Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ihnp4!ptsfa!ames!oliveb!pyramid!weitek!neal From: neal@weitek.UUCP Newsgroups: sci.electronics Subject: Re: Power Supplies for Tube Circuits. Message-ID: <485@jetsun.weitek.UUCP> Date: Tue, 7-Jul-87 07:53:06 EDT Article-I.D.: jetsun.485 Posted: Tue Jul 7 07:53:06 1987 Date-Received: Thu, 9-Jul-87 06:11:39 EDT References: <12887@rose.UUCP> <8240@utzoo.UUCP> Reply-To: neal@jetsun.UUCP (Neal Bedard) Organization: Weitek Corporation, Sunnyvale Lines: 42 In article <8240@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes: >> ... I could obtain the less than 330 volts needed by >> directly rectifying the line current and putting it through a voltage >> doubler, eliminating the power transformer. [...] > >The trouble is that the negative end of the power supply is not isolated >from the AC line and hence can pretend to be "ground" only if the whole >gadget is self-contained and well-insulated, and you avoid connecting >things like oscilloscopes to it. I believe some TV sets do their power >supplies this way, much to the irritation of TV service people who need to >haul around an isolation transformer if they want to connect test gear. They don't call 'em "hot chassis" for nuthin'! Yeow! > [...] For a pre-amp, you are presumably going to be connecting it to other >equipment. Isolating those connections is going to be hard, probably too hard >to be worthwhile. Accordingly, I would give this a big black NOT ACCEPTABLE. >Use a transformer. One way to reduce the bulkiness (but not neccessarily the cost) of the setup is to rectify the AC directly, and use that as the power supply of a DC-DC converter. Not only do you get the isolation (thx to the ET transformer) but you also get better regulation on the B+ (300V) supply. However, you still need filament voltage (a small/cheap transformer can do this.) Another approach is to use a low (filament) voltage transformer with enough excess capacity to drive a "flyback" DC-DC converter for the B+ supply. You get isolation, simple B+ regulation, and filament voltage too. However, if you're lazy (or dislike working with switchers), you can probably scrounge an all-in one B+/filament transformer pretty cheap from a surplus house. You'll have to filter the B+ supply with some big HV caps, tho'. Ick. One advantage: transformers tend to "swallow" line spikes. Henry's right, tho' - whatever you do, do not hook the circuit innards directly to the AC line. I wouldn't recommend the use of an "auto-transformer" either, since these connect you to at least one leg of AC (50-50 chance it'll be the "wrong" one.) Be careful. -Neal