What to do with obnoxious transformer?

[vigilante397]

I have a friend I do work for every now and then, both repairs and builds, so aside from paying me he picks up anything he can find at garage sales, thrift stores, etc. that he thinks might be useful to me. Yesterday he brought me an old analog oscilloscope that the tubes had been pulled out of and was in general "bad shape."

The innards, however, were immaculate! Very clean stuff, I was able to salvage some very good looking Sprague caps and carbon comp resistors. But the big thing I was excited about was the power transformer, as I'm trying to venture deeper into amp building. It didn't have any labels, and the only markings on it are "54-103 Magnetic Windings Co." and a serial number.

I knew from taking it apart which leads the primaries were so I VERY CAREFULLY plugged it in and VERY CAREFULLY took measurements at the secondaries. It looks like I have two sets of filament leads, both reading at ~6.3VAC, but the red wires, which I correctly assumed were the HV leads, measured 685VAC!!!

So my question is what could one possibly do with something like this? Is this even usable in an amp, or is it just too much voltage to reasonably work with?

[R.G.]

I knew from taking it apart which leads the primaries were so I VERY CAREFULLY plugged it in and VERY CAREFULLY took measurements at the secondaries. It looks like I have two sets of filament leads, both reading at ~6.3VAC, but the red wires, which I correctly assumed were the HV leads, measured 685VAC!!!

So my question is what could one possibly do with something like this? Is this even usable in an amp, or is it just too much voltage to reasonably work with?

Sadly, it's pretty much useless for guitar amp work. the high voltage is too high, and the current it provides is too small. And it's dangerous to even use it for the 6Vac filament power. It's not that you couldn't do something with it, but you'd be spending a lot of time and trouble working around it. Rectifying the 685Vac gets you nearly 1kV of DC, and there just aren't any practical guitar amps that use that. There are ways to cut the voltage down to the 350Vdc to 500Vdc that is common in the guitar amp world, but you throw away the excess as heat. The scope trannies I've looked at are too small to provide a lot of power, and it's worse when you have to throw away half to two thirds as heat in a regulator to get down to something useful.

It's not that you can't - but you'd probably be happier with a different power transformer as a basis.

[vigilante397]

It's not that you can't - but you'd probably be happier with a different power transformer as a basis.

And there's the answer I was looking for Though it's not a total loss as the filament windings have very long leads (about 1 foot) and they are cloth covered and insulated, so I think I'll clip those off for wiring to have on hand then toss the iron.

Thanks

[blackieNYC]

Side note- Before moving in deeper and giving it the juice, I'll put a 1.20 volt ac signal into a great big mystery transformer. If I was overseas I suppose I would use 2.20 volts ac. Then measure the secondaries, and you'll see what kind of voltages to expect (.5v would be a 50 volt winding if it were given 120volts ac) Or, maybe you'll find a burnt or shorted wiring. I may be just another monkey at a typewriter, but sometimes I have a bright idea regarding self-preservation.

[printer2]

On the secondary, was there two or three wires? Don't bother plugging it in if yes, check with ohm meter.

[vigilante397]

I'll put a 1.20 volt ac signal into a great big mystery transformer

This is an excellent idea for being even more careful and further avoiding death I also do my best to follow the one-hand rule. Working on amps = cool. Getting killed by amps = not as cool.

On the secondary, was there two or three wires?

Just two on the high voltage. I think I had the same thought as you, if it had a center tap on the secondary I could just run one of the red wires with the tap and get a very usable ~340V. Alas such is not the case.

[printer2]

I'll put a 1.20 volt ac signal into a great big mystery transformer

This is an excellent idea for being even more careful and further avoiding death I also do my best to follow the one-hand rule. Working on amps = cool. Getting killed by amps = not as cool.

On the secondary, was there two or three wires?

Just two on the high voltage. I think I had the same thought as you, if it had a center tap on the secondary I could just run one of the red wires with the tap and get a very usable ~340V. Alas such is not the case.

See you got here before me. No you would not use one red wire. With the center tap you use both wires but the circuit is a full wave rectifier. One half the cycle one winding works and the other half cycle the other one does. Depending on the rectifier you should get between 350-400V. Something like this.



400V on one side and 400V on the other. Just like a tube amp.

[PRR]

Tube 'scopes will generally have 6V, some 400V (too much for a preamp but not enuff for a Power amp), and a wee bit of 2,000V.

The 2,000V may be 600V or 10,000(!)V, but never more than a mA or two, so is more danger than help.

Fred's plan with CRT fed from hardly 500V means a very low-performance 'scope. Trace tends to be dim and un-sharp. (I'm not complaining, I restored a gutted DuMont with such low volts and used it in classroom.)

Always say what 'scope you scored before you tear into it!! Most 'scope manuals can be found, and the manuals typically tell all. A few (very few!) are even worth something, maybe even tube-less (special features, or just for repair parts).