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304TL VTTC

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U4R1A

Tue Feb 15 2011, 02:19AM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

Well this is my latest VTTC I've been working on. I just got it lashed together and got first light. Now for the fun part of tuning and trying to think of a theme for the final finished product. A question I've been meaning to ask is about the different classes of capacitors. For my tank capacitor I went with (2) TDK 400pf@30Kv (class 1) in parallel. For RF type frequency's I know mica is the king but if you don't have mica caps on hand wouldn't you want to go with class 1 caps since they seem to maintain steady capacitance over a wide array of temps? I may be thinking of this wrong but I see a lot of VTTC builds with UHV-TDK caps that are class 2 for the tank and the class 1 caps for the RF bypasses. Let me know if I'm going wrong and here is a link to my Semi-thrown together 304TL VTTC.

HazzWold 1993

Tue Feb 15 2011, 02:35AM

Registered Member #2563 Joined: Mon Dec 21 2009, 10:17AM
Location: Australia
Posts: 280

Very nice, what sort of tube is that you have the sparks shooting out of?

U4R1A

Tue Feb 15 2011, 02:41AM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

I bought several 340tl's off of ebay and of course since they are old and weren't packed well they all died(broken filaments) but one. So that is one of the broken ones on top. The interesting thing is that they don't all glow blue like that. I think in my final setup I'm going to leave that tube as the topload.

Weston

Tue Feb 15 2011, 03:47AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Thats a nice looking VTTC. Seeing the cheep prices on eBay for the 304tl, what was the failure rate exactly for the ones that you bought? The one on the top of the coil is gassy.

Do you have a staccato controller on the coil? On a few parts of the video it sounded like the break rate changed.

U4R1A

Tue Feb 15 2011, 04:00AM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

I bought a batch of 5 and only 1 survived. Which is a pretty crappy survival rate and the four that died all had broken filaments. Steve wards VTTC faq sheet pretty much says that all of the old tubes like the 304tl's have very brittle filaments that seem pretty easy to damage during shipping. I don't have a staccato on the coil yet. That's one part of the VTTC that I need to do more research on or break down and buy one of EVR's kits. The break rate did seem to change but all i was changing was the voltage applied via a variac. I'm sure one of the more experienced tubers have an answer to this.

Coronafix

Tue Feb 15 2011, 05:02AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Nice work. I see why Dr Spark loves tubes so much, they have a phat sound.

Weston

Tue Feb 15 2011, 05:19AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

What are your grid leak network values? You could be running close to cutoff, causing "sputter mode" operation.

As for the 304tl filiments being fragile, I think it might just be filament design or materials. I have tubes the same age as or only a few years younger. The two smaller tubes in my profile picture are proubaly from the 50's. I think it might be that the 304tl has a helical filament more prone to vibrations. Or possibly they put some more research into better tungsten filaments in power tubes before developing slightly more modern tubes.

Mads Barnkob

Tue Feb 15 2011, 08:11AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Great job on the vttc, it is a lovely topology to experiment with :)

I am a little worried about sending such a high voltage through a vacuum. Would it generate x-rays?

U4R1A

Tue Feb 15 2011, 03:27PM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

@ Weston- My gridleak values right now is 400pf/7.9KOhm but it seems to operate pretty much the same from 4KOhm-8KOhm. I picked up a few other tubes(3-200A3 Eimac, 4-400C Penta, 4-125A Eimac) in case the old 304tl gives out. I think I going to try a tetrode VTTC next.

@ Mads- I thought about X-ray generation and without some form of testing I wont really know. I'll probably make a post over in the radiation forum to see what those guys think. I hope not because I like that 304tl as a topload.

U4R1A

Mon Mar 07 2011, 10:22PM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

Here is a couple of updated pics of the 304tl coming together. The only real note worthy design feature is that the variac is hid in the black PVC that supports the upper section.

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