Well, if you use a bigger variac and a lot of capacitance in the power supply, its not bad.
I have two, both run around 2000 volts at about 400ma, and they don't have much problems. The RF deck is a steady load, so that's no problem, the modulator can peak up to 500ma, but with a swinging choke and 50 uf of capacitance, it does not seem to be a problem. The variacs are mounted on aluminum front panels, and DO generate a nice magnetic field, best kept well away from any audio stuff. I think I use 10 or 12 amp variacs, which gives me a little extra variac head room... The little variacs (2 amps) work great for things like screen voltage in RF decks, bias supplies, ncl supplies, where the load current is very light, and regulation is not important. On audio tube screens, you want something very stiff. I DO like the primary tap/relay setup, works great if you can deal with the fixed voltage steps it gives. I have a ton of 110vac relays pulled from equipment that did not actually operate them except in a power failure (UPS bypass relays), so used them in the 812 rig. I used smaller plug in relays that came with the nice sockets that mount directly to the chassis, with screw down wire terminals, a snap to add to the circuit. DPDT, 10 amps per contact, high quality relays. I parallel up the contacts, for 20 amps. The RCA transformer I use has 3 primary and 3 secondary taps, which gives plenty of range output. I think I have it on maximum power now, 1500, 1700, 1900 volts output. I keep going up, as the 812a rig seems to not mind higher voltage at all. I figure its fine as long as the plates don't show any color. I might use the same relay setup in the 4d32 rig, separate power supplies for rf and modulator, but run things on the low voltage taps. I bet there is a good and easy way to use some sort of solid state components to replace the variac, but I have not looked into it. Brett N2DTS > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of Donald Chester > Sent: Thursday, February 26, 2004 9:49 PM > To: [email protected] > Subject: RE: [AMRadio] Need Variac > > > > > > >they work well, > >but there is loss in the variacs, more sag under load > >than without one. > > > I think part of the reason is the carbon brush contact. You > cannot use a > metal contact because it would short out adjacent turns as > the variac is > rotated and overheat the coil and burn up the contacts as well. The > graphite brush acts as a current limiting resistor to protect > the winding, > but also introduces losses. I also found the voltage > regulation to be > noticeably better without the variac. The best tapped > autotransformer I > ever had was the voltage control from an old x-ray machine. > It had huge > contacts on a rotary switch, and there was no detectable > increase in voltage > sag with the transformer in line. Unfortunately, I swapped > it for some > other gear after I had acquired a nice new G-R 20 amp variac. > > Don K4KYV > > _________________________________________________________________ > Stay informed on Election 2004 and the race to Super Tuesday. > http://special.msn.com/msn/election2004.armx > > _______________________________________________ > AMRadio mailing list > [email protected] > http://mailman.qth.net/mailman/listinfo/amradio
