Im feeling isolated- advice- opnion?

[firechief]

Just scored today this power transformer. Must be good for something. Heck I know that this is the start of somethigf. I will post more info as soon as I have some time to make measurments. My benifactor said that it is 110 to 220 isolation. Sounds like I may have to find out if the secondary can be re-wound, or try reversing? He also said that there were two teriraray (sp) windings that should be good for a couple hundred watts each, but I would rather make use of the whole thing. Big SS amp? It was a gift. Wow!

[indianajo]

240VAC-120 VAC isolator can be used for a 55 VAC ( 75 DC) single rail amp if energized with 120 VAC. Dynakit ST120 rail was regulated to 80 VDC at 6.5A. Modern transistors can get nearer the rail than the 2n3772's the ST120 was designed for. There are other designs with output caps - look at the thread "output capacitors objective and subjective results" Output capacitor: subjective and objective views? for another design using expensive fets for better signal to noise ratio. Output caps are good for preventing DC blowout of your speakers if the transistors short out. If the amp doesn't cross zero volts, electrolytic caps don't make the funny noise while the chemical reactions occur as it does in the output of a split supply amp.

[firechief]

Thank indianajo: So see here, a picture of the windings seperated out by groups. I'm guessing that the red/black is the primary, then a group of gray/yel/brown, purp/whr/or, and some smaller guage secondarys blue/brown, and pink/brown. I could inject AC from my audio generator into the red/black and check with the scope to measure the AC on the other windings, or just put 110 on the red/black and check what I am asuning are the secondarys to detirmine the step up ratio. (with a current limiting lightbulb of course). Also I belive it is correct to load the secondarys with a resistor to get an accurate measurments.

[indianajo]

Originally Posted by firechief

I could inject AC from my audio generator into the red/black and check with the scope to measure the AC on the other windings, or just put 110 on the red/black and check what I am asuning are the secondarys to detirmine the step up ratio. (with a current limiting lightbulb of course). Also I belive it is correct to load the secondarys with a resistor to get an accurate measurments.

Uh, yeah. I put a 24 VAC secondary transformer in a steel box (PC power supply) with a fuse in , on off switch, a fuse out, and a 6 ohm onan generator resistor (1/2 pound) in series with the output for determining what transformers were. Clip your meter on, flip power switch on, read, off, no touch, no fire. Good luck.

[firechief]

OK so with 1VPtP(100HZ) on the red and black, I get .5V PTP with the scope ground on the brown to the yel and same from brown to gray. This appears to be a 50 % step down. Likewise the purp is the seconday center tap 50% step down. (Two matched secondarys)Then there is .5V PTP accross the pink/green and .5V PTP accross the blue/brown small taps. Hummm, looks encouraging. Assuming I run the input at 110V, after full bridge rectification and filtering, what can I expect for +- supply voltages? Anybody got some real big heatsinks for sale?

[indianajo]

Like I said, 50% winding make 60VAC RMS, 1.4x that - 1volt=83V Peak. If your really have two independent center tapped windings, then you could use double diodes instead of bridges. You can either parallel them for a super ST120, or stack them for +-83V to make something like a Peavey 1.3K. I've got six or ten motor drive heatsinks, but you can probably find a maintenance man at a food plant out there that can fish them out of the dumpster for you. (Food plants kill lots of motor drives with the washdown process).
You'll need something like +- 15 for op amps, the cheapest stuff I know is wall-transformers at a $1 each in a grocery cart at Salvation Army resale shop.
Finding speakers that will take 75 volts is not hard- Say Peavey SP3 or SP7BW. Owning a hall that needs them, and moving them there is the problem. A pair of SP7's came up on craigslist here yesterday, they weigh about 150 lb and are on casters. They would not fit in my car like the SP2's did, I'd have to rent a truck. Oh, BTW, I bought a real Peavey 1.3k amp in September, inop, for $65. My winter project. I'm making 3 ST120 channels this winter to transistorize my organ, +83 is perfect for a single ended ST120. Look at this link too, the single ended fet amp http://www.angelfire.com/ab3/mjramp/mjr8-2.html