Overhauling high voltage vacuum tube (valve) power supply - Page 1
Forum section: electronics / projects

author: proschuno
author_rank: Contributor
timestamp: February 16, 2014, 05:12:27 am
content: So I"m overhauling a 50 year old home-builtPSU in serious need of refitting. Using two pass tubes of them has a busted filament and the other was replaced about a year ago. Should I worry about getting a matched pair or just get a single tube?Also, the circuit uses two oil filled caps that I would like to replace for good measure (along with all the other ones in the schematic). Seeing that these on ebay are quite expensive along with any other electrolytic alternative, I'm wondering if I couldn't simply tie two 450V caps in series as shown for the +B supply? This would be much cheaper than getting another oil cap or similarly rated electrolytic ($US 80 on Mouser).. I notice one of the oil caps' leads is tied before the choke, so I wonder why so?ProschunoAnd for the missing piece: The OA2 tubes, along with the 1st and 3rd leads of the pot, are in parallel with the 30MF caps, and the cathode of the error amp tube is connected to the wiper of the pot.And BTW, taking apart the PSU I found the schematic; notice the date…. it's literally as old as my mother lol )The pass tubes are 1619 (or VT164), the error amp is 6sj7, the rectifier for +B is 5v4g, and the 2nd one is 6x4.

author: SeanB
author_rank: Super Contributor
timestamp: February 16, 2014, 06:17:28 am
content: Just be careful of those oil filled caps, they likely contain PCB oil, and should only be handled with gloves, and if there is any oil leakage clean it off with solvent and dispose of the rags, gloves and caps as dangerous waste. You could replace them with 4u7 1kV film capacitors with no problems.

author: calexanian
author_rank: Super Contributor
timestamp: February 16, 2014, 07:09:12 am
content: The 8uf oil cap and that choke form a VERY important network!!!!!!! That is a capacitor input to filter type circuit. Much of the ripple rejection of that supply comes from that combination of a low capacitance and the choke. Otherwise you will have a lot of ripple and the plate voltage will be way too high for the pass tubes to handle. Just bring the supply up slowly on a variac and I bet that oil cap will be fine. Monitor the current draw to the unit and with no tubes apart from the rectifier tube. Watch the voltage on that cap as well. As the filaments heat up in the rectifier it will self soft start so to speak. Depending on age those electrolytics are toast. If the oil cal is bad, replace it with as close to 8uf as you can get.

author: SeanB
author_rank: Super Contributor
timestamp: February 16, 2014, 07:55:08 am
content: Just looking 4u7 630V caps are hard to get. You can use 2 parallel 2u2 630V film capacitor to replace them, they are $4 each. The electrolytics are a lot cheaper, you can use 33uf 450V Nichicons as replacements for the 47uF ones, they are under $2 each. If you want to use to replace the 4uF oil then use 10uF 450V capacitors with a 100k 1W resistor across each capacitor like the other side.

author: proschuno
author_rank: Contributor
timestamp: February 16, 2014, 02:41:50 pm
content: Ok also too, my concern is wattage rating, if it should be one; is this also a reason oil caps were used too? Or can I literally replace them with the polypropylene/ethylene caps that I have, or are we talking some high power film caps?

author: SeanB
author_rank: Super Contributor
timestamp: February 16, 2014, 02:53:32 pm
content: Oil filled was used because the old capacitors were paper insulation, which needed the oil fill to keep water and moisture out which would go bang. Any film capacitor rated at 630V DC or more will work as a replacement. Just put a 10R to 22R 1W resistor in series with the capacitor, as the originals would have had a high series resistance and the diode rectifier valve would have been designed around it.

author: dannyf
author_rank: Super Contributor
timestamp: February 16, 2014, 03:00:20 pm
content: On high voltage caps: you can find them in camera flashes, or transformer-less smps (usually 470v or less, however).I would explore if a smps can be made to produce the regulated voltage - should be much smaller, lighter and more efficient.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 16, 2014, 03:06:11 pm
content: Agree with the comments on the oil capacitor. If it is bad/leaking, the replacement should be as close to the original value as possible. Excessive capacitance here will kill your rectifier tubes in short order.As far as the pass tubes go, no need for specially matched pairs. 1619s haven't been manufactured in probably 50 years, so NOS ones from eBay or similar are your only option. Fortunately, even though they are 6L6 variants, the 2.5V directly heated cathode kept them from gaining cult status among the audio and guitar amp crowd. They are still pretty easy to get, and cheap compared to the rest of the 6L6 family.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 16, 2014, 03:12:15 pm
content: Quote from: dannyf on February 16, 2014, 03:00:20 pmPhotoflash caps are intended for energy storage applications. They DO NOT like being used in ordinary filter applications, as they can not tolerate ripple current well, and have a very short life (often with rather spectacular failure mode) when used as such.

author: proschuno
author_rank: Contributor
timestamp: February 16, 2014, 03:20:24 pm
content: Quote from: dannyf on February 16, 2014, 03:00:20 pmI actually have considered this for building one, but at high voltages I'm to inexperienced to build it, and I would rather have the low-noise and transient response of linear regulators. The device is already made, and trust me, at nearly 10lbs its a brick! And also let's try finding a switching transformer at those rated voltages….But actually my main concern was the wattage rating for the oil caps… but they're not leaking so they seem fine.

author: dannyf
author_rank: Super Contributor
timestamp: February 16, 2014, 03:26:17 pm
content: Maybe it is a long-term solution and you can start working on smaller boost-up converters first, with the goal to build your own sometime down the road.Getting a transformer for a smps, or winding one, is not that difficult, especially if you don't need lots of power output.Transient response could be an issue, however. If it is used to power tube stuff, they are likely class A so transient response isn't a big deal.

author: calexanian
author_rank: Super Contributor
timestamp: February 16, 2014, 03:42:08 pm
content: The ripple in those input caps is HUGE in those circuits. It you are going film make sure you have a very low esr. Also I do not recomend series. Parallel is a great idea as it lowers esrAnd distributes heat. Yes...... They do heat up in that kind of service! Those regulated circuits are kind of an animal all their own.. They however can be extremely reliable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 16, 2014, 04:19:11 pm
content: FYI, the resonant frequency of that cap to the choke is 28Hz, so it's not a resonant filter. It's just a really awful C-input filter. This gives terrible supply resistance (VDC = 1.4 * Vrms for light loads, dropping to maybe 1.1x for... 30% loading maybe, down finally to ~0.85x for 60-100% loading), but the low capacitance is required to keep the rectifier happy while running near ratings.Tube diodes cannot withstand large peak currents; typically the absolute maximum cap-input value is 40uF, and that's with derated current capacity.The ripple through the capacitor is miniscule: we're talking 100s of mA here. Okay, fairly large compared to the ratings of an electrolytic that size, but sheesh, you can put whole amperes through a film cap, no biggie. The ripple *voltage* however is fairly large, much more than the 10-20% you'd want on an electrolytic. The voltage swing will be the full peak voltage at heavy loads (where the diode current is continuous, and the filter is acting as a choke-input filter), but less at lighter loads (cap-input filter action).In a regulated supply, I suppose the supply resistance doesn't matter much, so who cares. But that said, I don't know why they didn't just leave it choke-input altogether, why bother with an input cap at all? They certainly could've added a few more henries for the cost of that oil-can cap, probably getting better performance (less ripple, better regulation) in the end.Tim

author: calexanian
author_rank: Super Contributor
timestamp: February 16, 2014, 04:33:05 pm
content: Believe me. Put an ac meter in there and you will see much higher ripple than that! The resonance is supposed to be less than 120hz. Far less. It's not a dc filter in a traditional seance. It's to provide an ac component to the current flowing through the choke to provide proper filtering there. You just have to play with those circuits to understand. Look up the section in the RCA radiotron design handbook under power supplies. It's the best description and explanation of how those circuits work anywhere. You can see how they did it before our modern stuff. I highly recommend that book to everybody. Just the basic circuit theory in there is better than any modern textbook.

author: oldway
author_rank: Super Contributor
timestamp: February 16, 2014, 04:52:02 pm
content: Quote from: calexanian on February 16, 2014, 03:42:08 pmConcern with ESR for an old valve/tube equipment looks very funny to me: in this time, whe did'nt even know what ESR was...In the sixties, i have build a lot of vacuum tube audio amplifiers and repair hundreds of vacuum tube tv's (black and white and colors tv's) and i had never hear somobody speaking about ESR.Later, when I worked with high power fast SCR's inverter, concern was not to exceed RMS current in the electrolytics. It was low frequency ripple (120Hz/180Hz/360Hz)ESR concern was important for SMPS because the high frequency ripple.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 16, 2014, 08:27:19 pm
content: Yes, ESR in those old caps (the few that survive without horrendous ESR!) was on the order ofohms. Miliohms here or there on a film capacitor will go completely and utterly unnoticed.As I expected, my intuition was in the right ballpark:The cap's Irms = 158mA is quite reasonable. 108mA DC output at 432V, probably a bit over ratings (the choke says 100mA, but I don't know what it's actually expected to run at).As it turns out, the capacitance is too large to fully transition to choke-input mode operation at rated current (that'll take more like 200-300mA). So I'll admit to being wrong about that. Accordingly, the output voltage is more like 1.0 x Vrms at rated current, up to 1.4x at no load (and following some curve inbetween).134Vp-p is still kind of a lot of ripple voltage, more than an electrolytic would want to deal with. So yeah, film is still a good idea. One of those EPCOS "DC Link" caps would be more than enoughNote this simulation includes 200 ohms series resistance (representing the transformer and diodes' series resistance), and ideal capacitors and inductor. 200 ohms is probably being generous (that might be the transformer alone, not even including the 5R4/5V4), and the choke is probably 50 or 100 ohms again; actual output is probably more like 350V at the same load.Tim

author: proschuno
author_rank: Contributor
timestamp: February 17, 2014, 04:19:46 am
content: BTW, the choke is 375 ohms (it says so on the label and it measures that.). So are these components purposely designed, or could I actually tweak this design to improve performance slightly I wonder? This was homebuilt, so I'm sure the guy who built it just threw together what he had on hand, so I could go back and make it better….Second Question: Would these caps be suitable to replace the oil caps? I hooked one up to a 1khz square source on my oscilloscope in series with a resistor and it measured to be 4 microfarads, so I'll suppose the other is in the same condition (don't know if I stated this before, but noise, ripple, and nothing else like that was ever a problem with this. But in the event I would need to, I'm just curious, could the film caps below (attached photo) be used?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 17, 2014, 04:29:19 am
content: Whatever it is, it has to be rated for 600VDC absolute minimum; I'd probably get 1000V caps myself. Then, if the value is alright, doesn't matter much beyond that.Tim

author: oldway
author_rank: Super Contributor
timestamp: February 17, 2014, 10:09:07 am
content: Why do you want to replace the oil caps ?If it has no leakage, there is nothing wrong with this caps.It is doing his job and will do it for 50 years more.Never replace what is not really necessary.If ripple is low, you don't have to worry about the caps.You want to make better this power supply.It is a very old design and has a lot of shortcomings.To make it better, you should make a new one.A new design would be a SMPS with power FET, voltage regulation and current limiting, digital readings, ...For a sixties design, I think that is not a very good one:- there would be several voltage ranges with transformer secundary taps comutations for reducing anode power dissipation.- OA2 voltage stabilisators are not that good : noisy, not very stable, often oscillates, ...- 5V4 (more serial resistance) and 6X4 could be replaced with silicon rectifiers. In those times, I used BY100 silicon rectifiers.- I would prefer to use EL34/6CA7 in place of 1619. If it would be a later design (seventies), i would choose to use high power color TV pentode PL/EL519.An example of a far better design:An other one with 7 ranges:

author: dannyf
author_rank: Super Contributor
timestamp: February 17, 2014, 11:59:49 am
content: For high impedance / low current devices like tubes, high ESR is either irrelevant or desirable.

author: proschuno
author_rank: Contributor
timestamp: February 17, 2014, 02:18:53 pm
content: Quote from: calexanian on February 16, 2014, 04:33:05 pmFor the radiotron designer's handbook, would this be what you're talking about?http://www.tubebooks.org/Books/RDH4.pdfOh, and in general, can anybody tell me why the cathode of the 6x4 is connected to one of the hot leads of the High voltage leads? Never quite understood that..

author: richard.cs
author_rank: Super Contributor
timestamp: February 17, 2014, 03:06:25 pm
content: I would be inclined to replace the electrolytics but leave the oil-filled caps. They tend to last forever unless they physically leak oil. Film caps would be a perfectly acceptable replacement I just don't consider them necessary. One change that might improve performance is swapping the OA2's for a zener-string but personally I wouldn''t bother.If your purpose is to overhaul the supply and make it work as well as it did when do then just go with minimum change, if you want a better performing supply then you'd be best off starting from scratch.One thing to watch out for if you use film capacitors for high voltages is they tend to exhibit charge recovery much more than electrolytics or oil filled capacitors do. You should make sure that bleeder resistors are present and not open-circuit.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 17, 2014, 03:24:32 pm
content: Quote from: proschuno on February 17, 2014, 02:18:53 pmYes, that would be the one referred to. That was "the" classic circuit design manual for the vacuum tube era.QuoteThat is where the 6X4 gets its AC input voltage. It goes into the cathode rather than the plates, because the 6X4 is being used to develop a negative bias voltage. Both plates are tied together because the tube is being used as a half-wave rectifier.

author: calexanian
author_rank: Super Contributor
timestamp: February 17, 2014, 04:12:44 pm
content: Quote from: proschuno on February 17, 2014, 02:18:53 pmThats the one. Also. I don't know who started the idea that oil filled caps have high ESR. Tell that to the many thousands of them still being used in discharge welding service! They are still used for big stuff. Radio transmitters, BIG welding equipment. X ray supplies, anything really high voltage. Chicago Condenser is still in businesshttp://www.capacitorindustries.com/Have a look through their website for fun!

author: calexanian
author_rank: Super Contributor
timestamp: February 17, 2014, 04:21:06 pm
content: They actually have a foil film replacement for you. have a look at the CMP-805-6C. Looks like an old style one if you want to keep the warm fuzzy feeling of an old oil can.Might actually have to design something just to get one to play with! BTW. I am not trying to be snarky or obstinate in any way.. Many of these technologies were working well before we were born and will continue working well after we are dust. Just because something is old or deemed to be obsolete, or has a bad name based on feeling rather than science (Leaded glass for example) does not mean it still can have a place or be useful. Don't forget. One of the largest use for tubes still to this day are RF bombarders in the preparation of materials for semiconductors! hahahahahttp://www.capacitorindustries.com/capacitors/high-voltage-dc-capacitors/cmp/default.html

author: proschuno
author_rank: Contributor
timestamp: February 17, 2014, 04:47:05 pm
content: Oh nice, time to request a quote.. lol. But seriously I am going to leave the oil caps alone, they seem to work fine, but electrolytics seem to work fine too, but I'll replace those for good measure anyway (they're like $2 or what not…)Quote from: calexanian on February 17, 2014, 04:21:06 pmI totally agree, I love the fact that this old and outdated tech is still working… to me, it has so much more 'life' than semiconductor stuff for some odd reason, it's so much more fun to work with for me. It will be a sad day to replace these old suckers. And the fact that these were probably handmade, someone took the time to make these, and design the rest of the supply, putting there heart and soul into their passion. And I'm the little quirky college kid out of thousands that has the honor of using and repairing this… that's part of why I love tube tech in the first place, all that history and life, you know?And about the 6x4, I totally knew how the negative supply was made, just never realized it used from the HT windings… never a dull moment with me lol.But yeah, if I can avoid extra work definitely…. the bundle of wires is what I'll have to deal with, along with other photos of the PSU in general, and the caps are the masking-tape colored things. Yep, these photos are the little animal I regularly use:

author: proschuno
author_rank: Contributor
timestamp: February 17, 2014, 05:02:20 pm
content: OH! And can anyone also tell me if the pass pentodes are in pentode or triode connection? Usually one would see an entirely separate supply dedicated to the biasing of the screens, but this just has 100 ohm resistors from un regulated supply to the screens… wonder why?

author: oldway
author_rank: Super Contributor
timestamp: February 17, 2014, 05:18:49 pm
content: Not clean at all !You better look after an Heathkit IP-17 High Voltage PSU.http://www.sgitheach.org.uk/ip17.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 17, 2014, 05:52:08 pm
content: Eww...messy! And masking tape on the caps!The pass tubes are triode connected. Not really anything you can do about that, unless you specifically add a floating screen supply, or some fancy bootstrap or something. Not usually worth it, triodes aren't usually too bad in the end.Tim

author: proschuno
author_rank: Contributor
timestamp: February 17, 2014, 06:54:30 pm
content: Lol now we can all see part of why it was $60US at the antique shop…. the store owner said he got it from some radio guy who was getting rid of stuff…. this being one of them hahahagonna replace one component at a time

author: calexanian
author_rank: Super Contributor
timestamp: February 17, 2014, 07:40:25 pm
content: In a pass tube configuration where we are not amplifying, only bringing the cathode up to as close as we can to grid potential where is no real benefit to having a regulated screen supply as both plate and screen voltages will remain relatively constant. In cathode coupled load circuits plate resistance is so effectively low it does not really do you any good being able to move the screen supply around. As a matter of fact, many supplies used 6AS7's as the pass tube. Both sections in parallel. That tube can really conduct. Lots of cathode and its shot very fluffy for high emission density. Somewhere around here i have a photocopy of the original Tung Sol prints on that tube. Cool stuff..

author: proschuno
author_rank: Contributor
timestamp: February 18, 2014, 02:40:43 am
content: Yeah would totally love for you to upload those, that would be a treat! But yeah, i've been wanting to build a power supply using these bad boys, but I can't seem to identify what they are, and they puzzle many other tube enthusiasts like myself. They're clearly in the 276a/804/242 family, but now true way of testing

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 18, 2014, 03:24:34 am
content: Beefy! Probably not well suited for a power supply though... they typically need high plate voltages. Lots of dissipation means lots of raw output capacity, but only if you put in a thousand volts or so to start with..If nothing else, you can hook up the filament and get yourself an industrial grade night light.Tim

author: calexanian
author_rank: Super Contributor
timestamp: February 18, 2014, 03:29:23 am
content: Just use a variac and a step down transformer and increase voltage until near incandescence. the common filament voltages would be 5, 6.3, 7.5 or 10. There are some higher but for triodes it gets tricky. Thats usually reserved for big rectifiers and RF tubes. It looks like a sand blasted Moly anode. It may be zirconium coated but it looks a bit light in color for that. Moly anodes have a max dissipation of about 5 watts per square CM in the electron beam area and will usually glow a dull red at that. Get some meters and a few supples together and start making some curves and figure out what you can use them for. First run an Eb/Ib test. That is base voltage over base current. That is tube testing terminology for how much does the thing conduct in plate current. Tie the grid to the plate via a 1K resistor or so. Slowly begin increasing plate voltage and plot plate voltage vs current. Use a plate balast resistor so to not tear up your power supply but take the voltage measurement right from the plate. Start at a lower filament voltage and work your way up. If the tungsten filament has a white coating then it may be thoria bonded in which case the emission will be much higher than just thoriated tungsten, but it is most likely just thoriated tungsten. Bright yellow is about the optimal temp for thoriated filaments. Not quite white. Think light bulb with the dimmer switch turned down just a bit for mood lighting. Then apply say 250 to 300 volts on the anode and lower the grid voltage to cutoff. Don,t be surprised if at 0 grid volts its already near cutoff. Many specialty RF and oscillator tubes actually were used in grounded grid service and need a positive grid voltage to conduct at low anode voltages.Are there any marks on them? A manufacturer perhaps?

author: calexanian
author_rank: Super Contributor
timestamp: February 18, 2014, 03:45:13 am
content: I ran down a quick list of types those may be based on no plate cap and short pin medium metal ceramic shell base triodes.203A, 838, or some kind of variant of the 845 or 211. There were many firms out there such as United Electronics, ETI, Cetron, etc etc who made many special variants of standard tubes for various customers needs. All of those tubes mentioned have a 10 volt filament.

author: proschuno
author_rank: Contributor
timestamp: February 18, 2014, 05:00:04 am
content: Alright, totally forgot to say this but one of them (marked "short" on the bottom) glows blue on the inside, particularly near the plate, but also around the bottom of the structure; a few minutes with HT applied this goes away eventually, so I assume it's gassy?There is no manufacturer, but numbers are inscribed on the glass stem inside were the structure is mounted.On the non-shorted one:52552GSShorted:5187L13s91And I have put 10V through the filaments, and they glow a bright yellowish like you described, and consume about 3 amps.

author: calexanian
author_rank: Super Contributor
timestamp: February 18, 2014, 05:48:01 am
content: 10 volts at 3.25 amps is a standard filament. The blue glow and the one marked short has arced and has a high gas pressure (BAD) High amounts of gas have been liberated. The getter sorptions up what it can but other gases it cant sorption cause a gas current avalanche to a high current arc/short condition. Its a common failure for tubes, but they still look pretty.

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 18, 2018, 03:55:40 pm
content: Sorry for resurrecting a necro-thread, but this discussion was *very* interesting!Wonder if the OP finished this project.