board with more VTTC experience maybe they can comment on the anode voltage question for your particular tube. You can see some pictures of my VTTC at this link.

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