I haven't seen any post about the saturated core constant voltage XFMRs. I use them a lot. They keep the OSCs on the RCVR and XMTR stable. I have never used one on a filament source for a directly heated cathode tube but since the output wave form is not a sine wave but somewhat squared due to the saturation of the core, I would assume the voltage would need to be reduced to arrive at the same heat on the filaments. They were easy to make. A lot of the hybrid TVs of old use power XFMRs with saturated core technology to keep the output voltages constant so that the picture didn't shrink or jump when the air conditioners or heaters would kick on. There was a 2 or 4 uF oil filled cap across part of a secondary winding that some what resonated and saturated the core. The input voltage could vary for 100 - 130 with very little change in the DC from the rectifiers. I would use the filament winds to buck or add to the output to get it where I wanted it to be. With one manufacture, I actually came up with 115 volts as read on a RMS meter. The scope showed it to be clipped about 10-15 percent. A light bulb test was made and found to be brighter on the constant voltage XFMT than on a variac delivering the same RMS measured voltage. This could be compensated for I would think.
John, WA5BXO -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Donald Chester Sent: Friday, January 17, 2003 11:53 AM To: [email protected] Subject: Re: [AMRadio] Today's AC Mains voltage too high for many BA Rigs I have the problem here. I have seen the voltage as low as 108v and as high is 125. My main transmitter has a rheostat on the tube filament transformer line feeding several transformers, but I have the same problem, having to keep a constant eye on the thing and frequent adjustment. Incorrect voltage won't burn out tubes right away, but the manufacturers have warned for years that this will shorten useful tube life. One solution I have thought of, and probably have all the parts for, would be an automatic voltage regulator using a variac and reversible motor. I have a relay that looks like a meter, but the pointer has two sets of contacts, one of which will engage wenever the pointer is above or below the set value which is adjustable with a knob near the zeroing screw. It's just a matter of figuring out exactly how to build the thing and putting it together. Minus the special relay, I have thought about using a VR tube and comparing its voltage output to that of an unregulated power supply with the same nominal output voltage. Of course, it could also be done using solid state components. One thought about using a variac. With most of the ones I have used, even the HD ones, the regulation sucks. A tapped autoformer with a heavy duty switch is better. Instead of using the variac to drive the equipment direcly, it would be better to use a large, high current, low voltage transformer@ about 15-24 vac/ 30 amps as a bucking transformer and use the variac in the primaryof the transformer. That way you get better resolution of voltage and better regulation. The problem with variacs is the carbon brush contact. It must have a certain amount of resistance so that shorted adjacent turns don't overheat, since the wiper contact must make before break to assure uninterrupted output as the voltage is varied. Don K4KYV
