Re: A question about VTTC
hyparh, Wed Mar 06 2019, 08:09AM

No one knows if GS-35B can be used for VTTC? Come on guys, I have few days to decide buying or not this brand new triode for only 60$ or it will be sold to someone else... :(
I just want to know if I can make a VTTC with it. Pls!

The triode looks like this:

Re: A question about VTTC
Sulaiman, Wed Mar 06 2019, 09:01AM

It is a triode, rated for 1500 W anode dissipation and it has gain up to GHz therefore it will work in a vttc.

(using GHz devices at vttc frequencies the one extra thing to be aware of is the possibility of unintentional (and un-'scopable) very high frequency oscillations causing overheating)

Re: A question about VTTC
hyparh, Wed Mar 06 2019, 09:50AM

Thank you, mate! This is very useful information about possible VHF oscillations and overheating, I'll have it in mind.
I think I'll take it :)

Re: A question about VTTC
teravolt, Wed Mar 06 2019, 05:34PM

you will need a lot of support equipment like filiment transformers maby a socket witch will cost a lot more than 60$. I would use a gu-5b.

Re: A question about VTTC
hyparh, Thu Mar 07 2019, 06:15AM

Hi teravolt,
Yeah, GU-5B definitely is the better choise, but I think it won't fit on my budget right now... At the moment 60$ is the max I can affort and I think it is worthy to take the GS-35B.
I already have filament transformer (modified MOT) for my VTTC with GU-81M. I'm a bit surprised to find out that GS-35B filament requires much less current than GU-81M. Only 2,95A. I'm looking at the specs of GU-5B now and it requires much higher 20-27A.
As for the socket, I think I can use copper strips (cut from copper sheet) and wrap the triode's cylindrical leads with them. Then I won't need a socket :)

Re: A question about VTTC
teravolt, Thu Mar 07 2019, 02:15PM

I see some of these russain videos make HF tesla coil candles out of those type of UHF tubes. I cant waight to see the results. VTTCs are a dying art after the addvent of the QCW

Re: A question about VTTC
MRMILSTAR, Thu Mar 07 2019, 03:57PM

A re-wound MOT is a cheap way to implement a filament transformer. One thing to keep in mind though is the inefficiency. MOTs are built cheap. They have barely enough iron and copper to function properly. Even then, they run hot. You will find that a re-wound MOT will draw about 500 watts even when it is not connected to a load. Add the load power to that to get the full power consumption of a re-wound MOT.

For my 833A VTTC, I initially used a re-wound MOT for the filament transformer. It worked, but since the 833A was putting out about 2 KW by itself, that extra 600 watts (500 waste + 100 load) was putting me very close to tripping the breaker on the 120 volt circuit. For that reason I bought an actual Thordarson 10 volt filament transformer which is very efficient. I combined that with a small variac and volt meter to allow me to bring up the filament power slowly to extend the life of the tube.

I knew that MOTs were inefficient but I didn't realize how bad they were until I tested a few. If you don't mind the loss, they still are a cheap solution.

VTTCs were never about state-of-the-art technology. After all, VTTCs have been around since the 1930s. While QCW coils may operate in a similar fashion as VTTCs, they do not have the awesome look or presence of a VTTC with a large glowing vacuum tube. VTTCs with big tubes have sort of a steam punk look and are a perfect example of what yesterday's technology can still do.

Re: A question about VTTC
hyparh, Sun Mar 10 2019, 06:34PM

I took the triode :)
MRMILSTAR, you're right - MOTs really are very inefficient. Actually... can I use computer PSU for the filament? I have few PSUs from old PCs. It will provide stabilized 12V and the current will be more than sufficient for this tube. Stabilization is very important, because the voltage from our power grid often fluctuates significantly which will complicate the things by using ordinary transformer for the filament.

Re: A question about VTTC
MRMILSTAR, Sun Mar 10 2019, 07:05PM

I see no reason why you can't use a computer PS as long as it is capable of supplying the necessary filament current. Also insure that the voltage, under load, is very close to the filament supply spec for your particular tube. Some tubes are quite sensitive to the filament voltage being within a few tenths of a volt to get the best performance. Consult your spec sheet. You don't want to run the filament below its rated voltage. That can damage some tubes. If needed, you can run the filament voltage a little higher than normal. I wouldn't exceed the filament voltage spec by more than 0.5 volt.

If your computer PS is adequate for powering the filament there is another way, other than a variac, to provide for a soft-start filament warm-up to maximize your tube life. Just use a voltage-dropping power resistor that you can switch in and out during warm-up. Pick a resistance that will cause the filament to just barely glow. Then after a few seconds, bypass it for the full filament voltage. Not quite as good as a variac but much better than just hitting the filament with full voltage during warm-up.

Re: A question about VTTC
hyparh, Sun Mar 17 2019, 05:25PM

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?

Re: A question about VTTC
MRMILSTAR, Wed Mar 20 2019, 03:53PM

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.

Re: A question about VTTC
Proud Mary, Thu Mar 21 2019, 02:13AM

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.

Re: A question about VTTC
hyparh, Thu Mar 21 2019, 01:44PM


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


That's an interesting idea to think about :)

Re: A question about VTTC
hyparh, Thu Mar 21 2019, 09:49PM

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?

Re: A question about VTTC
MRMILSTAR, Fri Mar 22 2019, 03:25AM

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.

Re: A question about VTTC
hyparh, Fri Mar 22 2019, 03:31PM

Here are some values from the spec sheet:

Filament voltage: 11,9-13,3V

The next ones are "no more than…"

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 don’t know if a PC fan can achieve air flow of 2500l/min…

Re: A question about VTTC
MRMILSTAR, Fri Mar 22 2019, 06:04PM

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.

Re: A question about VTTC
hyparh, Sat Mar 23 2019, 09:36AM

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).

Re: A question about VTTC
MRMILSTAR, Sat Mar 23 2019, 02:06PM

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.

Re: A question about VTTC
hyparh, Sun Mar 24 2019, 01:08PM

Thanks for the analysis! Yep, I have one 5KVA variac - very handy for different experiments.

I think I found a very good solution for the cooling (price, dimensions etc.):
"Air Volume: 190.30CFM"

PS. That's one neat VTTC you've got there! :) (though it gives me an error when I try to open the images)

Re: A question about VTTC
Sulaiman, Sun Mar 24 2019, 01:49PM

Just be aware that the fan that you pointed to is 4-wire ... external motor drive/controller required. (cheap via eBay)

Fans of this type are reliable and designed for various ready made ducting options.
An axial fan should suffice, bigger = less noisy, e.g.120mm should be ok.

From memory: MOTs become more efficient at 10% to 20% reduced operating voltage.

Re: A question about VTTC
MRMILSTAR, Sun Mar 24 2019, 03:43PM

The image feature has been broken for a while on 4hv.org. Hopefully it will soon be fixed.

Re: A question about VTTC
hyparh, Sun Mar 24 2019, 10:02PM



Isn't it like a PC fan? I mean, no matter how many wires they have, as soon as you figure out the two "main" ones, you can run it at full speed? I did this with many PC fans.

Re: A question about VTTC
The Lightning Stalker, Mon Mar 25 2019, 12:17AM

Some microwave ovens have instead a centrifugal blower that flows more air than a standard rotary.