Steve WD8DAS said:
I'm resurrecting a very nicely-built homebrew grounded-grid 813 amp. I noticed that the filament voltage at the pins is 8.5 vac. Nominal on the 813 is 10 vac as I recall. Is the 8.5 OK or will emission suffer? I guess I could also be into an RMS versus average metering issue... hmmm. Reply from Jim WD5JKO, Steve, the accuracy of your voltmeter is always a question in cases like this. The load is resistive, so if the filament transformer is putting out a decent sine wave the True-RMS versus peak responding and RMS calibrated meter should give the same answer. More than likely you have some resistance between the transformer and the tube pins. There is likely a filament choke in there since this is a cathode driven amplifier (grounded grid). Look at all the connections, see if any get hot, or maybe measure across each connection looking for a voltage drop. What does the transformer secondary have across it? If the transformer is behaving, and the 1.5 vac drop is from the choke, then you might consider a 12 volt transformer with a resistor in series with the primary such that you have 10 volts AC at the 813 filament pins (not the socket). The series resistor will cushion the turn on current surge a bit, and that is a good thing. I was just going through my transformer junk pile, and I have a nice Stancor 10v CT @ 10 amps; gathering dust. Regards, Jim JKO -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.430 / Virus Database: 268.13.27/517 - Release Date: 11/3/2006 4:30 PM ______________________________________________________________ AMRadio mailing list List Home: http://mailman.qth.net/mailman/listinfo/amradio Partner Website: http://www.amfone.net Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net
