On Friday 22 May 2009, Joerg wrote: >John Doty wrote: >> On May 21, 2009, at 6:00 PM, Joerg wrote: >>> Chris Albertson wrote: >>> >>> [...] >>> >>>> I'm thinking about tube amps that had an output impedance of about 1M >>>> ohm that used transformers to drive 8 ohm speakers. About a 100,000 >>>> to 1 ratio. >>> >>> 1M? What kind of tube was that? >> >> Well, that's a typical plate resistance for a small signal pentode, but >> >> Power pentodes have lower plate resistance. > >Small signal, yes. But a 12AX7 won't be enough for a rock concert ;-) > And its not even a pentode, its a dual triode, designed for phono preamps and such.
>> For a large signal amplifier, the load generally isn't matched to the >> output resistance. Instead, it's roughly (peak output voltage)/(peak >> output current), which is different. If you match the output >> resistance, in most cases you'll clip at an output power well below >> the capacity of the amplifier. >> >> And finally, the real issue here is the current required. 100 amps >> will melt the wire in any audio transformer I've ever seen. >> >> Everybody seems to think Mark's soldering gun suggestion is a joke, >> but I don't know. I think I'd get one, pull the transformer, measure >> its characteristics, see if it might work (maybe a couple of them in >> series/parallel or something). They're light, cheap, and the closest >> thing to the requirement here I can think of. I'm sure they won't run >> at full power all day without overheating, but for a test set that >> might be OK. Full power? A minute thirty maybe then needs at least a 5 minute cooldown. >It's not a joke, quite viable maybe. But the solder guns I have used >can't quite get to 100 amps. Maybe the 100W Weller I got for a client >does, it cost around $30 at a hardware store. Another option is to use a >regular (fat) mains transformer that has a bit of clearance between the >packet and core. Run a wide sheet of thick copper through there, only >one turn and leave its usual secondary winding alone. Or if to be driven >from a generator drive that other secondary and leave the primary alone >(and don't touch it ...). This results in a huge current capability. >Another option may be welding transformers. Even my cheap one can >deliver 160 amps for quite some time. But those are huge. > >As usual, Levente needs to take every piece of metal off. Wedding band, >wrist watch, etc. Best not to have credit cards close by either because >their magnetic strip might later be stripped of its information. BTDT, >quite embarrassing when you take the guys out for lunch and the waitress >comes back with "Your credit card doesn't work". :-) -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) The NRA is offering FREE Associate memberships to anyone who wants them. <https://www.nrahq.org/nrabonus/accept-membership.asp> Life is a whim of several billion cells to be you for a while. _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user