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4-125A VTTC

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Arcstarter

Mon Jul 18 2011, 06:56PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Oh yea, you could use the MOT ground. But measuring cathode current would also include the screen current (and grid current though this is much smaller) would it not? For tubes like the GU-81M screen current can be as much as 200ma.

U4R1A

Tue Jul 19 2011, 04:51AM

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

Here is a spark vid of the vttc. I have the primary and secondary frequencies pretty close but I'm not getting the output I'd like. I'm sure after a couple weeks of tinkering I might start getting close. It will also help once I get some metering setup. I wonder how a dual 4-125A setup would perform?

Herr Zapp

Tue Jul 19 2011, 05:23AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

U4RIA -

Ah, a picture is worth a thousand words.

Your tank cap is comprised of DC-rated ceramic capacitors that are generally pretty lossy at RF. I think you'd see an immediate improvement in performance if you substituted a low-loss RF-rated mica transmitting capacitor for the string of doorknobs.

Herr Zapp

Weston

Tue Jul 19 2011, 07:32AM

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

I 2nd the idea of getting better tank caps. I was using ones similar to the red ones you have in a 572b coil. They heated up quick. You can get some Russian doorknobs for cheep. I got 2x 15kv 470pf and something like 20KVAR caps for around $30 shipped. Instead of a dual 4-125a, why not drop in a 4-400a? Its a massive tube and fits in the same socket. They are not that pricey if you check ebay often.

U4R1A

Tue Jul 19 2011, 05:18PM

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

Well after looking around a bit I found these two caps. The Aerovox is .005MFD/10Kv and the Faradon is .004MFD/2Kv so if I put these guys in series that should give me 2222pf/12Kv which is around what I need. Do you guys see anything wrong with using them in this manner?
@ Weston I have a 4-400C that I picked up awhile back but after looking at it more closely it looks like it has a couple holes in the plate around the top were it was spot welded to the bracket. Do you know if this could be a manufacture defect and how this might affect the operation?

Dr. Dark Current

Tue Jul 19 2011, 06:25PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The caps in series will indeed result in a 2222 pF cap, but the voltage rating will be only 3600 volts.

Arcstarter

Tue Jul 19 2011, 07:09PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well i used those (what looks like) exact same caps with that 811A coil of mine that you commented on. I only used one, too. It did heat a bit, but it never died. They may not be rated for AC, but i got fair efficiency it seems.

Why use a bigger tube to get bigger sparks when you can use a little tube and just tune for better efficiency? Tuning requires changing many different factors. If you change one aspect of the coil, all of the variables can change. For example, if you add more topload, you might have to re-tune the primary for impedance matching, then change the tank capacitance to get the right primary resonant frequency, and the feedback could change (which shouldn't make much of a difference just with a different topload), and grid leak values could need changing. It is rather hard to tune everything perfectly, but you can at least try to get everything close. It would be best if you had some big variable capacitor in parallel with the tank cap and variable resistors/capacitors everywhere else, but this adds a lot of cost so is impractical.

If you do not have proper feedback, your tube may not be turning all the way off, so while the current through it may change, it could be mostly on burning up alot of energy. The voltage on the control grid should swing sharply negative, with some positive bias to help turn the tube on hard. This video shows what the voltage on the grid should look like.

At about 5:30 he shows the grid on the scope.

Weston

Wed Jul 20 2011, 01:27AM

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

Do you have a picture of the 4-400c with holes on the top of the plate? If they are symmetrically, it is most likely built that way. Even if melted, it should still work to some extent if the grids are not melted.

Herr Zapp

Wed Jul 20 2011, 01:41AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

U4RIA -

I think the "holes" you are seeing at the top of the plate structure are just where the cylindrical sheetmetal was formed inward and welded to the plate lead-in wire.

When a plate get a hole melted in it, its generally near the center of the plate structure.

Here's a picture of Ross O's 3-1000 triode in his large VTTC, after burning a hole in the plate (spot of light near center of plate):

Herr Zapp

U4R1A

Wed Jul 20 2011, 03:51AM

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

The holes are symmetrical and at the top so I believe you are correct Herr Zapp. I'm pretty glad because its been setting on my shelf for a ~year just waiting to hop into some Tesla contraption. I noticed a large improvement once I installed a bypass cap on the screen. I assume there was some parasitic oscillations going on. Would it ever be necessary to put one on the grid?

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