250V DC power supply - Page 1
Forum section: electronics / projects

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 01:44:21 pm
content: I want to build a power supply for a tube phono preamp, but i can't find a high voltahe transformer.The preamp draws about 10 mA, possibly less, and i can't get the voltage. I would like, if possible a tube rectifier.The only way i find it is to use back to back transformers. What ratings should they be? I already have a 2x12 1a transformer.

author: singapol
author_rank: Frequent Contributor
timestamp: August 20, 2015, 02:01:18 pm
content: You mean connecting the secondary coils in series? A better way is to get your country's domestic ac voltage transformer...a stepup to industrial 415 ac volts.Who say there are no high voltage transformers or if you have creadit card can but frm ebay chinese ones or canadian Hammond transformers at reasonable prices.

author: amyk
author_rank: Super Contributor
timestamp: August 20, 2015, 02:07:18 pm
content: Tube circuitry tends to be very lenient with respect to high voltage, so if you live in a country with 240V mains and can make your design use ~340VDC, a rectifier + smoothing capacitor would be much simpler.(Usual disclaimers about high voltage apply.)

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 02:22:56 pm
content: I know, but i would lose the isolation. Isn't it quite dangerous to have the supply mains earth referenced?It would be extremely easy to make it run directly off mains.Any idea how i can get some isolation?

author: edavid
author_rank: Super Contributor
timestamp: August 20, 2015, 02:29:18 pm
content: Quote from: darkness_electro on August 20, 2015, 02:22:56 pmYes, don't do it! Totally inappropriate for a preamp.Back to back transformers are fine. You can calculate the primary current from secondary rating divided by turns ratio.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2015, 03:16:21 pm
content: That's pretty low power.How about using a lower voltage transformer and a voltage multiplier? A 24V transformer plus an eight stage multiplier would give about 250V.

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 03:22:36 pm
content: Shipping charges for a transformer are more than the item iself. I can find only small and cheap tranaformers that vibrate like an engine.I wrote in a separate thread, the thing with voltage multipliers is the losses just add upt too much. I will use the back to back transformer. Thank you. Cand anyone reccomend a cheap tube rectifier, eventually one that is aviable on ebay? I'm in the eastern europe.

author: LukeW
author_rank: Frequent Contributor
timestamp: August 20, 2015, 03:23:35 pm
content: There's also the possibility of making a switching converter, something like this:http://diyaudioprojects.com/Tubes/12AX7_Preamp/F02psschematic.jpg

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2015, 03:24:51 pm
content: Quote from: darkness_electro on August 20, 2015, 03:22:36 pmAnd I said at only 10mA that shouldn't be an issue.

author: darkness_electro
author_rank: Contributor
timestamp: August 20, 2015, 03:28:37 pm
content: The switching transformer is quite hard to find. I thought of a mc34063 supply, but a smps in a tubr amp just seems a bit out of place. A bit.

author: Gyro
author_rank: Super Contributor
timestamp: August 20, 2015, 03:45:25 pm
content: I don't know whether it's worth mentioning here but old UK (and presumably European) shaver outlets and shaving lights are a useful source of decent mains isolating transformers in the 20VA range. They usually have a 120V secondary tap too which could maybe be used as a CT for lower outputs.Enough to get you around 50mA at 320V after rectification and smoothing so ideal for pre-amps.They're well constructed because they are designed to maintain safe isolation in a steamy environment. Worth looking out for when disposing of other 'treasures'

author: AF6LJ
author_rank: Unknown
timestamp: August 20, 2015, 03:59:58 pm
content: Here is your transformer..http://www.mouser.com/Power/Transformers/Power-Transformers/_/N-8u9n5?P=1z0wd1a

author: AF6LJ
author_rank: Unknown
timestamp: August 20, 2015, 04:01:09 pm
content: I would also strongly discourage using switching power supplies, unless you really want the headaches of keeping the radiated EMI from the switcher out of your preamp.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2015, 08:10:46 pm
content: A voltage doubler and linear regulator can provide a very clean power supply.V1 & Rs model a 24V transformer with some series resistance.IL is the 10mA load.EDIT:Changed Q1's configuration for better ripple regulation.

author: Gyro
author_rank: Super Contributor
timestamp: August 21, 2015, 08:59:02 am
content: You need to add a country flag to your profile so that people can give you more useful sourcing information based your location. An economical source of an HT isolation transformer is hopefully not a million miles away...http://www.ebay.co.uk/itm/Contactum-Shaver-Outlet-2300-/141728190729?hash=item20ffa89109Of course you still need to arrange a heater supply but that shouldn't be an issue.

author: AF6LJ
author_rank: Unknown
timestamp: August 21, 2015, 01:23:15 pm
content: Quote from: Gyro on August 21, 2015, 08:59:02 amA heater supply is going to be easy, I would even go so far as to suggest a DC supply.

author: darkness_electro
author_rank: Contributor
timestamp: August 21, 2015, 03:46:29 pm
content: I'm thinking also of a dc heater aupply, but it would cossume more power than an ac supply. Can i connect the teaters in series? 2 6 v heaters at 12v supply?

author: AF6LJ
author_rank: Unknown
timestamp: August 21, 2015, 09:54:05 pm
content: Quote from: darkness_electro on August 21, 2015, 03:46:29 pmYes you can however here is the plot complication.....If you use an AC heater supply yhou need a center tapped transformer. The secondary winding needs to have a center tap that is grounded. this cancels out most of the hum the heater circuit may introduce into the preamp circuit.If you want to be super picky about the balancing out the hum caused by the heaters get rid of the AC altogether and use a DC supply. This was common in vacuum tube test equipment back in the sixties. Power the heaters of the low level stages off DC to eliminate the hum. Another method is to use a pot across the heater leads with the center arm grounded and use the pot to null out any hum caused by the heaters. My HP-412 VTVM uses this method.By the way;This project is a wonderful way to learn about the trials and headaches involved in grounding and avoiding ground loops. If you pursue this to it's conclusion you will come away knowing a lot about grounding you may not already know.

author: bills
author_rank: Frequent Contributor
timestamp: August 21, 2015, 10:49:30 pm
content: You can also source what you need from a vintage valve radio. or here.https://www.tubesandmore.com/products/transformers_chokes?filters=Brand%3DHammond