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A question about VTTC

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hyparh

Mon Mar 04 2019, 07:03PM

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

Hello all,
Today I was offered an unused vacuum triode GS-35B for 60$. Is this one good for VTTCs, because I can't find anything about using it for that purpose?
I have a VTTC with GU-81M capable of about 45cm long arcs and I'm wondering if GS-35B can do better.

Thanks!

hyparh

Wed Mar 06 2019, 08:09AM

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

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:

Sulaiman

Wed Mar 06 2019, 09:01AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

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)

hyparh

Wed Mar 06 2019, 09:50AM

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

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 :)

teravolt

Wed Mar 06 2019, 05:34PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

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.

hyparh

Thu Mar 07 2019, 06:15AM

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

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 :)

teravolt

Thu Mar 07 2019, 02:15PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

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

MRMILSTAR

Thu Mar 07 2019, 03:57PM

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

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.

hyparh

Sun Mar 10 2019, 06:34PM

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

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.

MRMILSTAR

Sun Mar 10 2019, 07:05PM

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

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.

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