Voltage multiplier current question - Page 1
Forum section: electronics / projects

author: darkness_electro
author_rank: Contributor
timestamp: August 19, 2015, 02:46:07 pm
content: Hello, i watched Dave's video about the Cockroft-Walton multiplier and i have a question about the current i can supply and the capacitors. I want to use it to power 2 12ax7 tubes at 250v b+. I have a 2×12 v 1a transformer but i can get a 2×24 one. Can i use the multipier for the b+ voltage? Can it supply the current, about 5 ma per tube? What value capacitors whould i get?

author: mathias
author_rank: Regular Contributor
timestamp: August 19, 2015, 06:03:35 pm
content: It won't work. It will hum.

author: darkness_electro
author_rank: Contributor
timestamp: August 19, 2015, 08:16:13 pm
content: Then what can i use to get 250 vdc? I can't get any 1:1 transformers. Only a 230-110 transformer. Would two transformers back to back work? Also, what is the voltage drop of a tube diode?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2015, 08:33:03 pm
content: Hammond and Edcor make "vintage" style transformers which may be of interest.A multiplier on 12VAC won't get you there, because the diode drops add. Different multiplier designs can avoid that issue (at some expense to capacitor requirements or complexity), but you're really better off using a higher voltage. (48VAC wouldn't be too bad.)Yes, back-to-back transformers work, and even if you had only 120V transformers, a doubler on one of those would do a fine job.Not sure what "will hum", that's just a matter of using enough capacitance for the required filtering.You might also consider using a much lower voltage circuit. 6DJ8/6922 is quite popular, having less voltage gain but far more current gain than 12AX7, and is capable of quite reasonable performance under 100V supply.Tube diodes perform better than silicon diodes, at low currents; but for the most part, perform much worse, especially when heater power is included in that assessment. What were you considering?Tim

author: darkness_electro
author_rank: Contributor
timestamp: August 19, 2015, 11:35:08 pm
content: I am soing this circuithttp://diyaudioprojects.com/Schematics/7025-12AX7A-Tube-Phono-Preamplifier-Schematic.htmUnfortunately, the transformers you mentiomed are extremely hard to find in eastern europe, i can barely find a 2×12v. Absolutely no way i can get one of those. I have the tubes, i have the parts, but i can't find a way to get the b+. How critical is the voltage and how clean shoud it be? 20v ripple is acceptable? I may be able to get the voltage from directly rectification from mains, but then i won't have any isolation. Why would tubes be bad for rectifier? I am aiming at a full tube design, but for starters, anything is ok. I have a big problem because i can't seem to find the tube diode voltage drop. What voltage will i get from the rectifier tube. Is a normal inductor, say max 500mH work as a choke?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 20, 2015, 06:46:40 am
content: A fully vintage circuit would use a 6X4/6X5 rectifier, and the forward drop would be in the 20V range. A small power amplifier would also be included. Typically a 250-0-250V 50mA transformer would be used, giving about 300VDC for the power output stage (which might be 6AQ5/6V6 or 6BQ5/EL84).A third preamp/driver tube might be needed, which could be 6C4, 6AV5, another 12AX7/7025 (two sections could be used to drive a tone control stage), etc.A phono stage needs to have very clean power, usually by hanging it off some RC filters. Typical values are 10k ohms and 10-40uF. The voltage drop across the resistors also results in lower voltage, so you might expect 200-250V on the phono stage, as labeled.There may be nearby sources, for transformers like this, but of course I wouldn't know about them; you might search some larger hifi groups, like diyAudio, to see if anyone knows. If nothing else, I know a few in western Europe who wind transformers; of course, a custom transformer probably wouldn't be a good price.Tim

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 07:58:27 am
content: 250 v is quite close to mains voltage, so i will try to use two transformers back to back to get 230v and use maybe a hybrid rectifier.I have this tube rectifier.http://frank.pocnet.net/sheets/138/c/CK1005.pdfDo you suggest that the circuit may need another stage? I would use a 12ax7 or 6dj8 cathode follower, but i don't understand how to build that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 20, 2015, 10:57:08 am
content: I wouldn't recommend it, with 450V maximum peak rating, it won't be able to handle 250VDC (>500V peak rating is required).Tim

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 12:54:29 pm
content: Ok. The last question is what transformers shoud i use? Do i connect the 12 v windings in parallel? What rating should be the transformers?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 20, 2015, 02:05:42 pm
content: For just this circuit, a few mA at 250V will work, which means a few 100mA at 12V will be enough. It's a pretty small transformer, so not a big cost (as long as you can find it, in stock, in a catalog, of course).The first transformer (mains down to 12V) needs to be larger, because it supplies the 12V heaters too. (And anything else you might need at low voltage?)It might be worthwhile using slightly larger transformers anyway, because there will be more voltage drop than usual when cascading transformers, especially small ones. Probably anything around 200-500mA at 12V (2.4-6 VA rating) will be fine. If you use a larger size, you can use the same part for both transformers.Tim

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 02:25:01 pm
content: Thea heaters draw 340 ma each tube, so 700 in total. Would a 2amp be enough for step down, and the 1a as step up?Please, can you reccomend me a cheap tube rectifier?

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 02:29:09 pm
content: Also, do i connect both 12 v windings in parallel?

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2015, 03:21:35 pm
content: I don't see why a voltage multiplier wouldn't work.You only want 10mA so just use large capacitors.I wouldn't advise tube rectifiers though.A voltage regulator could be added to get rid of the ripple and hum.

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 03:23:58 pm
content: I aim at a full tube design. It's just that. I just want to experiment with tubes.

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 04:16:01 pm
content: http://diyaudioprojects.com/Schematics/7025-12AX7A-Tube-Phono-Preamplifier-Schematic.htmThis is the schematic. Can anyone figure how clean the 250v DC should be? I see some RC filters there, so i assume that it can have some ripple.

author: Gyro
author_rank: Super Contributor
timestamp: August 20, 2015, 05:23:29 pm
content: Why start 2 threads on exactly the same subject / requirement???You already have one thread running on this Today: "250V DC Power supply"