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Planning to build benchtop 500V tube PSU, help needed.

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duak:
Probably hard to find these days are television horizontal sweep tubes.  These are tubes optimized for switching applications - in a way like power FETs and IGBTs are optimized for low RDS,  high breakdown voltages and minimal drive and switching losses.  Radio amateurs use them in power applications where non-linearity isn't such a big deal.  There are a few articles on the tubes here:  http://www.tetrode.co.uk/sweep.html  I think one of them says that to get low plate resistance, the cathode is larger than one would expect. Scanning the vast wasteland of the 'net, I see that tube audio people that have tried them, especially the single ended Class A open loop folk, don't seem to care for them.  I wonder if the thrash guitarists would?

I would think that as long as the transfer function is monotonic, a good power supply could be designed using a sweep tube for the pass device.

T3sl4co1l:
Nonlinearity isn't such a big deal, it's mostly the 3/2 power law, typical of any tube.  Frame grid and planar types tend to be worse actually, because of cathode space-charge physics.

The relevant properties are: very high perveance (large cathode, small grid cross-section), close anode spacing (low Rp), and relatively high anode dissipation.  This gives low mu, or relatively low gm, but as the cathode-grid voltage drop is usually used for the anode resistor of the error amp, it really only affects the headroom of the error amp.

Which means if you are okay with a 250-500V supply, you probably won't need a negative supply, but if you need 100-500V say, you'll likely need a -50 to -100V supply, and then you might as well go slightly further and make it a -250V supply and go for 0-500V output range instead.  (And, mind that now you're talking a >800V range on the error amp, and you may want to use a small power tube just to handle that; like, say, 6S4 but if it had higher mu?)

Sweeps are the same way, but with the plate replaced by the screen of course (in terms of grid-anode / grid-screen spacing being similar), the grids aligned (to reduce screen current), and somewhat tighter grids to give higher Gm and mu_g2g1.  Here the peak current is as important as the high perveance.  Some other tweaks to beam tetrode structure and you get a useful saturation voltage under 100V.

Tim

David Hess:
Tektronix used tubes like the 6080 (sometimes with both triodes directly in parallel) and the 12B4 (sometimes lots in parallel).

The 6080 was apparently intended for series regulators and the 12B4 was often used for them.

H713:
I would use tubes that are cheap and available long-term. Look at plate dissipation ratings. The 6L6GC might be a good choice- lots of good power supplies have been built around them, and there's a Russian equivalent that is very cheap.

There are advantages of using tetrodes and pentodes instead of triodes for the pass elements, most notably what Pete Millett describes as a "simple, brain-damaged half-assed current limit" that can be implemented by limiting the screen voltage. The other big advantage is that big triodes are very costly in comparison. The 6AS7 was a very common pass tube back in the day, but both new production and NOS tubes are very expensive. Demand has brought down the price of new-production tubes used in guitar amps, and as such 6L6GCs in particular are pretty economical and have a 30W plate dissipation rating. BTW, for those unaware, there are multiple different tubes called the 6L6, and not all of them have the same ratings. Throw an original 6L6 into some circuits designed for the 6L6GC, 5881 or 7581 (all closely related to the original 6L6 in terms of characteristics) and there's a good chance of it red-plating or melting the screen grid (UL operation is very hard on the screen grid- the early tubes weren't designed to take that kind of abuse). Plate dissipation ratings are also lower on the old tubes.


--- Quote from: 001 on June 08, 2019, 04:59:59 pm ---
--- Quote from: H713 on June 07, 2019, 06:25:37 am ---  and a lot of the parts in that power supply are slightly odd choices and look suspiciously like they were originally specced for other equipment. 

--- End quote ---


never heard of special " benchtop 500V tube PSU" parts  :-DD :-DD :-DD :-DD

--- End quote ---

Of course there are no special parts for a bench 500V power supply, but there are parts that one would expect to use and parts that aren't. There are a lot of parts in that particular power supply that I probably would not have chosen if I were designing a bench power supply, but suspiciously turn up in other pieces of Heathkit gear. In other words, they were originally chosen and purchased for other kits.

T3sl4co1l:
Also 6BG6 (sweep version), 807 (coke bottle, RF), 1614 (metal, RF), etc.

Mind that new "6L6s" are all over the place, at least in the cheaper brands.  Everyone makes 6L6GCs, because of course they do, but they don't all handle 30W like a 6L6GC is supposed to.  Derate generously (say 30-50% below rated), treat them more like 6L6GBs if that.

Even then, the material quality is poor, so don't expect great lifetime.

That's a nice advantage to industrial and PQ types: better materials, higher ratings, longer life.  A $100 tube that lasts 10khr is a better deal than a $50 tube that lasts less than 5k -- assuming you have the budget to do so, and will be using the equipment frequently.

Or conversely, if not, it may actually be a better deal to use the cheaper part, and then maybe keep spares handy, accepting that when (not if) the originals fail, you'll have the annoyance of changing them out, but you won't be SOL for a week awaiting delivery of new parts.

One last thing.  Mind that poorly constructed tubes are more likely to melt or arc over and fail shorted.  Have some protection method in place (a crowbar?) to deal with this.  It's most likely to happen at the lowest output voltages, where the load will be most vulnerable to a surge to full B+...

Tim

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