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Old 14th Mar 2014, 5:58 pm   #21
Radio Wrangler
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Default Re: HT Supply (Test Set)

Let's see...

I wouldn't recommend a high voltage PSU with valves because the types best suited all have a zillion audionutters chasing them, offering stupid money. So this way isn't going to be cheap. Besides it's all been done before and that reduces the fun.

Doing a switcher is more interesting. The issue comes down to making something which works, which is robust enough to do the job and not occasionally destroy whatever you're powering with it, or set the house on fire, or electrocute someone. Some people go overboard on safety, but it's something you do need enough of. Too much is a bind, too little is a disaster.

The problem with PC power supplies is that they are very precisely targeted at devices where a CPU takes a hundred amps at a bit below 1 volt, and there are other higher voltage rails at much lower power. This is reflected in the turns and gauges (and foil thicknesses) of the windings on the transformer.

The mains input mesh, the rectifier, reservoirs, PFC if it has it, are all useful. There are lots of good components in there. The control chips are good too.

It's the transformer secondary which is the exact opposite of what you want. Re-winding would be the nice way out. Unfortunately, for efficiency they wind the high power output in foil at the bottom of the stack for lowest length and resistance. This means a rewind has to take off the input winding for you to have to make a new one when you've redone the secondary.

The cores are good, but the windings would be a pain to re-use.

The output rectifiers won't include any high enough voltage parts, and you'll need fast rectifiers.

If I needed to do a high voltage bench power supply for myself, I'd nick an input section from a scrap supply to give me 350-ish volts power factor corrected or maybe just from a straight rectifier. I'd need a high freq chopper and a ferrite transformer to do DC isolation from the mains connected bits. PC SMPS will donate transistors and cores. I'd need to wind something myself. Then I'd use a SEPIC fed from the isolated HV stage to vary and regulate the final output voltage. SEPICs are interesting circuits and work well. They convert upwards as well as downwards. PC SMPS transistors should do for that, but high voltage fast rectifiers will cost. A re-wound PC transformer would need gapping to make the SEPIC transformer. Having the SEPIC running isolated from mains allows you to easily get feedback from the output voltage and to also sense output current. Being current mode arrangements, SEPICs have integral current limiting, but an accurate one needs care.

The SEPIC converter was mentioned earlier. It and the Cuk came along a generation after the Forwards and Flyback topologies.

David
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Old 14th Mar 2014, 8:48 pm   #22
kellys_eye
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Default Re: HT Supply (Test Set)

Quote:
but high voltage fast rectifiers will cost.
....for the 10's of amps you usually associate a PC SMPS with but for <1A the UF4005 is available for pennies (10 of them to be exact )

DD - not sure if you're pulling my leg but since you're on about it..... I'm new to the antique radio/tube (valve) scene so my parts bin doesn't include the necessary without resorting to purchasing them (shudder.... ) but if I DID have the bits I'd be doing the tube/transformer dance right now. In time and by virtue of my hoarding instinct this probably won't take too long to acquire.

I work with what I've got and within the constraints of my seemingly endless curiosity but nothing is ruled out. I'll go with the flow for now pending the results of my recent blagging requests via friends and neighbors.
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Old 17th Mar 2014, 4:15 pm   #23
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Default Re: HT Supply (Test Set)

I've been following these threads with interest as this is something I intend to build at some point. I had settled on a fairly simple design with a bipolar-based linear regulator (2 transistors stacked to give a decent amount of margin on Vce) 0-500V, 0-100mA voltage/current regulated and a simple -100-0V 1mA bias supply (pot + emitter follower). A single tap change or switched doubler to reduce the dissipation at low output voltage. The hard part with that design is getting it to survive a short applied when the output voltage is at the maximum but with little load as the full unregulated supply appears accross the output transistors and needs to be shared equally.

The alternate design I considered was a valve pass element (probably something like a dirt-cheap TV line output PL509 which is happy with a few kV a-k and can easily handle the 100 mA cathode current) and an SCR preregulator on the transformer primary to keep the anode dissipation to 10W or so. This could be done with tap-changing too but it'd be a bit of a mess. I still like this design because it's much better able to survive odd transient events. The instantanious anode dissipation might hit a few hundred Watts for half a mains cycle, it's thermally massive enough to shrug that off.

If you'd like to go for a switcher look at the cheap ebay inverters, if you tap off before the final H bridge you've got a non-isolated 12V to 300V dc-dc that's happy at half an amp or so. Add a doubler to the output and put a twidley pot in the feedback path and you've got a 0-600V variable supply. Current limit can be added too.
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Old 17th Mar 2014, 8:03 pm   #24
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Default Re: HT Supply (Test Set)

Quote:
Originally Posted by richard.cs View Post
tap off before the final H bridge you've got a non-isolated 12V to 300V dc-dc that's happy at half an amp or so. Add a doubler to the output and put a twidley pot in the feedback path and you've got a 0-600V variable supply. Current limit can be added too.
Wouldn't the fact that's it's non-isolated be a safety problem? Or are we plugging it into an isolated 12V source?

Last edited by daviddeakin; 17th Mar 2014 at 8:09 pm.
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Old 18th Mar 2014, 10:14 am   #25
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Originally Posted by daviddeakin View Post
Wouldn't the fact that's it's non-isolated be a safety problem? Or are we plugging it into an isolated 12V source?
Sorry, should have clarified that. Most of these HV bench supplies seem to have the 0V output tied to mains earth (unlike low voltage bench supllies), I was imagining a mains-isolated 12V supply with the -ve earthed driving the modified inverter although there's no reason it couldn't be floated.

My reason for mentioning it was partly to make it clear a mains-isolated 12V supply is needed for safety*, and partly because it makes modifying the feedback and adding current limit simpler.

*Not that a bench supply capable of 500V 100mA can ever be truely safe.
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Old 18th Mar 2014, 7:39 pm   #26
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Default Re: HT Supply (Test Set)

Just downloaded the March 1964 issue of Practical Wireless to find a stabilised PSU based on a pair of 807's.

Simple enough design but not sure about the availability of 807's. The usual suspect EL34's are waaaay too expensive thanks to the audiophiles.....

Any ideas on availability and/or price of these 807's?
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Old 18th Mar 2014, 8:36 pm   #27
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Default Re: HT Supply (Test Set)

There are a number of valves that are functional-equivalents to the 807 that could be used: the 1625 is the same but with a 12.6V heater and there's a similar VT/CV-numbered military beast with 25.2V heater designed for use in aircraft with 28V power supplies.

The 6L6 is sort-of an 807 in a single-ended envelope - hence if you can go single-ended the 1619 is sort-of a "6L6-with-2.5V-heater"; I've used these in a couple of 80-metre 'classic' AM transmitters [one valve as xtal-oscillator/output, the other as driven-by-a-carbon-mic modulator and using the primary of a centre-tapped push-pull output transformer to couple them] and they'll happily dissipate 20 Watts or so.

If you truly want double-ended, you could do worse than look at the PL519 TV line-output valve - but you'll probably need to add some 'stoppers' to tame them as they're rather high-slope and have a tendency to see their life-mission as oscillators.
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Old 18th Mar 2014, 8:37 pm   #28
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Default Re: HT Supply (Test Set)

Quote:
Originally Posted by kellys_eye View Post
Simple enough design but not sure about the availability of 807's. The usual suspect EL34's are waaaay too expensive thanks to the audiophiles.....
6L6GT are pretty darn cheap, and are almost the same as 807. Though I really don't see the point of a valve regulated supply when solid state can do it better at a fraction of the cost?
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Old 18th Mar 2014, 11:59 pm   #29
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Default Re: HT Supply (Test Set)

807s ought to be available at reasonable prices because the audio brigade haven't latched onto them. They don't seem to like things with anode top caps.

807s are very robust valves and survive amazing abuse. The 6L6 as has been said is the same thing on a octal base without top cap. Its voltage rating is a bit lower but otherwise it's the 807 electrode structure.

Another 807 variant, less common but smaller is the 5B/254M and these got used in some high quality stabilised lab supplies. The 807 in the full size envelope will run cooler and probably longer.

807s are rabbits of the valve world. I believe they form spontaneously in junk boxes. You don't even need a breeding pair to get them going. EF80s do the same thing.

PL519 as said is also a good bet (and I second the advice on stopper resistors)

The audio cultists and the guitarists are after every thermionic rectifier going, so they're almost all at crazy prices except for single-diode TV ones with awkward heater voltages. As long as your religion doesn't demonise silicon, a few cheap silicon diodes will do the job for pennies.

Capacitors are easy, this leaves you with the need for a nice transformer.

David
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Old 19th Mar 2014, 11:14 am   #30
richard.cs
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Default Re: HT Supply (Test Set)

Quote:
Originally Posted by daviddeakin View Post
Though I really don't see the point of a valve regulated supply when solid state can do it better at a fraction of the cost?
For a linear regulator at >500V there aren't a lot of cheap solid state options, it can be done but takes some thought to make something that won't go pop under odd conditions. For a HV bench supply a valve pass element has some advantages even if the rest of the supply is solid state.

Or just build a switcher.
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Old 26th Mar 2014, 8:01 pm   #31
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Default Re: HT Supply (Test Set)

I wrote about my variable PSU some time ago, here:
https://www.vintage-radio.net/forum/...ad.php?t=27926
I have used it quite a lot and find it very useful. It is quite simple to build and nothing amazing as far as the circuit is concerned.
Bob
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Old 31st Mar 2014, 4:11 pm   #32
richard.cs
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Default Re: HT Supply (Test Set)

I stumbled across this today: http://www.diyaudio.com/forums/power...egulators.html

Whilst these are fixed voltage regulators the design could be adapted for a variable bench supply. A nice starting point anyway.
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