A better way to a regulated variable HT Bench supply?

I've been thinking I need to build myself a High Voltage supply for tube projects instead of having to build a supply each time I just want to test something out.

I want to make this a very versatile do it all supply that's semi-continuously variable (well, with range selecting switches) from 60-300V. I would also incorporate an LT supply into the same box but that's another story. I have a rough design worked out but the problem is I haven't seen pots that are rated for these high voltages. The basic idea I came up with is to series several, say 30V zeners, and have pots adjust inbetween the multiple 30V ranges, and have a range selector rotary switch select inbetween the series zener nodes (or perhaps I should say the pots coming off each zener junction). The only problem I can see with this plan is the pot voltage ratings... Is there a better way to go about this?

I know this goes against the DIY grain, but you might want to go look for a Heathkit bench supply on e-bay, or if you're real lucky maybe a Lambda rack-mountable variable HT supply. I found one of the latter years and years ago, and it's been my mainstay ever since.

Otherwise, I'd do the switched-zener thing, and a switched-resistor substitute for the pots, using HV-rated switches. Not infinitely adjustable, but probably close enough.

Peace,
Paul

Get one on eBay, or at a hamfest.

I got one from eBay for less than 50 bucks, shipping included.

I also got a Beckman Duostat for $10 at the MIT swapfest.

Peace,
Al.

Thanks guys.

Yeah I was thinking about looking for old heathkits, but none of the ones I've seen are as versatile as I want, they have larger than I would like gaps in the ranges.

Otherwise, I'd do the switched-zener thing, and a switched-resistor substitute for the pots, using HV-rated switches. Not infinitely adjustable, but probably close enough.
Yeah that was what I was thinking would be the only way. The only drawback is that all those switches can get pretty expensive. Ah, hell, I'm just gonna do it, I'll end up using it for the rest of my life might as well build it right.

You must be looking at the wrong Heathkits. The one I have does 0-400V at up to 100mA continuous, 125mA intermittent, plus a 0 to -100V at 1mA bias supply and a 6.3VAC/4A filament supply. Mine is model IP-32. They made other, later models that are similar.

I also have a Lambda supply with similar outputs, but much heavier-duty (I believe the high-voltage output is good for 250mA or maybe more). It weighs a freakin' ton, though. The Heathkit is light and portable.

Hey Ethan,

I scanned an article that was printed in Gl*ss Audio a few years ago, then was reprinted in a book by the author sometime later. It's a design for a test bench power supply, which is adjustable, regulated, and has built in protection circuitry. It's a pretty cool design.

I made it into a PDF which you can get here. It's just under 1MB.

There is no mention of copyright anywhere in the book, strangely, so I'm assuming it's okay to pass it around.

Enjoy!

Here's another circuit designed by John Broskie from the Tube Cad Journal. It's a fixed, regulated high voltage supply, but it can easily be converted to a variable supply by switching out different values for the bottom resistor in the feedback voltage divider. It's another really cool circuit, and it is apparently quiet enough to power an MC cartridge.

http://www.glass-ware.com/tubecircuits/High_Voltage_Regulator.html

Thanks a lot Electro!

For "those what care", I've placed a PDF of the Heath IP-32 schematic and abbreviated manual HERE. It's a 970kB file.

Awesome... thanks Dave!

- Fellow IP-32 owner

DAMN! These things are pricey these days! I think I got mine for forty bux or something...

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=32720&item=3869055337&rd=1

Another thank you to Monsieur NYD in the corner with the red trunks.

Electro, would you happen to have a date for when that Rozenblit article was published?

Hmmmm... not sure. I don't have the magazine, I just have the reprint from Rozenblit's "Beginners Guide to Tube Audio Design" book.

Ethan, if you want something quick and simple, just get a small variac to power a transformer. Use solid state rectifiers.

6.3/12.6 is a no brainer.

Minus 100 bias? who needs it.

If you want regulated, you can build the Fairchild 670 supply and put a knob on the trim pot. That is what I use.

There are also designs in RDH4 if you have it. Let me know if you want to download the power supply section.

I have to say this forum split is working just as intended.
Looks like about a 7:1 ratio. Just enough to keep the design section habitable. :thumb:

cj

[quote author="Ethan"]would you happen to have a date for when that Rozenblit article was published?[/quote]

The Rozenblit supply appears in the December 1990 issue of Glass Audio (Number 2, Volume 2).

Peace,
Al.

awwwwwwwwwwwesome.

Thanks guys.
Yeah, I have RDH4, but have yet to even read a page in it yet, I'm working on Morgan Jones' Value Amplifiers at the moment. I'll have to take a look at that, Thanks CJ.

AH! I have Rozenblit's Beginner's Guide... How quickly I forget.

Thanks everyone for all the info. Give me about a month or maybe even two, and I'll get some money together to finish a BEAST of a bench supply (It might take that long to convince the woman why this is a better investment than a payment on the house). This should be fun. If all of a sudden you don't hear from me, call 911, I may have disillusioned myself into thinking that my fingers are volt meter probes. :shock: (that was a joke in case any of you non-jokers are lurking thinking that this might be my cry for help :grin

I've just missed a faulty 800V psu on evilbay... damn!
It was faulty but with a nice variac, big transformer and a big choke...

I think the problem with this project is the transformer, or is there some reversing trick?

Frank

[quote author="electronaut"]DAMN! These things are pricey these days! I think I got mine for forty bux or something...

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=32720&item=3869055337&rd=1[/quote]

That's a ridiculous asking price. Notice that there are zero bids? There's a good reason for that!

That guy has a lot of nerve, slapping about $10 worth of new caps in the thing and then trying to charge such a jacked-up price for it. I'll bet he didn't even replace the selenium rectifiers!

I think I paid $5 or $10 for mine at a flea market. I haven't used mine in a while because it's developed a malfunction, but it should be fairly easy to fix once I finally get around to it.

Could someone kindly take a look at T5 http://www.electronaut.info/public/PowerSupply.pdf and explain to me why the Center Tap is brought out to a terminal instead of being grounded? When would I need the floating 0V center tap reference?

Also, it looks like T4 and T5 could be consolidated into one transformer with a filament winding. Is there a reason why this should not be done?
Thanks!

T4 is the filament supply for the 12AX7 in the power supply, which is biased up to some DC voltage. T5 provides the external filament voltage, so it has to be isolated unless you want any filament circuit you're powering to be biased the same as the 12AX7 inside the power supply.

[quote author="Ethan"]When would I need the floating 0V center tap reference?[/quote]

Anytime you build a tube circuit where the cathode voltage is higher than the heater-to-cathode rating (like sometimes in the "top" tube in an SRPP circuit) you have to bias the filaments up to some point within the rating. For example, a 12AX7 is rated at 200V, so if you build a circuit where the cathode sits at +225V, the filaments should be biased up about 50V or so, so the filaments aren't too far away from the cathode.