Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watmath!clyde!cbosgd!hal!cwruecmp!neoucom!wtm From: wtm@neoucom.UUCP Newsgroups: sci.electronics Subject: Re: Power Supplies for Tube Circuits. Message-ID: <622@neoucom.UUCP> Date: Thu, 9-Jul-87 10:37:12 EDT Article-I.D.: neoucom.622 Posted: Thu Jul 9 10:37:12 1987 Date-Received: Sun, 12-Jul-87 07:41:27 EDT References: <12887@rose.UUCP> <8240@utzoo.UUCP> <485@jetsun.weitek.UUCP> Organization: Northeastern Ohio Universities College of Medicine Lines: 51 Summary: DC power for tube heaters Hi, While we're on the topic of tube preamps... I learned a lot about the nearly lost art of tube construction methods by looking at old Tektronix 500 series oscilloscopes and their letter series plug-ins. The Tek scopes were about as far as the state of the art of tube technology went. In their hight gain amplifers, they often used DC on the tube heaters to help reduce 60 Hz modulation of the signal being amplified. The high gain preamps were also usually isolated from the chassis by rubber or springy metal strips. The idea was probably to combat microphonics from picking up vibrations of the fan. (Quite a big fan was necessary to cool down a 551 with its complement of 111 tubes! -- and it had a separate power supply with about a dozen more tubes for regulating all voltages). Shock mounting the amp would probably be useful in combating feedback from the speakers. One thing that has puzzled me is that some Tektronix plug-ins have flat rubber coated resistors. I've wondered what the purpose of doing so was. I've never seen any references as to why. One thing to watch out for in tubes is the potential difference between the heater and the cathode. 12AX7s permit about 100 vdc between the heater and cathode. The main impact is on the life of the heater, and possible dielectric breakdown of the barrier between the heater and cathode. You should be aware of this if you are designing a circuit that runs one or more of the stages with the cathode significantly above 0v. If so, you can float the filament xformer. The got-cha is if you feed DC to the heaters, and the DC supply is ground referenced. I like the idea of using a switcher to supply the B+. By using a switcher, you eliminate bulk and the higher frequency means that smaller capacitors can be used to provide equal ripple removal. Be sure to use good construction, to prevent RF from escaping. Also, you need to use good quality capacitors with a high Q rating. Regular aluminum electrolytics aren't very good. A small mylar capacitor in parallel can be used if you can't locate high grade electrolyic capacitors. (Save your $$$, and don't bother buying esoteric polyproylene caps for your power supply). Who knows, the polypropylene capacitors may make a difference in interstage signal coupling-- you be the judge of that. To me, mylar caps sound equivalent for audio coupling. Hope this was of some use. Bill (wtm@neoucom.UUCP)