High voltage, high current diy power supply for tube amp prototyping. - Page 1
Forum section: electronics / projects

author: Greg Robinson
author_rank: Regular Contributor
timestamp: November 16, 2014, 05:09:15 am
content: Hi everyone.I picked up a big, beefy transformer recently, it's 1kVA, and meant for industrial settings to allow 115/230V equipment to be used on any number of different supply voltages, so has a number of primary windings (configurable for 208V, 230V, 380V, 400V, 415V, 440V, 460V, 480V, 500V, 525V, 550V, 575V & 600V). The secondary is 2x 115V for 115V or 230V equipment.Here's the datasheet:http://www.farnell.com/datasheets/1648839.pdfI'm planning on running it backwards (it is an isolation transformer, not just an autotransformer), so that I can get a number of different high voltages, and build a bench power supply around it for prototyping tube circuits. I'm planning on using a microcontroller to control it, using relays to select the operating voltage of the transformer, and an emitter follower output (controlled with an optocoupler) to control the voltage (using a 10 bit PWM for 0.5V steps). I'm hoping to get a number of adjustment ranges from 200V up to 800V (there will be a gap in the range between 275V and 400V to limit power dissipation in the output devices, I've got other power supplies that can handle this range), at 1A max current (stupid amount of power, I know). I don't want to bother with a constant current control.I've designed it with 8 parallel output devices to help keep individual dissipation down and reduce junction to case resistance so that I won't need liquid nitrogen to run the damn thing.Here's a link to my design as it currently stands:https://www.dropbox.com/s/yimf2gx8wzd0850/HIGH%20VOLTAGE%20POWER%20SUPPLY.pdf?dl=0I haven't started prototyping it yet, but I'd really appreciate any comments, insights, suggestions, or especially the pointing out of any obvious flaws! I haven't designed a power supply this big or high voltage before, so I'm sure I've made a bunch of bone-head mistakes.It's going to take me a while to justify purchasing all the parts, so I think I'll just be tacking bits and pieces onto other orders for a while... Might be a LOOONG time before anyone gets to see this completed!

author: oldway
author_rank: Super Contributor
timestamp: November 16, 2014, 08:22:34 am
content: For more safety, I would choose valves as series regulator (for example, 3 or 4 PL519)Think about what could happen in case of a short circuit !As you don't need more than 600V, (typical range for such HV power supply : from 250V to 600V) , a 2 ranges switch with a triac preregulator could be a better solution than all those relays.

author: German_EE
author_rank: Super Contributor
timestamp: November 16, 2014, 09:32:27 am
content: "I haven't designed a power supply this big or high voltage before, so I'm sure I've made a bunch of bone-head mistakes."This is the wrong place to make a bone headed mistake.1) Use Google Image Search for internal pictures of high voltage units, you can also use tube based RF power amplifiers as examples. Look carefully at things like layout, distances between HV connections and thicknesses of cable used.2) Use the right cable internally. If you have a 800V supply then you will probably need 1000V rated insulation. Use a double layer of heatshrink over every connection, yellow for the first one and black for the second, that way you can tell if something has been damaged.3) Have a minimum of two safety interlocks on the case that should be impossible to defeat. The idea here is to remove mains power as soon as the cover is opened (and you have two safety locks in case one fails).4) Fit a 230V neon bulb inside the case to show if the circuit is energized.5) Relays might work but the combination of inductive switching the coil and that 800V load means that your suppression circuits need to be almost perfect. Consider opto isolating everything on the control side and DON'T buy cheap relays on Ebay because the isolation between the contacts and the rest needs to be maintained. Also consider using a HV rated rotary switch and a stepper motor as an alternative to relays.6) Fit a big power switch on the front panel so that you or anybody else can find it in a hurry.7) What sort of output connectors are you going to use and do they have a safety interlock if they become disconnected by accident?The golden rule: Never work on energised HV circuits alone.

author: Greg Robinson
author_rank: Regular Contributor
timestamp: November 16, 2014, 11:07:39 am
content: Hi Oldway,I think I'll stick with solid-state for the regulator for efficiencies sake, I'm a pragmatist when it comes to vacuum tubes.I need to add more detail to the schematic, the one I linked to before is very, very basic, and missing a lot of information and parts.I'll add short circuit protection, probably using the microcontroller.Not sure why you think I "don't need more than 600V". It's quite common for consumer tube amplifiers to run at up to 750V (take a look at some MusicMan examples). Serious audiophile stereos (not an area of interest for me, like I mentioned, I'm a pragmatist when it comes to tubes, where they outshine solid-state is in the non-linear region, so musical instrument amplifiers where distortion is intentional are great, but music reproduction with tubes is a waste of time and electrons as far as I'm concerned) are often run at voltages approaching a couple of kilovolts!I think a triac circuit with the amount of bulk capacitance I'm going to need for the power levels I'm wanting will be very tricky to design. Also, triacs are not something I'm very familiar with, I don't think this is a good project to try and cut my teeth with them. Anyway, I don't see any problem with the relays, they're cheap, easy and conceptually simple.Hi German_EE. Hopefully I'm not too bone-headedI build and repair tube amps for a living, so I'm intimately familiar with high voltage safety practises.I have 1000's of feet of PTFE insulated wire with appropriate insulation ratings, so don't worry about that.I think an interlock is implied for this type of application. I will very definitely use some.An LED connected in a bleeder resistor circuit on the bulk capacitance I think would satisfy your "circuit energised" requirement.With regards your concerns with the relays, I do not intend to do "live" range switching. Rather, a primary-side relay would connect/disconnect mains voltage (via normally open terminals of course). Only when the primary side is disconnected would the secondary side relays be switched, before turning the primary side back on. I also plan on including a switched NTC thermistor in the primary circuit to reduce inrush (relay would bypass NTC after bulk capacitance charging period to allow it to cool, could also have microcontroller prevent range switching without NTC cooling delay).Of course, over-voltage protection will be included on the primary, my preference is for TVS diodes.Ebay electronic parts are a never for me, nothing grey market (that includes house-brand stuff from some of the big distributors for me, like Element14's "Multicomp" or RadioSpares "RS" branded stuff). I'm planning on using Omron parts.Power switch is a no-brainer.I was planning on using 4mm sheathed banana connectors (easily available in voltage ratings above 1kV), but a safety interlock is a good idea. I'll have to have a think about that.I'm very comfortable (not complacent, important distinction) around HV, I always wear rubber soled shoes, don't wear a watch or jewellery, don't work if I'm sweating, etc.I have on occasion gotten shocks from 240VAC mains, 600VDC and up, and while it hurts, I have yet to even be injured thanks to my other precautions. To pretend that it's not going to happen, that you'll always be at 100%, that it's never going to be the case that some idiot hasn't been inside something before you and wired the chassis live (I have actually seen that before, and god was it scary), that you won't clumsily brush your hand against something, that's the real danger. Better to accept that it WILL happen, and try to minimise the danger when it does.I'm much more frightened of RF burns, they hurt a LOT more than simple AC or DC if you're lucky, but if you're unlucky they'll cook your insides before you even notice. And it doesn't need to be particularly high voltage or current to do that... Good old dielectric breakdown...Anyway, here's an updated link with some more details:https://www.dropbox.com/s/yo4s0nzb7jdho7i/HIGH%20VOLTAGE%20POWER%20SUPPLY%20v2.pdf?dl=0Thanks for the comments so far!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2014, 11:28:03 am
content: A couple of things:1. Switched supply is nice, as long as it's in sync with the output of course (don't want it to run out of range). But it doesn't save you from short circuit voltage drop. For that, you need sheer voltage capacity in the transistors, or an aggressive foldback limit (which means it needs additional circuitry to actually start up).2. You don't have enough base supply current. BU508A isn't rated for hFE, but even if it's over 20 at 1A (as the typical graph suggests), you need 50mA minimum base current at full load. With up to ~90V drop in the intended range, you need 4.5W dissipation in the driver, but CNY17-2 is only rated for 70V and 0.15W, 30 times too small. (It would actually handle the current, assuming enough LED current is available, which an OPA364 probably won't do, but an L272A is more than capable of; in fact, it's more than capable of exploding the LED and 10 ohm resistor, which isn't a very good idea.)3. You don't have enough SOA or dissipation. BU508A runs out of SOA at about 200V, 0.12A, which will be fine for normal conditions, but won't handle current limit conditions -- at least, not without some serious foldback (completely turns off for >150V drop?) or other protection circuitry (a current limit / shutoff / auto-reset thing?).You also don't have enough filtering (should be more like 500uF total, not 33uF, for an amp load at that voltage), and the C mult. isn't wired right (the zener couples ripple peaks already, and for ripple > 12Vpp just lets it all through anyway).And, why such a particular voltage divider chain, if you're just going to throw up your hands and say "I don't know what the hell this should be!" with that huge trimpot? The proper way to use trimpots: calculate all resistors so the ratio is exactly what you need, then use a trimpot (in series / parallel whatever as needed) to cover exactly twice the toleranced range. So +/-2% for 1% resistors. That way, even if the trimpot is set just absolutely wrong, it only messes up the fine calibration, without affecting fundamental operation (whereas, as shown, at the extremes of range it will either saturate trying to output a voltage it can't reach, or explode from trying to drop too much voltage).The suggestion of using tubes is great, except that you still need to dissipate that power (~80W as drawn I think?), which is a couple sweeps in parallel or something like that. Transmitter tubes are almost all expensive and high voltage drop, so, not as useful.Tim

author: oldway
author_rank: Super Contributor
timestamp: November 16, 2014, 01:23:54 pm
content: QuoteYou areQuoteand you don't want to use tubes in this power supplyQuote...If you care with efficiency, stop with prototyping tube circuits, they ALL are an huge waste of energy.Using tubes as PL519 (line output pentode) is far more secure: no problem of SOA, no risk of short circuit between input and output of the serie's regulator, ...etc...in such a high voltage power supply, security is more important than efficiency...and still more here because your schematic shows that YOU ARE NOT CONFORTABLE with prototyping power stages with semi-conductors.

author: richard.cs
author_rank: Super Contributor
timestamp: November 16, 2014, 02:04:10 pm
content: I am working on something similar, 0-250V @ 200 mA, 250-500V @ 100 mA, plus a 0-200V negative rail at a few milliamps and various heater outputs. I'm using paralleled PL504's as my series element, they're about the cheapest you can get per Watt of anode dissipation, if I wanted higher dissipation like you need I'd go for PL519's, their big brother to keep the number down. On my supply the grid drive is also valve but the rest of the control loop, and the negative regulator is solid state. I have been around the design loop a few times now to come up with a design that I expect to be reliable and to survive the rigours of normal bench use.T3sl4co1l has raised many of the points I was going to but I'll go over SOA some more. The BU508A is a good choice but it still suffers from second breakdown. With 125 mA per device you're safe up to 200V forward drop but you need to catch the current limit condition (includes startup into a capacitive load) and either foldback or shutdown. This is doable but tricky and you'll have to think through all the possible conditions that may lead to SOA being exceeded. The short-circuit case is much worse, picture the situation where your input capacitors are at 800V and you accidentally short the output - loose wire, dropped screw, valve under test flashes over, anything. It will happen on a lab supply and you should design it to survive. At this point much current flows, limited to about 8 kA by your 100 milliohm emitter resistors but to hFEx0.8A so perhaps 10A or so by your base resistors until the transistors fail C-E short*. If that happens it will die, probably dramatically with smoke and flame. Unless your overcurrent trip can limit the short-circuit current to a few tens of amps and slam the whole thing off in microseconds (as designed now it won't) the output transistors don't stand a chance, best make them a replaceable assembly and build up some spares. If it happens anywhere but a mains zero crossing you'll likely loose your input diode bridge too.I would strongly push you towards a valve design, probably with paralleled TV line output valves as others have suggested. In the same short-circuit situation they just don't care, they'll be emission limited to a few amps each but cathode stripping takes way longer than that, and the anodes have enough thermal mass to deal with it until your short circuit trip or current limit gets out of bed and starts to react. If you really must build a solid state design you should:1) Scatter resistors around to limit peak currents. I would start by changing your 100 milliohm emitter resistors to some tens of ohms but also put some series resistance in the transformer outputs. This of course eats into the efficiency advantage of solid state.2) Consider a design with the pass transistors in series. If you can bring it down to less than 70V per device then you're dissipation limited rather than second breakdown and that gives you headroom. If you do this then you'll have to shunt bases with capacitors so in the event of a sudden short they're all pulled down together and in proportion. I considered this for my supply but dismissed it as too fragile.3) Consider using more transistors.4) Put a lot of work into the design and test of your current limit circuit and your short-circuit shutdown. It will need both and they will have to befast. Getting them stable may be tricky, you can't do the normal hack of just making your current control loop way slower than your voltage one.5) Give the whole thing a thyristor output clamp. If you find yourself working at low output voltages (when dissipation will be bad and failure perhaps more likely) and for any reason an output transistor fails you don't want to kill your downstream circuit. Though I suppose if your downstream circuit will always be valve it may just shrug it off.With that transformer I would give strong consideration to going multi-tapped unregulated. If you're only doing valve stuff not HV semiconductors then that's still a very useful thing to have.*Semiconductors normally fail short-circuit as the dopants diffuse around and your nice transistor structure is replaced with a lump of conductive silicon. If you're lucky the bond wires will vaporise soon after protecting your equipment under test.

author: Greg Robinson
author_rank: Regular Contributor
timestamp: November 16, 2014, 02:21:11 pm
content: Hi T3sl4co1l,1. Yes, I need to address the short circuit issue. I haven't settled on my output devices yet, the choice of BU508A was mostly convenience for simulation in LTSpice because the model was already in my library, and mostly I just wanted to simulate the topology to see if it was at all workable.2. Regarding base current supply, I forgot to add a driver, I'll add one to form a darlington.3. SOA, yes, I'll address that when I settle on component selection.I'm still working on making the schematic more clear, but I do have more bulk capacitance than is immediately obvious. I have a 30 x 100uF capacitors in each tier, for a total of 1mF bulk capacitance. I'll re-draw it for more clarity.Not sure how I goofed up the capacitor multiplier. Funnily enough i worked great in simulation. Always good to have a fresh set of eyes look something over. Again, I drew the bulk capacitance in the multiplier strangely, there's actually 3 x 100uF in each tier, for a total of 100uF.The trimmer circuit needs optimising, that was just a rough "first values that pop into my head" design, really just to remind me not to forget it.Thanks for your input, it's brought up a number of issues I need to address.Oldway,No need to be rude. You've quoted me completely out of context to suit your point, and skipped over everything I had said explaining my reasoning.QuoteNow that is a useful contribution, you made a suggestion, and provided good reasoning for it. That type of input I appreciate. It's just a shame that you're such an jerk about it.I guess I should point out (although I didn't think it should be necessary) that this is a WORK IN PROGRESS, not at all intended to be something I'm going to be going out and building as it is currently depicted. I am literally just spit-balling the basic topology and looking for some input, hoping that I've got some basics right and that IF I've made some botch of the topology, someone will point it out to me, and explain why (without being a jerk, in case you need to be reminded).I guess next time I'll just wait until I have everything completely done before letting anyone have an eyeball. Not unreasonable that I be expected to do my homework first, just guess I'm used to other forums with more of a "community" vibe to them.

author: oldway
author_rank: Super Contributor
timestamp: November 16, 2014, 04:45:12 pm
content: QuoteYou are wrong to post such a thing here...If you would have post your "work in progress" (in my opinion, you are nowhere !) in "beginners", it should be different.QuoteI remember of some public adress amplifiers with EL34 and 800V anode voltage, but that's not quite commun.With a triac preregulator, you can maintain constant the voltage drop of the serie regulator , with relais not.QuoteToo slowI don't recomend to use a microcontroller in such a high voltage power supply, or it should be fully isolated from the power stage.

author: richard.cs
author_rank: Super Contributor
timestamp: November 16, 2014, 04:59:28 pm
content: Quote from: oldway on November 16, 2014, 04:45:12 pmI disagree, this (the work done so far) is clearly beyond "beginner" and this is not acompletedprojects forum. Lots of initial ideas and works-in-progress get posted here. HV design is a specialised area and people experienced in other areas commonly fall into traps, DC safe operating area being a big one.Quote from: oldway on November 16, 2014, 04:45:12 pmI'll agree with this though, for a HV supply with semiconductor pass elements the short-circuit protection needs to be very fast. The destruction is all over in 10 microseconds (though other effects like cracked cases and flying sparks might not follow for some milliseconds). With a valve pass element it would be ok to implement SC protection in the microcontroller, just make sure that if the micro crashes there's a HBC fuse on the transformer output that'll still bring it down without flames.

author: oldway
author_rank: Super Contributor
timestamp: November 16, 2014, 05:48:00 pm
content: The work done so far ?....A project with14 relays?For me, that's just a project of a beginner, not more.More complicated it is, more dangerous it became...that's a basic rule.Power on controlled by microcontroler....(NB: because relais can't switch with applied voltage), what a dangerous solutionIf he don't like triacs (new 3 quadrants triacs are safe with inductive loads.), why not ajust the voltage drop on serial regulator with a variac in the primary ?When I read such comments, I am totally amazed ... it's utter recklessness and irresponsibility.Dare boast been electrocuted and still be alive .....QuoteHonestly, if one of my technicians should behave so dangerously, I never let him work on a project of a 800V 1A power supply ..

author: holko
author_rank: Contributor
timestamp: November 16, 2014, 06:45:12 pm
content: I've had good results with power supplies designed for electrophoresis.I've got a few of these ones:eBay auction: #291291990911eBay auction: #251720487060-Holko

author: Greg Robinson
author_rank: Regular Contributor
timestamp: November 16, 2014, 08:05:33 pm
content: Hi richard.cs,Sorry, I was typing a reply when you posted.SOA: At this stage, any device types on my schematic are really just place-holders while I get the topology right.Anyway, I think I'm going to scrap the BJT regulator, it's just going to be impossible to keep it within SOA. I think mosfets would be a much better choice for a series regulator, I'll post a new schematic later. But thanks for the detailed information and explanations!I really wasn't thinking when I saidQuote-that was really dumb, I know very much better, call it a brain-fart.I'm considering a number of options for short circuit protection, I'll add something to my next (mosfet) schematic version.I know there are lots of good reasons to argue for using tubes instead of solid-state devices for this application, but lets just call this a design exercise: this is what I want to do (800V, 1A variable power supply), and I want to achieve it with solid state. If I get a workable design out the end, hey, maybe I'll build it?Hi 3roomlab,Thanks, I'm familiar with the ARRL publications, although I haven't really looked into them much. I'll file that with my "must read at some point" list.Hi holko,I'm familiar with electrophoresis supplies (I saw Dave's video on them). Not quite what I'm after, I mostly just want to design something myself, utility at the end of that process is just a bonus. Thanks for the reminder about them though.Oldway,You really have a talent for being exactly the opposite of constructive, don't you?Thanks, but I'm just going to ignore you from now on, I'd appreciate it if you ignore me too (which given your attitude to this thread, you probably should have done in the first place).Thanks to everyone else for their contributions! I'll be back later with a much more sensible topology.

author: oldway
author_rank: Super Contributor
timestamp: November 16, 2014, 08:25:21 pm
content: If I can avoid you kill yourself, I will consider that I have been sufficiently constructive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2014, 08:45:24 pm
content: MOSFETs are a much more likely candidate here, but beware, they are susceptible to 2nd breakdown as well (for much the same reasons as BJTs: negative tempco in the linear range, and high power density). Which was never not true, it's just that really old MOS wasn't hot enough to get into the unstable / runaway regime, so it unfortunately turned into a saying ("MOSFETs free of 2nd breakdown"). But now that they are comparable in power density, 2nd breakdown is a real thing.You do have the advantage of needing little (DC) bias, though you still need to supply Cdg to make the follower move. Not really a problem for a power supply, as you can soak up slowness of the regulator in a bigger output filter cap. But a proper regulator really shouldn't need that, the output impedance should be low enough at most frequencies by design.Note that current limiting in a power supply application is best implemented as a constant (and adjustable) limit (little or no foldback, no shutdown), or a moderately excessive (but perhaps fixed and poorly controlled) limit with time-out and retry. The former is good at charging capacitors and limiting power into a fault; however, it's bad at limiting its own power, and requires a lot of dissipation rating. The latter is great at charging capacitors, maybe not very fast on average, but the burst of current helps power through SMPS startup conditions as well. No, probably not a concern here, but it's still an option. Foldback limiting is generally either slight (reduce output current by 20-50% into a short?), or avoided altogether; using too much means your capacity for charging empty caps or recovering from short circuits goes down, or all the way to zero if you have it set that high (meaning, you connect it to a load and it just sits there, never charging). Note that the traditional implementation depends on Vbe, so has a huge tempco; this can be compensated with an NTC, but you'll want to run some temperature sweeps to verify that you've got the right proportions.If you make the relays programmable based on actual output voltage, you can at least save dissipation on the milliseconds time scale -- since relays take 5-50ms to operate, and capacitors must charge or discharge over several line cycles until returning to quiescent voltage. This helps save heatsink and ventilation costs, though you still need enough total transistor dissipation to handle the full output (transistors exceed ratings in 10s of us; capacitors discharge in 10s of ms; fuses blow in 10s of ms; heatsinks are way out there in the seconds).Tim

author: Greg Robinson
author_rank: Regular Contributor
timestamp: November 17, 2014, 06:34:47 pm
content: Hi T3sl4co1l,Thanks for your input!I'll make sure to pay attention to possible second breakdown, and will seriously consider using linear mosfets to help account for this. I'm not going into production with this, and I want it to work correctly and safely, so the added expense of linear FET's is a no-brainer.Cdg will be of particular concern if I do decide on linear FET's, they have much higher gate capacitance than switching FET's. I'll just have to make sure I can drive them hard enough.I think I'll go with adjustable constant current, with time-out shutdown, and manual retry. I don't really want it to keep cycling and turning back on into a short-circuit until it blows up. I wont be using foldback, I plan to use this mostly with capacitive loads. I'll just make sure to massively over-build it, and include shutdown as a belt-and-braces approach.I don't plan on doing live switching of the secondary relays, rather have the voltage range first selectable on the controller, which sets the secondary relays, before turning it on using the primary relay. But yes, I'll have the micro sample the actual output voltage and shut down the primary if it's outside of the selected range. As you explained, it'll probably mean I can save a bit on the heatsink. I'll be making all my thermal calcs based on a 50°C ambient temperature (it can get pretty hot here in Australia, even way down south here in Melbourne), which should leave me pretty conservative anyway.Fuses are to prevent fires, not protect circuits, that's what crowbars are forAnyway, I've given more thought to the regulator topology, and I think I've got something that will prove workable. I still need to scale it for the power I'm hoping for (need to parallel up the pass device/s), and make some device choices, but I think I'm on the right track with this:https://www.dropbox.com/s/e24bqdcgmewpb9t/A%20better%20regulator.pdf?dl=0Remember, this is just the regulator circuit. There's some passive current limiting, but I won't be relying on this in the expanded circuit, I just feel it's good practice. Any comments or feedback is appreciated!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 17, 2014, 06:54:33 pm
content: Why not simply connect gate to:~100k pull-upQ4 collector to gateQ4 emitter to R to GNDQ4 base to op-amp (B, as shown)And use HV BJT or MOS for Q4.I don't see what you gain with Q2-Q3 and associated parts. As shown, Q4 is suicide biased, meaning the op-amp will have a hard time finding the correct operating point (the op-amp output will vary from 0.6-0.8V in the linear range, saturating at 0V (150% below the sweet spot) or +12V (6000% above the sweet spot!).Tim

author: TubiCal
author_rank: Newbie
timestamp: November 24, 2014, 12:24:18 am
content: Hi Greg,i checked your design for some 2 minutes and i simply don´t getting it....what are you doing?I would suggest you to begin with a simple linear regulator for some 200V and test it thoughout until you familiar with it and than simply stack it, that´s exactly how the industry handle things like that.No one is going to do something like 0-800V / 500mA in just one piece.I done some MosFet based PS with the IRF830 (in every unit i built, besides PowerAmps, i used this method) and had some tube based heavy units, all gone by now due to limited space. But i still got and use an unregulated 0-400V/200mA Pro build heavy unit having just a huge tranny with 10 taps and some 100W Pots "regulating" the AC until it´s rectified and fed to the outlet via a big choke and another C-R-tiny C stage. Works way batter than thought and it´s very ruggedly constructed. Even short circuit, regardless of which tap is selected, doesn´t affect it much, you only hear the tranny a tiny bit more. Of course it is fused, but it never blew off since i got it, some 15 years ago.If you´re designing big TubeAmps anyway you can simply rectify the AC supplied from your tranny; and with a good filter chain feed this into your Amp. In case you´ve test the amp with lower voltage prior to final test. For more savety you can hook up some 100W light bulbs in parallel/series to have some sort of current limiting.I use a 800V/1.4kVA sec. bridge rectified tranny powered from my 2kVA Mains isolated variac to supply the plate voltage for my serious Tube Amps, monitoring the Amps and volts while rising the plate voltage via the variac slowly. No semicondictor is inbetween which will may go wild and do "funny" things and set my lab on fire.Remeber you´re doing some serious energy things here and a good filter chain incorporates some 100-400müF capacitance. And this is attached to some 800-1200Volts and just think of the energy stored in them. Nothing will stop this when short circuit, the stored energy will burn out and melt down everything. It´s like putting a big wrench between a car batteries´s terminals...AC pre regulation prevents all this, as you limit the energy before it getts stored.....Cheers,TC

author: oldway
author_rank: Super Contributor
timestamp: November 29, 2014, 05:43:45 pm
content: Another fine HV stabilized power supply project: type 8619.Replace OA2 and 85A2 voltage stabilizers (too noisy) by zener diodes and EL34 by PL504, PL509 or PL519.EF94 : Va max = 300V too low for your application.Perhaps replace by a pentode such an EL81 (for TV line output)http://www.pa3esy.nl/meetinstrumenten/vdheem/html/vdheem_set.html

author: richard.cs
author_rank: Super Contributor
timestamp: November 30, 2014, 09:59:28 am
content: Quote from: oldway on November 29, 2014, 05:43:45 pmWhile something like the EL81 with it's top cap anode is a better choice it's probably worth noting that Va max is a very flexible rating when operating at low current on something like this. Really there's very little in the physics of the valve that limits it so it's more about how many volts you can stand between adjacent pins.

author: oldway
author_rank: Super Contributor
timestamp: November 30, 2014, 12:28:53 pm
content: In this project, whe need a Va max of 800V !EF94 is not intended to withstand such a high anode voltage.There can be problems coming from lack of enough distance between anode and other electrodes and internal connections and risk of arcing.Pentode for TV line output are able to block high voltage as high as 7KV (PL519 for example)...

author: richard.cs
author_rank: Super Contributor
timestamp: November 30, 2014, 02:18:23 pm
content: And yet a non-gassy EF94 with the anode running cool would be happy until there's a flash-over outside the glass, probably on the socket contacts as these are closest. If the vacuum is good the electrode spacing inside has almost no effect on the maximum voltage that the valve can stand. You'll notice that one of the suggested circuits in the datasheet has the valve driving a tuned transformer in the anode from a 250V d.c. nominal supply, peak voltage there will be around 500V and indeed the datasheet actually suggests a peak voltage on the anode of 550V is acceptable, similarly the curves go to about 480V. This is normal and an artefact of the rating system where it is just assumed that you may get double the d.c. voltage in some circuits and that it should work. I think you'll find the 550V is actually a limit on the B7G base given the pins are only 3mm or so apart, but if kept clean and dry (not something that always happened inside a domestic TV set!) I wouldn't worry about exceeding it by a fair margin.I'm not claiming an EF94 is the best choice in this circuit but if the current is low it would probably loaf along for decades with 800V d.c. on the anode. There just isn't a hard voltage limit in the way there is with silicon where it drops out of the device physics.

author: oldway
author_rank: Super Contributor
timestamp: November 30, 2014, 05:36:32 pm
content: You try to prove you are right when you are wrong.I am not fluent enough in english (it's not my mother language) to write a lot, but, what you argue show that you make a wrong project."If everything is 100% allright, nothing will happen", thats what you are using as explanation...that's not a good manner to project something.I have worked with power electronics (tubes and semi-conductors) for more than 40 years long, I projected many industrial power electronics equipments and they have been produced for years, some for more than 10 years without need of modification.Something very important in HV / high power project, is to consider that if something has to go wrong, it will go.QuoteThose words are proving that you are making wrong statements and wrong project...When you make a HV/High power project, you MUST be 100% shure that nothing wrong can occur, not be hoping that everything will be OK !It's a 800V 1A dc power supply, not a 12V power supply....

author: richard.cs
author_rank: Super Contributor
timestamp: November 30, 2014, 09:09:33 pm
content: I was making the point that the data sheets give a good starting point for typical use, but that if you have an understanding of how the devices work and the potential failure mechanisms then you can push the limits slightly. The facts are that 1) nothing inside a tube with good vacuum really limits the allowable anode voltage and 2) The pin spacing is about 3mm, which in air will hold off several kV forever.There is nothing wrong with putting 800V on a tube with 550V Va_max if you understand what the potential failure mechanisms are and how to avoid them. In this case they are:1) A gassy tube will fail earlier than it might in a circuit with a lower anode voltage, or to put it another way the levels of residual gas acceptable are lower, but still well above that normally found in production valves. A nearly-dead tube will become apparent sooner but a healthy tube is unaffected.2) If the socket is damp, covered in dust, flux from soldering or otherwise contaminated current may track along the surface causing the circuit to misbehave. This is easily avoided by good construction technique and maintenance of the finished supply.3) If the wiring technique is poor then the clearance between adjacent pins will be compromised and there will be the potential for flashover. Again easily avoided.There is a tendency for people who've grown up with semiconductors to apply the same "The datasheet maximum is the absolute limit" philosophy to valves. Ideas like "design centre maximum" get forgotten and sometimes there's a failure to appreciate that the limits aren't so hard. If you apply 800V to a 550V transistor there's a good chance (but no certainty) that it will avalanche, some localised area of the die will get hot, diffusion will occur and within a few microseconds all you've got is a lump of conductive silicon with all 3 legs connected together. If you put 800V on the anode of a 550V valve the electrons will hit the anode a bit harder. It might not say in the datasheet that 800V is OK if you keep the anode cool and the socket clean but it doesn't mean it will magically fail at 551V either. The ratings give something that works reliably caked in dust and filth inside a TV and are not necessarily the limits of the possible.Let's not forget that we should consider the consequences of failure as well as just their probability. If a valve in this position in the circuit were to flashover for some reason the output would collapse, there would be no danger to the operator or the connected equipment. And having blown the unfortunate spider who crawled into the valve socket open-circuit normal operation would most likely resume a fraction of a second later. :-)I understand that in a production design it has to still work with that 1 valve in 1000 that only just made the specification, and not fail just because the assembly technician sneezed all over the socket, I just don't agree that applies to a personal project of which only one will ever be built.

author: timb
author_rank: Super Contributor
timestamp: December 01, 2014, 12:35:51 am
content: I think @oldway doesn't understand English fluently and isn't grasping some of the nuances in the OP and others posts. Likewise I don't think he intends to come off like a dick, either. His advice in the thread is good if you read it from a neutral point of view.Sent from my Tablet

author: oldway
author_rank: Super Contributor
timestamp: December 01, 2014, 06:40:35 am
content: @richard.cs: Once again, almost a page to show that you do not respect a fundamental principle of a high voltage electronic project, whether it be industrial production or a single copy: we must respect the safety limits and never use a component at is absolute maximum ratings or above.You must use a safety factor.Using an output pentode (as EL/PL81) intended to withstand high L x di/dt blocking overvoltage from flyback is far more secure than choosing an EF94 !If you can't understand that, you should not make any project in power / high voltage electronic field because you don't work with safety.NB: PL81 is cheap, I have found a new one for only 2.8 € on ebay.de

author: BennVenn
author_rank: Regular Contributor
timestamp: December 05, 2014, 01:06:34 am
content: Quote from: richard.cs on November 30, 2014, 02:18:23 pmI'm no expert, but I've done a little bit of vacuum tube design and construction and the electrode spacing is directly related to the potential it will withstand before breakdown. There are various means to emit electrons from a material, including Field Emission.

author: richard.cs
author_rank: Super Contributor
timestamp: December 05, 2014, 08:06:20 am
content: I'd rather not get into further arguments about the wisdom or otherwise of running things past their datasheet limits but I find the physics fascinating and I'll happily discuss it. :-)Field emission is interesting, it depends on work function and on E-field, but it is the local E-field at the surface and therefore geometric features like sharp edges come into play. There are whole books on the subject but it's not really that well understood. I suspect in any discussion about breakdown from excessive anode voltage in a pentode we really mean electron emission from g3 and the various support structures (at least to start with).I wouldn't have expected any clearances smaller than 0.1mm in a signal pentode, though I'm happy to be corrected on that and field emission needs around 10^9 Volts per metre, a kV on a 0.1mm gap in vacuum feels safe then at 10^7 V/m even if there are sharp bits where the field is higher. The main breakdown mechanism so far as I'm aware is bombardment of these structures by ionised residual gas knocking electrons free, hence my comments about gassy valves.I guess given some knowledge of how valves work and their manufacturing tolerances I find it hard to believe any but the most borderline would fail with a mere factor of two in anode voltage. Otherwise the whole thing would be so marginal (given the poor control of certain parameters) that production yield would have been terrible.

author: oldway
author_rank: Super Contributor
timestamp: December 05, 2014, 01:59:49 pm
content: All this is only theory but in practice, this is rubbish.In a PL81, there are several mm's between the anode and the other electrodes.Let say 2mm for example.According to the theory, it may therefore be applied 20KV (1KV / 0.1mm) between the anode and the other electrodes.If you read the technical specifications of the manufacturer, it can be applied only max 7kV with maximum pulse duration of 15% of one cycle, with maximum of 18µs.That's far different !The difference between you and me is that you speak about theory and I about practice."Anything that can go wrong, will go wrong » (Edward A. Murphy Jr.).

author: richard.cs
author_rank: Super Contributor
timestamp: December 05, 2014, 02:09:03 pm
content: And this 3:1 discrepancy was exactly the point I was making. I believe the 7kV limit is the maximum based on tracking across the outside of the glass in worst-case intalled conditions: dusty, perhaps even damp, as would be typical of a domestic TV set. This is similar to the different ratings EHT rectifiers have depending on if they are given a moisture resistant coating or not. And if you can avoid such worst-case conditions, as might be reasonable in test gear, then it won't magically break at 7001 Volts. Understanding the reason they wrote 7kV in the datasheet tells you if it's safe to push it or not.

author: oldway
author_rank: Super Contributor
timestamp: December 05, 2014, 03:56:52 pm
content: I think your approach to HV projects is totally unacceptable from an engineer.Safety rules, standards and maximum ratings MUST be respected.This is not to be discussed.As an engineer, if you do not respect this and if someone is killed by your fault, you may go to jail.I frankly tell you what I think: you're dangerous to yourself and others and you stand here with an unacceptable reasoning.You should stay far away from HV and power electronics projects.You should set a good example, instead, you encourage readers to do wrong and dangerous things ...The total voltage of this power supply is nearly 1KV. (600Vac + 10% = 660Vac , this is 930Vdc,as high as the metro third rail voltage !)There is no place for kidding.

author: richard.cs
author_rank: Super Contributor
timestamp: December 05, 2014, 08:38:47 pm
content: Yes it's a kV, and if you touch it you're no more dead than 230V a.c. We are discussing a specific point in the circuit where if the component were to fail it would cause the supply not to work, I can't see any situation in which a failure here would make the supply any more dangerous to the user rhan it al

author: richard.cs
author_rank: Super Contributor
timestamp: December 05, 2014, 08:53:26 pm
content: Yes it's a kV, and if you touch it you're no more dead than at 230V a.c. We are discussing a specific point in the circuit where if the component were to fail it would cause the supply not to work, I can't see any situation in which a failure here would make the supply any more dangerous to the user than it already is.I can accept if you think it's poor practice to exceed datasheet limits, but I do not believe thatfor this component in this positionit is in any way dangerous to do so. I have never suggested one should ignore ratings whilly nilly, but that with sufficient consideration of both the failure modes and theconsequencesof those failures it can be a reasonable approach, particularly for something that will not be mass produced (and therefore the full range of component variation is never seen).Given we've both now made our positons clear I'd rather not argue further in a public forum over a single component, but instead let the thread revert to a more general discussion of the topic.

author: oldway
author_rank: Super Contributor
timestamp: December 05, 2014, 10:31:50 pm
content: QuoteI will not answer anymore at your such stupid and ridiculous statements because you touched the bottom off the well and you're really starting to let me nervous and irritated.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 06, 2014, 12:21:16 am
content: I've worked on 480VAC / 700VDC systems... arc flash is neither pretty (except perhaps from a sufficient distance), nor kind. I've also made accidental contact (a light touch, fortunately) to 480VAC on one occasion, which is also nothing like 240V or below: even a light touch leaves a small burn. It's angry stuff, no matter how you approach it. Best not to.Of course, that's line operated, where even a semiconductor fuse (= very fast melting fuse) lets through quite a show. But even just some film capacitors at that voltage will leave quite a crater in your screwdriver, let alone, say, the line frequency filtering caps.One nice thing about an active regulator, at least: you can omit the output bypass capacitor, so at least you don't get a huge arc flash every time the output is inevitably shorted. Can't say the same for the circuit under test, though.Tim

author: richard.cs
author_rank: Super Contributor
timestamp: December 06, 2014, 10:49:01 am
content: Eliminating the output capacitor is good, but the design should account for capacitance across the output, both in that it must be stable into such a load and that is should provide a discharge path (secondary to the one that should already be included in the circuit under test).On the subject of HV capacitors, not just those on the output but everywhere in the design, charge recovery is a very real hazard. You can take a capacitor that's been sitting charged for a while, discharge it safely, short the output and there will be no volts. Remove the shorting link and wait 10 minutes and that may no longer be true, it is possible to recover a good fraction of the initial voltage as trapped charges bleed out of the dielectric. This, far more than any worry about accidental charging from static, etc, is why HV capacitors are stored with shorting links. It's also one of the more non-obvious HV safety traps, and therefore often the one that catches people out. It's true not just of capacitors but things like long lengths of cable, etc.You can model it as a capacitor of a few % that of the bulk with a many-megohm ESR in parallel with the wanted capacitor. After you've discharged the capacitor it slowly bleeds charge back into the main one which can reach a high enough voltage and from a sufficiently low impedance to be lethal. It's probably of borderline significance here as a 1kV capacitor is unlikely to recover much past 50V, being limited by the ratio of the trapped charge to the bulk capacitance but definitely one anyone working on high voltage stuff should be aware of.

author: BennVenn
author_rank: Regular Contributor
timestamp: December 07, 2014, 10:19:35 pm
content: Why not go SMPS for testing? Low output capacitance will save a few components if something goes bad?I modified a 400w 240v inverter to give a 0-400v DC supply. It is still 400v but only a couple hundred uF to deal with when things go wrong