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Registered Member #4953
Joined: Sat May 19 2012, 11:56PM
Location: Australia
Posts: 12
Hey Guys,
I got started building a dual 811a VTTC, but after some delivery issues with the original tubes I'm now wondering if I should splash the extra cash on a pair of 572Bs instead because they are rated for a higher plate voltage.
I was just wondering though, if I am going to use staccato and overdrive my 811A tubes anyway, will there be any noticable benefit with the 572B's? Like can I get longer arcs, or will they just allow a larger duty cycle without burning out?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well they have the same heater so will push about the same amount of current. As for the voltage, direct heated tubes can usually stand a lot of (over)voltage at frequencies much below the maximum ratings. What confuses me though is the plate dissipation rating, which is 4 times as much on the 572B than on the 811A but the tubes have similar sized envelopes. The 572B will surely need forced air cooling if you want to reach the maximum plate dissipation. If the price on the 572B is right, I would go for it, but if theres a big difference I would keep the 811A's.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Steve Conner wrote ...
I believe the 572B has a heavy graphite plate that allows higher dissipation, for short periods at least.
If I remember correct, the 811A should only be run with cold plates, no red spots at all. The 572B with its graphite plate can run red hot. But bear in mind that the relation between money spent on 811A vs 572B will not give you the same relation in improved spark output.
If you like to experiment on the edges of what your VTTC can do, 572B is a sure bet not to loose a tube.
Registered Member #187
Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140
I struggled with that same question when I was constructing my VTTC. I ended up getting the 811As because of the cost, and I had to heat up my MOT on a hot plate and pull out a bunch of secondary windings (the varnish gets really soft when it's hot). I probably could have run the 1,200 (or so) VAC on the plates, but I didn't think the tubes would have been able to handle running full rectified and filtered, which would have been 1,600+ Volts. If had gone with the 572Bs I think I could have gotten away with using the MOT in its original state.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
As for any Tesla coil, I really do advice on getting a variac as fast as possible, best tool for testing, avoiding explosions, meltdowns and you can use components rated for something else than mains or if the outputs does not fit your application.
Registered Member #1316
Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
Mads Barnkob wrote ...
Steve Conner wrote ...
I believe the 572B has a heavy graphite plate that allows higher dissipation, for short periods at least.
If I remember correct, the 811A should only be run with cold plates, no red spots at all. The 572B with its graphite plate can run red hot. But bear in mind that the relation between money spent on 811A vs 572B will not give you the same relation in improved spark output.
If you like to experiment on the edges of what your VTTC can do, 572B is a sure bet not to loose a tube.
The inverse is true. The 811a is rated to run red hot. Any tube with a graphite plate should never reach red heat.
I think the 572b costs close to twice as much as a 811a. If you plan to get 2 572b tubes, you might as well buy a single larger tube such as a 833 or a gu-81m.
Registered Member #4953
Joined: Sat May 19 2012, 11:56PM
Location: Australia
Posts: 12
Mads Barnkob wrote ...
But bear in mind that the relation between money spent on 811A vs 572B will not give you the same relation in improved spark output.
That was my initial opinion when I was underestimating the overall cost of the project. I'm looking at $45 for two 811A's or $90 for two 572B's delivered (both Shuguang brand). At first I thought "double the price? forget it!" but then when you tally up the sockets, caps and other components in the coil, the extra $45 is acceptable IF it will give better results.
Mads Barnkob wrote ...
As for any Tesla coil, I really do advice on getting a variac as fast as possible
Good advice. I have a 1KVA variac on its way from China right now. I hope it will suffice.
Weston wrote ...
If you plan to get 2 572b tubes, you might as well buy a single larger tube such as a 833 or a gu-81m.
Unfortunately I've already built a nice cabinet to suit the 811A/572B size tubes and splashed out on all the mounting hardware.
If I started again I would probably go for a bigger tube but my issue with the 833A's was their price. I can't get them to Australia for < $160. And I was a bit afraid to go with the complexity of a tetrode for my first VTTC so I ruled out the GU-81Ms despite their good price.
My main goal is good arc lengths for visual factor. If I can match kaizer's results of ~30cm I'll be thrilled, and if paying an extra $45 on the 572Bs will allow more voltage and add a few cm I'll consider the money well spent. But if they only increase longevity, reliability, or power dissipation without giving better arcs then I'd rather save my cash if you know what I mean?
Oh hey, I just realised Mads Barnkob made the Kaiser VTTC I'm copying :)
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The graphite plate is usually really rated to run red hot. I was testing tubes for their rated dissipation. Take for example the SRS 457 beam tetrode , at its 500 wats the bottom of the plate gets more orange than red. The plate of the GU-81M gets red at 450 watts, I didn't test the 600 watt short term dissipation but it could probably get orange too. On the little RE125C (equiv QB3/300) the plate gets slightly red at the rated dissipation.
It is metal plate tubes which usually aren't rated to run red, especially the smaller tubes (such as TV sweep tubes) must not get red. A red plate on those means a heavy overload.
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811As or 572Bs for VTTC?
